Thermal Bridge Development For Uneven Surfaces Degree Thesis

Thermal Bridge Development For Uneven Surfaces  Degree Thesis
Thermal Bridge Development For Uneven
Surfaces
Degree Thesis
Plastics Technology
2012
Sisay Mesele
DEGREE THESIS
Arcada
Degree Programme:
Plastics Technology
Identification number:
Author:
Title:
Supervisor (Arcada):
9678
Sisay Mesele
Thermal Bridge Development for uneven surfaces
Mikael Paronen
Commissioned by:
Abstract:
A conductive particulate filled polymer composite is developed by blending Alumunium
oxide (Al2O3) powder as a conductive filler with various proportions by weight
(5,10,15,20,30,40)%wt and Silicone resin as a matrix for thermal bridging application.
The material is to be mounted between wall surfaces and flux-meter that was developed
at Arcada to measure energy in the form of heat-flux in buidings.
Most building surfaces are rough and hold air as thermal insulator and this has a significant effect to create a reading error for the device to give exact value due to the fact that,
the rough surface holds air creating a thermal resistance. In order to avoid this thermal
insulation by air, kair=0.0027W/m2.K, the composite material has to fill the rough surface
to enhance thermal conductivity between the surfaces.
The compostion materials were selected based on the application requirement to have
characteristics of good thermal conductivity,surface comformablity, soft,flexible, self
tacky and user freindly to the wall without leaving a stain on the area.
Each specimen were tested for thermal conductivity based on experimental mearement
and the result revealed as the amount of filler increased thermal conductivity decreased
because of increased filler resulted in tough and hard material that incomplete comformation on the wall left air gaps to exist. All samples achieved to conform to surface
irregularities, thereby eliminating air voids and improved heat flow through them to conduct heat from the wall surface to the flux meter by illumination of air contained.
Keywords:
TIM,
Composites,
Number of pages:
Language:
Date of acceptance:
54
English
15 Nov. 2011
2
Fillers,
thermal
conductivty
Acknowledgements
This project would not have been possible without the precious support of my thesis
supervisor Mr. Mikael Paronen, head of the department of Energy and Materials technology, who was abundantly helpful and offered me invaluable assistance, support and
guidance.
I also would like to thank Mrs. Marian Holmberg senior lecturer at Arcada University of
Applied science, for her kindness and helpful advice she has contributed for my me.
Deepest gratitude is also due to the people who have shown me their concern, support
and courageous assistance throughout my studies.
3
Contents
1
Introduction ........................................................................................................... 9
1.2
2
LITERATURE SURVEY .................................................................................... 11
2.2
Modes of Heat transfer in materials............................................................... 13
2.2.1
Conduction ............................................................................................ 14
2.2.2
Convection ............................................................................................ 14
2.2.3
Radiation ............................................................................................... 15
2.3
Heat flux ....................................................................................................... 15
2.3.1
Heat flux sensors (meter) ....................................................................... 15
2.3.2
Applications in building physics ............................................................ 16
2.4
3
Objective ...................................................................................................... 10
Factors affecting heat flux measurement on uneven surfaces ......................... 16
Material Selection ................................................................................................ 17
3.1
Overview of material selection...................................................................... 17
3.2
Thermal properties of Materials .................................................................... 18
3.2.1
Thermal conductivity ............................................................................. 18
3.2.2
Thermal Expansion ................................................................................ 19
3.2.3
Thermal resistance ................................................................................. 19
3.2.4
Thermal Transmittance .......................................................................... 21
3.3
Polymer Blends ............................................................................................ 22
3.4
Thermal Interface Materials and Conductive Composites.............................. 22
3.5
Particulate Filled Polymer composites .......................................................... 24
3.5.1
Types and Composition of Polymer Composites .................................... 25
3.5.2
Polymer matrix properties and selection................................................. 26
3.5.3
Particulate Filler materials ..................................................................... 27
3.6
Particle packing ............................................................................................ 31
3.6.1
3.7
Particle Packing Theory ......................................................................... 32
Additives ...................................................................................................... 32
4
3.8
Theoretical Background of Conductive Polymer Composites ........................ 33
3.8.1
4
Theoretical Models of Thermal Conductivity in Composites .................. 33
METHODOLOGY .............................................................................................. 35
4.1.1
Resin ..................................................................................................... 35
4.1.2
Filler (Aluminum Oxide): ...................................................................... 37
4.2
Fabrication of particulate filled polymer composite....................................... 37
4.2.1
Matrix Preparation ................................................................................. 37
4.3
Thermal conductivity analysis of Samples .................................................... 43
4.4
Characterization of the composite Materials.................................................. 48
5
RESULT and DISCUSSIONS ............................................................................. 49
6
CONCLUSION ................................................................................................... 50
7
REFERENCES .................................................................................................... 51
5
List of Figures
Figure 1.......................................................................................................................12
Figure 2.......................................................................................................................19
Figure 3.......................................................................................................................22
Figure 4.......................................................................................................................38
Figure 5.......................................................................................................................39
Figure 6.......................................................................................................................40
Figure 8 .......................................................................................................................41
Figure 9.......................................................................................................................42
6
List of Tables
Table 1.......................................................................................................................36
Table 2.......................................................................................................................39
Table 3.......................................................................................................................41
Table 4.......................................................................................................................44
Table 5.......................................................................................................................45
7
Nomenclature
K=Thermal conductivity
Q=heat
A=Area
Q’=heat flux
L=thickness
t=time
dT=change in Temperature
Rth= Thermal Resistance
Ccontact = Contact resistance
TIM= Thermal Interface material
Tg= Glass transition Temperature
Tm= melt transition temperature
Vf= Filler volume fraction
Vm= Matrix volume fraction
wf= weight fraction of filler
wm= weight fraction of matrix
f= density of filler
m= Density of matrix
F= Packing factor
Kc= thermal conductivity of composite
Kf= filler thermal conductivity
Kef= effective thermal conductivity
8
1 INTRODUCTION
1.2 Background
When the heat flux sensor is to be mounted on top of the rough surface of a wall, there
is difficult to find the correct measurement of heat energy loss rate, due to the fact that
the air trapped in the surface affects by creating a thermal resistance. Since air has one
type of thermal insulator with lowest thermal conductivity resulting in lowest conductivity of heat, a thermal interface material is used to fill this air holding spaces on the
surfaces. The air gap filler material has to be formulated from a blend of heat conducting materials that can enhance heat transfer from the measurement surface to the measuring device heat flux meter. The composite material is developed from a mixture of
two materials, highly soft silicone resin as a base matrix and Aluminum Oxide (Al2O3)
powder as a filler material having high thermal conductivity that can maximize the heat
transfer rate from wall to the bottom surface of the flux meter. To minimize the roughness of a wall surface for heat transfer measurement of building surfaces highly soft,
flexible and highly conformable silicone elastomeric is chosen as a matrix or base material, the selection is also based on chemical inertness, stain free, naturally tacky and
good thermal stability than other polymers are factors for using this material. In addition to this Alumina (Al2O3) fine powder is incorporated as thermal conductive enhancing filler. The composite material functions as a thermal bridge that interfaces the two
solid surfaces, i.e. the heating surface and the bottom surface of the flux meter.
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1.2 Objective
The main objective of the thesis is to develop a thermal bridge composite material that
can be used as a thermal interface material for the heat flux device found at Arcada to
enable to measure the energy transfer through wall surface of a building.
To select and formulate the composition materials based on the required parameters
such as thermal conductivity and stability, good filler-matrix integration, flexibility, surface conformation and tackiness as well as user friendly.
The material is used to avoid the air gaps created by the rough surfaces to make smooth
and avoid thermal insulation by air.
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2 LITERATURE SURVEY
2.1 Literature review
The literature review is to provide background information on the issues to be addressed
in this thesis and to emphasize the relevance of the present study. This embraces some
related aspects of polymer composites with reference to their thermal conductivity characteristics. The topics include brief review on:
•
Thermal properties of composite materials
•
Particulate Reinforced Polymer Composites
•
Thermal Conductivity of Polymer Composites
2.1.1 On Particulate filled polymer composites:
Hard particulate fillers consisting of ceramic or metal particles and fiber fillers made of
glass are extensively being used these days to dramatically improve the mechanical
properties such as wear resistance, even up to three orders of magnitude [20]. Various
kinds of polymers and polymer matrix composites reinforced with metal particles have a
wide range of industrial applications such as heaters, composites with thermal durability
at high temperature [22] etc. These engineering composites are desired due to their low
density, high corrosion resistance, ease of fabrication and low cost [10-12]. Similarly
for over two decades, ceramic filled polymer composites have been the subject of extensive research. The inclusion of inorganic fillers into polymers for commercial applications is primarily aimed at the cost reduction and stiffness improvement [12, 16]. Along
with fiber reinforced composites, the particulate filled composites have been found to
perform well in many real operational conditions. Important role in improving electrical, mechanical and thermal properties of the composites is played by silica particles
when they are added into a polymer matrix to form a composite, [27, and 28]. Mechanical, thermal and electrical properties of the composites have greatly been affected by the
shape, size, volume fraction, and specific surface area of such added particles.
11
2.1.2 Thermal Conductivity of Polymer Composites
Polymers with particulate filled composite can be classified as commercially available
thermal interface material and blend of polymer resin with conductive fillers. In both
cases thermal conductivity is enhanced by the particulate fillers since polymers alone
cannot transmit heat efficiently; hence polymers have low thermal conductive property.
However; some polymers are capable conducting heat through them in low rates that
takes time for effective heat transfer. And role of conductive filler particulates are to
facilitate heat transfer rate through the composite by forming clusters of conducting site
and network by uniform distribution in the polymer matrix [3, 11].
Considerable work has been reported on the subject of heat conductivity in polymers by
Hansen and Ho [32] and Tavman [31] etc. But most of these studies were based on the
thermal behavior of polymers only and not to their composites. Reports are available in
the existing literatures on experimental as well as analytical studies on thermal conductivity of some filled polymer composites [1,29-31]. The fillers most frequently used are
Copper particles, short carbon fiber, carbon particles, graphite, Alumina (Aluminum
oxide), Aluminum nitrides and zinc particles. The thermal and mechanical properties of
copper powder filled poly-ethylene composites are found by Tavman [31] while on
thermal properties such as thermal conductivity, thermal diffusivity and specific heat of
metal(copper, zinc, iron, and bronze) powder filled HDPE composites in the range of
filler content 0–24% by volume were investigated experimentally by Sofian et al. [35].
A moderate increase in thermal conductivity up to 16% of metal powder filler content
was observed. The improvement in electrical and thermal conductivity of polymers
filled with metal powders was reported by Mamunya et. al [36]. Weidenfeller et al. [37]
studied the effect of the interconnectivity of the filler particles and its important role in
the thermal conductivity of the composites. They prepared PP samples with different
commercially available fillers by extrusion and injection molding using various volume
fractions of filler content to systematically vary density and thermal transport properties
of these composites.
12
They measured that the thermal conductivity of the PP has increased from0.27 up to
2.5W/mK with 30 vol% talc in the PP matrix, while the same matrix material containing
the same volume fraction of copper particles had a thermal conductivity of only
1.25W/m-K despite the fact that thermal conductivity of copper particles have a value
approximately 40 times greater than that of talc particles. Tekce et. al [37] noticed of the
shape factor of fillers has a strong influence on thermal conductivity of the composite.
2.2 Modes of Heat transfer in materials
The three fundamental types of heat transfer are: Conduction, Convection and
Radiation. All engineering surfaces exhibit some level of microscopic roughness. The
resistance to heat flow through a contact interface occurs because only a small portion
(usually 1-2%) of the nominal surface area is actually in contact [14]. Heat may pass
through the interface via three paths: conduction through the contact spots, conduction
through the gas present in the gap between the surfaces, and radiation across the gap.
Also, radiation does not play a significant role at temperatures below 500°C [18], the
working temperature of building surface usually does not exceeding than room temperature .
Figure 1. Conductive Thermal Interface material positioned between two solid rough
surfaces [19].
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2.2.1 Conduction
It is the transfer of heat from one part of a body to another part of the same body, or
from one body to another in physical contact with it, without appreciable displacement
of the particles of the body. Fourier’s law is the fundamental differential equation for
heat transfer by conduction.
Where
= −( )
(1)
(quantity per unit time) is the rate of flow of heat, A is the area at right angles
to the direction in which the heat flows, and − is the rate of change of temperature
with the distance in the direction of heat, i.e., the temperature gradient. The factor k is
the thermal conductivity, it is characteristic property of the material through which heat
is flowing and varies with temperature.
2.2.2 Convection
Convection is the transfer of heat from one point to another within a fluid, gas or liquid
by the mixing of one portion of the fluid with another .This fluid motion is associated
with the fact that, at any instant, large number of molecules are moving collectively or
as aggregates. Such motion in the presence of temperature gradient, contributes to heat
transfer [9]. Convection is neglected due to the small length scales involved [18], between the thermal interface material and the substrate surface. The convective heat flux
q’ is expressed by;
′ = ℎ( − )
(2)
Where q’ is heat flux, h is the convective heat transfer coefficient, is surface temperature, is temperature of fluid.
14
2.2.3 Radiation
It is the transfer of heat from one body to another, not in contact with it, by means of
wave motion through space [8]. And given by;
′ = ɛ(T4s –T4sur)
(3)
Where, Ts is the absolute surface temperature (K) and Tsur is the surrounding
Temperature, ɛ is the emissivity constant, is the Stefan Boltzmann constant
( =5.67 *10-8W/m2.K4)
2.3 Heat flux
The heat flux q’x (W/m2) is the heat transfer rate in the x direction per unit area perpendicular to the direction of heat transfer, and it is proportional to the temperature gradient, dT/dx [9]. For one dimensional plane wall, having a temperature distribution T(x),
the rate of equation is expressed as
′ = − (4)
The measurement of heat flux is most often done by measuring a temperature difference
over a piece of material with known thermal conductivity. This method is analogous to
a standard way to measure an electric current, where one measures the voltage drop
over a known resistor [10].
2.3.1 Heat flux sensors (meter)
Heat-flux meters are devices used in both fluid mechanics and thermal engineering to
measure local directional heat transfer of heat. They can provide information on the
thermodynamic, chemical or mechanical state of a fluid, but they can also be used energy savings analysis building sciences. So the sensor should be mounted on a wall surface through which the heat flux has to be determined [5].
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2.3.2 Applications in building physics
In a world ever more concerned with saving energy, studying the thermal properties of
buildings has become a growing field of interest. One of the starting points in these
studies is the mounting of heat flux sensors on walls in existing buildings or structures
built especially for this type of research. In fact building wall surface temperature is
mostly set to room temperature, where there is no a tendency to get high surface temperature involvement. In such case, the rate of conductive heat transfer rate is not faster
even though the thermal conductivity of the thermal bridge material is higher [10]. Observing Fourier’s law of conduction provides good explanation of the principle of heat
transfer through the material with a given thickness.
Thermal properties of a wall generally do not change and that it is not always possible
to insert the heat flux sensor in the wall, so that it has to be mounted on top of the wall.
When the heat flux sensor has to be mounted on top of the wall, one has to take care that
the added thermal resistance is not too large [10]. In order to avoid or minimize as much
as possible use of thermal bridge (Thermal interface material) between the wall surface
and the sensor should be adequately implemented. The performance of this material is
selected based on surface conformability to fill air gaps that can increase thermal contact resistance to reduce and facilitate heat transfer from the wall through the sensor device. In addition to this the selected material should be able to conduct heat as desirable.
2.4 Factor affecting heat flux measurement on uneven surfaces
Surface texture, as it is discussed above surface roughness creates a thermal barrier by
holding air, which it thermal insulator. It is known that most building are made from
concrete and other construction materials that the outer surface are not smooth and finishing is usually difficult and even “real” surfaces are not perfectly smooth and flat.
When two solids are joined as fig [1] illustrates, imperfect joints (interfaces) are formed.
The imperfection can only be avoided by interfacing a material with a better thermal
conductivity in the contacting solids avoids the unevenness and facilitates thermal transfer.
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3 MATERIAL SELECTION
3.1 Overview of material selection
The vast availability of material existence with several characteristics is the main challenge in material development and compositions world. Existing wide range of materials as well as variety of requirements in the design procedure are roots of the difficulty
of the selection procedures. Suitable material selection based on the requirement for the
optimum application of heat transfer is the key idea of this paper. It is known that materials of high thermal conductivity are needed for the conduction of heat for the purpose
of heating or cooling. However; it needs through look to choose the compositions i.e.
Matrix and filler to fulfill the required task and to prepare a composite material.
Several types of thermally conductive materials can be used to eliminate air gaps from
rough surfaces such as, thermal interface, including greases, reactive compounds, elastomers and pressure sensitive adhesive films. All are designed to conform to surface irregularities, thereby eliminating air voids and improving heat flow through the thermal
interface [3, 14, and 15].
As it was mentioned the aim in this thesis is to find material composition in both polymer and filler particulates that enhance thermal conductivity. The final product likely to
have the following important qualities: possible higher thermal conductivity, Easily deformed by small contact pressure to contact all uneven areas of the surface, so that eliminating Rcontact., minimal thickness to help maximize the rate of heat transfer, non-toxic
to avoid any hazards to the user, maintain performance indefinitely, and manufacturing
friendly (easy to apply and remove).
A blend of two materials to create the required composite structure is studied in order to
understand and fulfill the desired properties. In this case, since several polymeric materials exist with most of them having unique and diverse properties and the rest similar
characteristics, it is exhaustive in order to identify the appropriate polymer matrix based
on the properties they have for the required application. In addition to this, most poly-
17
meric material properties are influenced by external factors such as temperature, light,
pressure and also their chemical compositions that determine properties significantly.
Secondly, flexible property of the polymer matrix is a property needed. This means, the
polymer that acts as a matrix should able to be rework able with no effort to the desired
shapes, better elasticity, thermal stability and malleability are the characteristics required for the application.
It is also necessary to put into consideration thermal properties of polymeric material
based on their intermolecular orientation. The rate of heat transport in and through polymers is of great importance. For good thermal insulation the thermal conductivity has
to be low, on the contrary for conduction purpose it has to be higher [13].
3.2 Thermal properties of Materials
3.2.1 Thermal conductivity
Thermal conductivity is an inherent or absolute property of the material. The time rate
of heat flow through a unit area producing a unit temperature difference across a unit
thickness.
∗
= ∗
(5)
Where, k is the thermal conductivity of the material, dq rate of change in heat transfer
(W/m2.K), L is material thickness (m), A is heat transfer area (m2) and dT is change in
temperature(0k)
It is the intrinsic property of materials to conduct heat. Normally one-dimensionality,
steady state operation, homogeneity of the material are given as simplifying premises.
The thermal conductivity of any homogeneous material is independent from its thickness. This is not the case with non-homogeneous materials such as elastomers or composites being reinforced by engineering plastics or glass fibers [14, 19].
18
3.2.2 Thermal Expansion
Upon heating, some polymeric materials experience very large thermal expansions on
heating as indicated by the values that range from ~50 * 10-6 to 400 *10–6/°C. Linear
and branched polymers show highest values because the secondary intramolecular
bonds are weak, and moreover, there is a minimum of cross linking. The magnitude of
expansion coefficient diminishes with increased cross linking. The lowest values are
found in thermosetting network polymers, e.g. phenol-formal-dehyde in which the
bonding is almost entirely covalent [41].
3.2.3 Thermal resistance
The opposition to the flow of heat through a unit area of material across an undefined
thickness. The thermal resistance through the material itself Rth Material is an internal
resistance and is defined after Fourier series development of conduction:
Rth Material = ∆T / Q or
L / (k A)
(6)
Factors that affect thermal resistance of the conducting material are material thickness,
the thermal conductivity and surface area [15, 17].
The thermal resistance of soft materials such as compliable elastomers and gap fillers
are subjected to pressure to achieve surface gap conformations.
19
Figure 2 Thermal gradients along the thermal path [14].
In addition to the internal material resistance, the temperature gradients on both contact
surfaces are decisive [4, 14]. They affect two contact resistances R th Contact 1 and R th Contact 2.
So the overall thermal resistance can be defined as:
Rth Total = Rth Material + Rth Contact 1 + Rth Contact 2
Here the equation can be modified based on the assumption that, using thermal interface material can avoid the contact gap by creating adhesive character in both contacting surfaces firmly
and both will be ignored . So thermal resistances reduced to:
Rtotal =Rth material
Thermal contact resistances are mainly caused by incomplete fitting and air interstitials
affecting the border areas with kAir = 0,027 W/m2-K at the acting as thermal insulators
[14,19]. Figure below shows the principle of linear temperature drop from surface temperature where heat dissipation occurs through thermal bridge to heat sink, i.e, and flux
meter.
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3.2.4 Thermal Transmittance
Thermal transmittances commonly known as the U-value, is the measure of the rate of
heat loss of a building component. The U-value is calculated from the reciprocal of the
combined thermal resistances of the materials in the element, air spaces and surfaces, air
gaps. It is expressed as watt per meter, per degree Kelvin (w/m2-k), and defined by expression analogues to Newton’s law of cooling.
Q=UA∆T
(7)
3.2.5 Specific Heat Capacity
Heat capacity of a solid material is a property that is indicative of material’s ability to
absorb heat from the external surroundings; it represents the amount of energy per unit
mass required to produce a unit temperature rise. Mathematically, heat capacity (C) of a
body is defined as
= !
(8)
Where dQ refers to the quantity of heat transferred to the body and ! is the rise in
temperature. Ordinarily, specific heat capacity(C) is specified per mass of material (e.g.,
J/g-oC, or J/kg-oC).
21
3.3 Polymer Blends
There is a trend toward specialization in the polymer products industry. Since the industry is expanding globally, a sufficient market is available for polymer blend products.
Blending is a convenient route to time-efficient and cost-effective upgrading of commodity and engineering resins and to tailoring these resins to specific performance profiles for the desired application [20].
Thermal and rheological behavior is affected by addition of fillers, and thus these have
impact on processing. Adding conductive fillers such as glass fiber or Aluminum oxide
increases the thermal conductivity, but only slightly affects the specific heat. The presence of fillers adds new phases to the system (the reinforcement and the sizing agent)
and creates several new interfaces: between the new phases and each of the phases already present in the system. Each of these interfaces has to be managed. Proper interactions have to be ensured between the different phases to obtain the desired product performance [20, 11].
In industrial fabrication, polymers blending with other non polymeric materials are
composites. The blending process and types of matrix filler reinforcement influences the
characterization of material for desired application; hence for the purpose of thermal
conductivity enhancement it can be categorized as thermal conductive composites.
3.4 Thermal Interface Materials and Conductive Composites
TIM are thermally conductive materials, which are applied to increase thermal conductance across joined or parallel solid surfaces in order to increase thermal transfer efficiency. Using a TIM of high thermal conductivity that can conform to the imperfect
surface features of the mating surfaces.
Any interstitial material that can fill the gap between contacting surfaces, and whose
thermal conductivity exceeds that of air will decrease the contact resistance [6]. The
working principle of thermal bridge can be described as, when two component surfaces
are brought together, a very few percent of the surfaces make physical contact. As much
as large percent of the surfaces are separated by a layer of interstitial air.
22
Some heat is conducted through the physical contact points, but much more has to transfer through the air gaps. Since air is a poor conductor of heat, it should be replaced by
amore conductive material to increase the joint conductivity and thus improve heat flow
across the thermal interface [3].
Therefore, by use of thermally conductive interface materials, both the contact resistance and the overall resistance are lowered. The better the material complies with
both surfaces, the less air interstitials there are. It minimizes the reading error that occurs during energy measurements [14, 15]. The overall target is to lower the thermal
conduct resistance created by air in the rough surface and increase the maximum heat
conduction between the surface and the device.. However, choosing a material with
higher thermal conductivity dominates for such application in terms of heat transfer
compliance.
Figure 3 Thermal interface materials in place with flux meter
As shown in Fig. the interface resistance is composed of three resistances in series,
assuming little effect of thermal resistance between the thermal bridge and heat flux meter Rcontact 2 will be ignored in the analysis. Rcontact1 is the contact resistance between the
TIM and the wall surface, Rcond is the conduction resistance across the thickness of the
TIM, Rcontact2 is the contact resistance between the TIM and the heat flux meter base
23
plate and also be ignored in the analysis, A is the surface area of the wall component
across which heat transfer occurs, and t is the thickness of the TIM.
The heat flux measurement is set according to the figure 3 shown, where the actual
thermal bridge (TIM) material is mounted on the surface of the wall to fully conform on
roughness, to achieve this, mounting pressure is needed to fill the thermal bridge on the
void surface. It can be applied either by hand or by other means that can assist handling
the material without altering the material temperature that may lead to measurement errors. Increasing contact pressure can also decrease the conduction resistance, Rcond, of
the interface material itself, by reducing its thickness. The pressure required to reduce
the thermal interface resistance of a given TIM is a function of its compressibility and
surface properties as well as the surface properties of the mating surfaces [4].
3.5 Particulate Filled Polymer composites
Modification of organic polymers through the incorporation of additives yields, multiphase systems containing the additive embedded in a continuous polymeric matrix. The
resulting mixtures are characterized by unique microstructures or macrostructures that
are responsible for their properties [12].
Polymer composite filled with metal are of interest for many fields of Engineering. The
interest arises from the fact that the thermal and electrical characteristics of such composite are close to the properties of metals [1]. The transfer conditions of heat flow determine the thermal conductivity level in heterogeneous polymer filler system, in which
the conductive phase is formed by dispersed metallic or carbon filler. The influence of
the type of polymer matrix and filler interaction has a great property characterization
effect [13]
24
3.5.1 Types and Composition of Polymer Composites
Polymer composites are mixtures of polymers with inorganic or organic additives having certain geometries (fibers, flakes, spheres, and particulates). Thus, they consist of
two or more components and two or more phases. Additives for polymer composites
have been variously classified as reinforcements, fillers, or reinforcing fillers. Reinforcements, being much stiffer and stronger than the polymer, usually increase its
modulus and strength. Thus, mechanical property modification may be considered as
their primary function, although their presence may significantly affect thermal expansion, transparency, thermal stability, and so on [6].
Continuous composites contain long fiber or ribbon reinforcements mostly in thermosetting matrices; when prearranged in certain geometric patterns, they may become the
major component of the composite (they can constitute as much as 70% by volume in
oriented composites) [10].
In discontinuous composites, the directional reinforcing agents (short fibers or flakes)
are arranged in the composite in different orientations and multiple geometric patterns,
which are dictated by the selected processing and shaping methods [11]. In order to be
able to select the material composition according to the needed use, both the matrix and
the filler material should have characteristics that are suitable for the desired application
25
3.5.2 Polymer matrix properties and selection
The characteristics required for the base matrix material is the most influential component in developing polymer matrix composite for this specific task. Therefore in order to
select the best alternative, the objective is to find a polymer or polymeric derivative material that has a flexible, soft, easy deformability, good elasticity, non toxic property.
Additionally conformability to roughness is the main characteristics desired, due to the
fact that, identifying a suitable polymer and conductive filler that can avoid the volume
of uneven surface of a wall without the requirement of additional force to mount on the
wall surfaces. The filling of polymer with metallic particles results in an increase of
both thermal and electrical conductivity of composites obtained [1]. So that selection
and formulation of suitable matrix (polymer) for such application is important part of
this work.
Adhesive matrix is used to form a mechanical bond at an interconnection. Thermoplastic adhesives are rigid materials at temperatures below the glass transition temperature (Tg) of a polymer. Above the Tg, polymers exhibit flow characteristics [17]. Thus,
the Tg must be sufficiently high to avoid polymer flow during the application conditions
[11,6], but the working surface area is at room temperature i.e, there happens to be
steady state heat transfer to the surrounding environment, and polymers depend on their
Tm and Tg to initiate their chrematistics are not useful for this purpose.
Thermosetting adhesives, such as epoxies and silicones, form a three dimensional crosslinked structure when cured under specific conditions. Cure techniques include heat,
UV light, and added catalysts. As a result of the cure reaction which is irreversible, the
initial uncross-linked material is transformed into a rigid solid. The thermosetting are
stable at high temperatures and, more importantly, provides a low contact resistance.
This results from a compressive force that maintains the conductive particles in intimate
contact after the cure.
26
That is, the shrinking caused by the cure reaction achieves a low contact resistance
with long time stability. The ability to maintain strength at high temperature and
robust adhesive bonds are the principal advantages of these materials. However,
because the cure reaction is not reversible, rework or repair of interconnections
is not an option [10,17].
Silicone elastomer, and other similar elastomers can be available or developed in a sheet
form with ceramic particles as conductive filler and or fibers imbedded in them to increase the thermal conductivity. These are especially good for temporary assemblies or
setups since TIM removal is simple and often reusable. There are also gap filling versions of these sheets such as those made by Fujitsu electronics manufacturer. These
TIMs require a moderate to high compressive force to work effectively. Thermal conductivity ranges from 0.5 W/m-K to 3.5 W/m-K [17].
3.5.3 Particulate Filler materials
Fillers are different from additives, in the sense that they are only mixed with the polymer matrix to impart specific physical properties instead of chemical properties such as
mechanical, strength, electrical conductivity or dielectric properties, thermal insulation
and they never take part in the macromolecule chain. Because fillers together with the
polymer matrix are considered as composite materials. Fillers can constitute either a
major or a minor part of a composite [12].
Fibers and whiskers are not discussed here because they are generally regarded as reinforcements, not fillers, although a majority of the fillers have reinforcing effects. Also,
fillers and additives that primarily modify or impart electromagnetic properties, such as
electrical conductivity, are not discussed.
27
3.5.4 Characteristics of Particulate fillers in composites
In formulating filler polymer composite, the developer should consider important properties and qualities the filler must have before the blending process. This is because filler can define the final properties of composite product that should be needed. It is necessary to raise and answer such questions, what property benefits are being sought, deleterious changes that may occur and how can they be tolerated, how easy is the filler to
handle and how might affect processing and so on[11].
Understanding the chemistry of compounding the filler into the matrix determines the
interaction between polymer and filler results the predefined product, while the chemical nature of the filler has little direct importance to its use in composites. They can be
divided in to two categories inert and reactive, while the later can reacts with the environment.
The other factor that affects the behavior of a composite is surface chemistry and surface interaction because polymers have a very much higher (20-30 times) thermal expansion coefficient than mineral and metallic fillers. Thus, in many well-dispersed, hot
processed composites, a compressive stress develops as the polymer cools leading to an
intimate interaction between matrix and filler [12].
The overall value of filler is a function of intrinsic material characteristics, such as particle size, particle shape, and chemical composition; of process-dependent factors, such
as particle-size distribution, surface chemistry, particle agglomeration, and bulk density;
and of cost [10] determine the structural properties of composite material.
3.5.5 Filler Loading
The amount of filler in a filled polymer is termed the loading and is expressed quantitatively although the quantitative measures vary from industry to industry. In plastics and
rubber industries, filler loading is formulated according to parts of filler used per 100
parts of polymer, weight percent (wt %), or volume percent (vol%) [11]. The optimal
loading of fillers in a polymer is a balance between property enhancement such as thermal and other physical and processing and material cost over the filler loading range.
[12].
28
3.5.6 Density and void fraction:
The average mass per unit volume of the individual particle is the true density or specific gravity of the filler. It is used to calculate the volume fraction of fillers. Densities of
finely divided, porous, and irregular fillers are typically measured by a gas pycnometer
that ensures all pores and crevices of filler agglomerates are penetrated. Apparent, or
bulk, filler density refers to the total amount of volume occupied by a given mass of dry
fillers and it includes the void volumes in the filler aggregates and agglomerates. Bulk
density is used in weighting fillers during filler purchasing, shipping, and storage [10].
The theoretical density of composite materials in terms of weight fraction can be obtained by equations given by Agarwal and Broutman [32].
"# =
1
&' &
%
+ ,
ρf ρm
(9)
Where, w and "# represent the weight fraction and density of the composite respectively. The suffix f, m and c stand for the filler, matrix and the composite materials respectively.
3.5.7 Mixing and Dispersion
In preparation of filled polymer composites, the process of uniformly distributing fillers
is most important process. Component mixing consists of three basic elements: incorporation, distribution, and dispersion. These occur simultaneously throughout the mixing cycle. Incorporation, which predominates the early stage of mixing, involves blending of separate ingredients into a coherent, although still inhomogeneous, mass. In the
distribution step, homogenizing occurs in which the ingredient become randomly distributed throughout the polymer matrix [12]. In industrial production all can be done by
electrical drive machine, the so called mixers and homogenizers. Since lab scale is carried out in this task all the blending mechanism are done by manual mixing of ingredients, that involves thorough mixing components until uniform dispersion of filler particle is achieved in the silicone resin.
29
3.5.8 Effects of Filler on composite structure
Filler particulate can significantly affect the structure of the matrix polymer itself and the properties of the final composite. Matrix modification can occur and the effect on properties can be
at least as important as that arising from the interfacial bonding effects. Ways in which particulate phase may affect polymer structure are; Molecular weight reduction during processing and
crosslink modifications due to interference with curing processes, adsorption of polar, low molecular weight species such as surfactants, plasticizers, stabilizers and antioxidants. [13].
Rheology and Process ability effect; the presence of fillers affects the flow characteristics of the filled polymers. The immediate effect of filler is to increase viscosity, interfere with the polymer flow pattern in a given process [11, 23]. Therefore it is necessary
to study the characteristics of filler and the interaction with a base polymer resin to
make a uniform dispersion, and so prevents cluster formation of particle concentration
in one place, it also upgrades the quality of the composites especially designed for
thermal performance applications [16]; hence it is a preliminary and important to study
the behavior of both filler and polymer aggregates in order to achieve the most out of
the performance required.
Other important filler effect to consider; since polymers generally have a much larger
thermal expansion coefficient than most rigid fillers, there is a significant mismatch in
thermal expansion in filled polymer composites. This mismatch could lead to generation
of thermal stresses around filler particles during fabrication and, most severely, induce
micro cracks at the filler interface that could lead to premature failure of the filled polymer [25].
30
3.6 Particle packing
The packing principles are of great value in preparing filler size distributions capable of
being processed at high loadings in polymeric composite systems. The packing behavior
of particles in a polymer matrix determines at what loading particle/particle effects become important and is a critical factor in the understanding and design of polymer composites, especially when highly filled systems are involved.
One of the most important characteristics of the filler-polymer composite is the filler
packing factor F [1]. The value of F depends on the particle shape and on the possibility
of the skeleton or chained structure formation. F is a limit of system filling and equal to
the highest possible filler volume fraction at a given type of packing.
./
- = (./0.1)
(10)
Where Vf is the volume occupied by the filler particles at the highest possible filler fraction and Vm the volume occupied by the polymer matrix (space among filler particles)
The maximum packing fraction, Ff, is a particularly useful concept. This is the maximum volume fraction of particulate that can be incorporated before a continuous network is developed and voids begin to appear in the composite. In addition to indicating
the maximum practical loadings obtainable, Ff is also useful in understanding and describing the effect of filler loading on composite properties. Many properties change
rapidly as one approaches Ff, almost in a percolation way.
31
3.6.1 Particle Packing Theory
The aims of particle packing theory are to predict the maximum volume fraction that
can be obtained under a given set of circumstances and the structure of the particle assembly at this point. Different ways of packing the same collection of particles, each
one resulting in a different maximum volume fraction. Ordered packing, for instance, is
the circumstance where there is a perfect particle packing with no free space occurs.
However; in random packing, the situation where pieces are put in container and shaken
and it is unlikely that they will ever discover this perfectly packed structure. Nevertheless, it will be found that, providing there are enough particles, a certain packed density
can be reproducibly obtained implying that some statistically balanced structure is
achievable[13,22].
Random packing is more relevant to particulate-filled composites, is important in understanding the application of packing theory, and two classes of random packing are
recognized, loose and dense random packing. Loose packing refers to the sort of packing obtained when particles are randomly assembled under conditions where they cannot easily move past each other, while in dense random packing, conditions are such
that movement is possible [11,26].
3.7 Additives
Polymers can incorporate in their structures additives that enhance some specific properties of
the polymer. However; most of them are chemical compounds that affect chemical properties,
and more common types of additive are, dyes, plasticizers, solvents, stabilizers, lubricants and
antioxidants. The primary reasons for using additives are, Property modification or en-
hancement, overall cost reduction, Improving and controlling of processing characteristics [6].They are also added to polymers to characterize and enhance physical, and chemical
properties according to the required performance of a given composite product [10,32].
32
3.8
Theoretical Background of Conductive Polymer Composites
Polymeric matrices have significant amount of thermal and electrical properties
and thus are thermal and electrical insulators. The conductive fillers provide both thermal and electrical properties and the polymeric matrix provides both thermal and mechanical properties. Therefore, thermal, electrical and mechanical properties are provided by different components, unlike that of metallic solders that provide all these properties [17, 23].
When a polymer matrix having a conductivity Km is filled with dispersed filler having a
conductivity Kf , the composite prepared gains a conductivity value Kc. when the volume filler fraction Vf reaches a critical value Vc (so called percolation threshold), an
infinite conductive cluster (IC) is formed and consequently , the composite becomes
conductive [1]. As the filler concentration increases from Vc to the filling limit F, the
value of Kc increases rapidly over several order of magnitude, from the value Kc at the
percolation threshold, the conductivity change is negligible and the conductivity of the
composite is equal to the polymer conductivity Kc or slightly higher.
3.8.1 Theoretical Models of Thermal Conductivity in Composites
Due to the fact that Polymers have lower thermal conductivity compared to metals and
many inorganic materials the thermal conductivity can be enhanced by filling a high
thermal conductive filler particulates, or metallic powder such as Aluminum oxide, silver, Zinc or graphite [12, 25].
Theoretical models have been proposed to predict the effective thermal conductivity of
two-component composites. For two-component composites, the simplest consideration
would be that the components are arranged in either parallel or series with respect to
heat flow, which give the upper or lower limits of the effective thermal conductivity of
composites. For the components in parallel,
23/ = 41 − 5/ 621 + 5/ 2/
33
(11)
and for the components in series,
(12)
For composites with irregularly dispersed fillers in a continuous matrix, Maxwell [18],
studied their effective thermal conductivity. The resulting effective thermal conductivity
of this system is expressed as,
(13)
Where ke is the effective thermal conductivity of the composite, km is the thermal conductivity of the matrix, kf is the thermal conductivity of the fillers and Vf is the volume
fraction of the fillers.
Thus, once the mixing proportion of both filler and resin are known, according Maxwell correlation, the thermal conductivity of the emerging composite material can also
easily be determined. The thermal conductivities of filled polymers typically lie in between the limits set by rule of mixtures.
34
4 METHODOLOGY
This chapter describes the materials and methods used for the processing of the composites under investigation. It presents the details of the characterization and thermal conductivity tests which the composite samples are subjected to. In addition to this thermal
conductivity of the prepared Aluminum oxide filled composite samples are tested for
thermal conductivity and compared the theoretical assumptions with the experimental
values obtained.
4.1 Materials
4.1.1
Resin
Silicone elastomers are elastic substances which contain linear silicone polymers cross
linked in a 3-dimensional net work. In most cases this network also contains filler which
acts as a reinforcing agent or as an additive for certain mechanical, chemical or physical
properties. In general all silicones (usually we refer to silicones as polydimethyl siloxanes) are noted for their high thermal, stability, biocompatibility, hydrophobic nature,
electrical and release properties. When silicones are cross linked to form a silicone rubber their characteristic properties are still prevalent [7]. The nomenclature classifies silicone elastomers by their curing mechanism and curing conditions. They are essentially
divided into two groups of materials, i.e., room temperature vulcanizing (RTV) and high
temperature vulcanizing (HTV). RTV systems are able to cure at room temperature
and HTV systems at temperatures well above 100 °C. A number in the name indicates
the number of components that upon mixing will form a curable composition,
e.g., RTV-2. HTV rubbers are mainly so-called solid silicone rubbers that are not suitable for desired application. They have a very high viscosity in the uncured state and appear as solids. This behavior has also led to the creation of the term ‘High Consistency
Rubber’ [7, 26].
35
Low viscosity silicone resin from Wacker, was used as a matrix material. Known by its
trade name Silicone Elastosil M4601 A, Pourable, addition-curing, two-component sili-
cone rubber that vulcanizes at room temperature (RTV-2) and the corresponding hardener
Elastosil M4601B (platinum Catalyzed) are mixed in a ratio of 9:1 by weight as recommended by the manufacturer [26].
It is chosen primarily because of the above mentioned reasons as well as it happens commonly used polymer for it has got a thermal stability and conductivity in comparison to other polymeric material, about 0.2 W/m-K, also depending on the curing process and the
amount of catalyst used, tacky (adhesive property), easily deformable to the desired shape,
flexible, elastic and chemically inert compound can be a achieved, if property characterization is applied to fulfill the desired application.
Property
Inspection Method
Value
Elongation at break
ISO 37
700 %
Hardness Shore A
ISO 868
28
Mix ratio at
A:B
9:1
Tensile strength
ISO 37
6.5 N/mm²
Viscosity at 23 °C, after ISO 3219
15000 mPa s
stirring
Viscosity at 23 °C
ISO 3219
10000 mPa s
Density at 23 °C
1.14 g/cm³
Table 1 Silicone properties from manufacturer product sheet [26].
36
4.1.2 Filler (Aluminum Oxide):
It is a white colored powdered with 95.5% pure Aluminum Oxide. It has high thermal
conductivity (K) of 31 W/m.k and for its ability to interact with resins and enhance conductivity of polymer composite structures. In addition to these , there are also key properties it offers for composite fabrication such as availability in powder form with low
agglomeration property that helps good dispersion in the matrix, resists strong acid and
alkali attacks ,excellent size and shape capability, non corrosive, non-toxic property and
low cost. Hence for our purpose Aluminum oxide has been selected as the filler material
[40].
4.2 Lab Fabrication of particulate filled composite
To develop particulate filled composite material by blending low viscosity Silicone
Elastomer with Aluminum oxide by predefined proportion of sample preparation. And
to measure the thermal conductivity value of the prepared samples and to identify the
most suitable composition for the application of thermal bridge material that fills uneven surface of a building walls to facilitate thermal flux measurement.
4.2.1 Matrix Preparation
Before mixing Aluminum Oxide powder (Al2O3) with silicone resin to make the composite product, blend of Silone resin with viscosity of 1.14g/cm3 and the catalyst is prepared as a base matrix that cures in room temperature. The two types of elastosil M resins with their respective catalyst included in the package.
37
Figure 4 Low viscosity silicone resins and catalyst from Wacker chemical
To prepare the catalyzed mix, the resin had to be mixed with the recommended catalyst by the manufacturer, before filler material Al2O3 is incorporated. Room temperature vulcanized (RTV) rubber is the result after mixture is cured with incorporation of
Aluminum oxide powder resulting in composite sample.
The table summarizes the standard proportions of catalyst that should be added to get
room temperature cured silicone pads.
Elastosil
catalyst
M Elastosi
M ElastosilM,B(catalyzed)
4511
4601A
5
-
-
5
-
-
5
-
-
9
1
T21(w%)
catalyst
T51(w%)
catalyst
T47(w%)
Table 2 Standard Resin preparation according to manufacturer recommendation (26).
38
.
Figure 5 Addition vulcanization of RTV-2 silicone resin.
The amount of the base matrix silicone Elastosil and the respective catalysts to be used
in order to get RTV (room temperature cured) silicone rubber pads or sheets molding.
Before the curing process start it is recommended to use the catalyzed mix to blend with
the filler particles as faster as possible, since the viscosity increases it can result in difficulty to incorporate the filler material and obtain uniformly dispersed in the matrix that
affects the characteristics of the final composite.
4.2.2. Filler (Al2O3) incorporation
1. The mass of the filler material alumina powder (Al2O3) calculated according to the
required weight fractions of the filler materials which are; ( 5,10,15,20,30,40) wt%. of
the resin (silicone Elastomeric). Sample composition is prepared by measuring the
weight fraction (%wt) of the filler Al2O3 and mixing with the weight fraction of the silicone resin (%wt).
39
Density of Alumina (Al2O3) is 4g/cm3, and conversion the mass to volume fraction enable us to know what amount of gram to mix to obtain the required volume fraction.
Figure 6 Aluminum oxide powders as thermal conductive filler.
2. The mass of the resin (silicone elastomer) is mixed according to the required volume
of blend and the size of the composite material required. In addition to this the density
of the composites could be determined from equation (6).
Filler (w %)
matrix (g)
Vf(%)
Vm (%)
Composite
Density (c)
5
50
1.5
98.5
1.18
10
50
3.1
96.9
1.23
15
50
4.8
95.2
1.28
20
50
6.7
93.3
1.33
30
50
10.9
89.1
1.45
40
50
16
84
1.6
Table 3 Sample preparation by varying the amount of filler percentages by weight.
40
Figure 7 Mixture of RTV-2 Silicone after through stirring.
3.
Accelerator (catalyst) and the hardener are added as weight % according to the
manufacturer recommendation given in the product sheet.
4. The filler and the catalyzed matrix are mixed for about 10 minute at room temperature continuously and slowly to avoid bubbling during mixing, and then Al2O3 filler is
added to the mixture with gentle mixing until uniformly distributed.
5. The mixture is poured from one corner into the mould (to avoid bubbles formation
which causes air to hold) and the uniform pouring is continued until the mould is filled
to the required level.
6. The mixture is left in the mould for (24) hrs at room temperature to solidify. Then the
cast is placed inside a room temperature.
41
Figure 8 Mixture allowed curing at room temperature.
8. The specimens are de-molded accordingly and cut in to the standard dimensions for a
test.
9. Thermal conductivity test is conducted on each sample using flux meter .After this
test the thermal conductivity k value of each samples were determined.
42
4.3 Thermal conductivity analysis of Samples
Thermal conductivity of pre-formulated composite samples were analyzed based on
both theoretical models and experimentally determined values. The theoretical values
were calculated using Maxwell model and using equation (13), to determine the effective thermal conductivity (kef) of randomly dispersed filler particles in the matrix. Additionally experimental values were obtained using measurements obtained by using heat
flux meter found at Arcada.
The average thickness of the specimen is 1.5mm, and the U values are given from the
device readings. In addition to this heat flux is calculated by the relation:
78 =
7
9
= k∗
:;
<
Using equation 3 and substituting heat flux dq’ values obtained from the device gives
us the thermal conductivity of the samples.
==
>7′. <
>@
The dq’ values are given in the table below while; the thickness L for all the samples is
1.5mm and the area A is the heat transfer area that is equal to the area of the flux-meter
and it is 0.01m2.Temperature change was recorded during the measurement on the outer
surface of the composite and inside surface, Tout=21.6oC, Tin=21.3oC, so dT=0.3oC.
43
U-value calculation
An experimental result based on measurements taken by heat flux meter was given by q
values in milliWatts (mW). First “Dry Runs” have been made to get mean U values of
the wall, and then the actual test was made by placing the silicone between the wall and
the meter. The ‘dry run’ q value was found to be 50 mW, is the flux from the wall surface on the area A=0.01m2. The mean U-value of the wall is,
q= Uwall*A*∆T
∆T,
Tout surface=21.6oC
Uwall=∗
Uwall=
Twater=7.8oC
AB1C
B.BD1E ∗(FD.GHI.J)KL
Uwall=0.36 W/m2.k, the mean U-value of the wall without silicone TIM
U-value calculation from simulated aquarium wall layers using Silicone.
Figure 9 Simulated wall layers for experimental measurement of heat and U values.
As the figure above shows, the wall is constructed from three layers, glass layer, polyurethane layer and the TIM layer, and it also shows the flux meter.
44
To calculate the overall U values from these wall layers, the corresponding thermal circuit is shown below.
Since, Rtot = 1/(Utot*A)
The overall thermal transmittance equation is given by:
OPQ ORSQT O
UV
1
1
=
+
+
+
M ℎN PQ RSQT UV
However; since the effect of water in the tank has small effect on the system, it can be
ignored and the above equation will be given as
OPQ ORSQT
1
=+
+
M
PQ RSQT
Known thermal property values are;
Tw= inside water temperature =7.5oC
Kpolyurathane=0.03
Lpolyurathane=35mm=0.035m
Kglass=0.96 W/m-k
Lglass=5mm=0.005m
Illustrating as example below for sample 1,
1 OPQ ORSQT
=
+
M PQ RSQT
1
MNQQ
=
0005 0.035
+
0.96
0.03
45
Therefore, \]^__=0.85W/m2.k
The difference between the values from the flux-meter and from calculation is found to
be 0.4.
The experimental U-value for each sample is calculated accordingly and the result can
be referred from table 4.
Samples (%wt) Filler
Theoretical
Theoretical
kef Values
U-value
(W/m-k)
1
Resin+5%
of
samples
(W/m2-k
0.21
140
0.22
146
0.23
153.3
0.24
160
0.27
180
0.3
200
Al2O3
2
Resin+10%
Al2O3
3
Resin+15%
Al2O3
4
Resin+20%
Al2O3
5
Resin+30%
Al2O3
6
Resin+40%
Al2O3
Table 4 Theoretical and experimental results of k and u value based on the composite
samples.
46
Specific Heat determination of the composite samples
Specific heat capacity of the samples were also determined by using equation 8
>7 = `[email protected], the mass (m) of the each sample were found by weighing (g). Example
oon the first sample is illustrated as follow,
dq= 41 mW
Mass (m)=52,5g , !=0.3oC,
Substituting values in = !
C sample 1 = / ! = 0.041efggh/(0.05252j ∗ 0.3B )
=2.62 J/kg-oC
Filler (w %)
Matrix (g)
Mass (g)
Experimental dq Specific heat capaci-
of composite
values (mW)
ty (C) in (J/kg-oC)
5
50
52.5
41
2.6
10
50
55
38
2.3
15
50
57.5
31
1.8
20
50
60
28
1.55
30
50
65
24
1.23
40
50
70
21
1.0
Table 5 Specific heat Capacity of the composite samples.
47
4.4 Characterization of the composite Materials
After first, trial of matrix preparation based manufacture’s instruction and incorporation
of filler, the resulting cured silicone composite did not fulfill the parameters required for
the desired purpose. For instance, it was found tough that has lower flexibility that could
not fill the roughness.
The main important characteristics of this material development is to obtain a material
as soft as possible to be able to fill a air gaps on the surface of a building walls, so that it
must be as conformable as possible, that means, flexible and soft enough, in addition the
material should be able to stick to the wall that can be removable at any time without
leaving a stain or spot on the wall and user friendly without causing any hazard.
Having these important properties in mind, silicone elastomeric was chosen as a matrix
material. Adding Al2O3 fine powdered into the catalyzed silicone matrix was one of the
important steps in fabrication process. After addition, mixing thoroughly enables to obtain uniformly and randomly distributed filler particles in the matrix that prevents cluster formation in one place resulting in poor uniform density of material. Mixing also
helps good surface texture and structural visibility. Hence, during mixing the two phase
materials integrate so that thermal conduction is also be enhanced to a greater extent.
.
48
5 RESULT AND DISCUSSIONS
Having done the experiments in Arcada chemistry lab, polymer composite sample could
be formulated based on the application requirement for thermal bridging purpose to be
used as an interface material between rough surface of a building and thermal flux
measuring meter that was designed earlier in the School to measure the energy dissipation from a building walls. The base matrix is prepared according to the above procedure by blending the silicone resin with the catalyst. And the preceding step was to mix
the amount of Aluminum oxide powder needed by weight in to the catalyzed resin to
form good dispersion and distribution by mixing.
After several trials and varying the amount of catalyst needed to cure the resin, it was
found that soft, flexible and elastic polymer composite material samples could be formulated. At first higher amount of catalyst added, resulting in tough and harder sample
after curing process is undergone. This property is not desirable for the required purpose, while step by step addition of catalyst with varying amount could result in desirable composite product. The amount of catalyst added was less than half of the recommended by the manufacturer 5%.
Experimental Thermal conductivity measurements using the flux-meter revealed that,
increasing the amount of filler decreases the conductivity in a very significant manner;
the result also shows contradictive figures with the theoretical value, this is because as
the amount of filler percentage increases the toughness also increases resulting in
incomplete conformability into the wall leaving air gaps, secondly as the filler increases
there is also a tendency to form a multiple layer of particles in the matrix that results the
heat transfer rate slow. However; this property is important for composite used for other
applications where a toughness and hardness property of a material is significant and the
theoretical value is universal for such composite materials.
The Aluminum oxide particulate filled in silicone composite shows good enough to fill
the roughness of a wall, self tacky on the wall to mount the measuring device. It was
found that higher Elasticity, flexibility and good surface texture were observed with
lower filler loading. It was seen that increasing the amount of filler fraction decreases
the softness and increases the rigidity that also decreases thermal conductivity.
49
6 CONCLUSION
Having frequent trials of sample preparation by varying the amount of catalyst (hardener) to be added, it was found softer, flexible, user friendly material that fills gaps on the
rough surfaces to facilitate energy transfer in the form of heat through itself by avoiding
air as thermal insulator.
According specific heat capacity (C) results shown in table 4, the lower value shows
that the property that is indicative of material’s ability to absorb heat from the external
surrounding wall with small energy and temperature difference.
Significant change in thermal conductivity value was not observed. This could be
caused by addition of small amount of filler volume fraction in the matrix resin. In other
cases, increasing the amount of filler volume fraction results to a material with undesirable characteristics such as hardness and toughness also increases. The better the material complies with both surfaces, the less air interstitials there are. However; even
though small thermal conductivity change is observed in the test result, thermal contact
resistances are mainly caused by incomplete fitting and air interstitials affecting the
border areas with kAir = 0,0027 W/m-K, that acts as thermal insulator, but the composite
conductivity values are much higher than the air to use as thermal interface. The better
the material complies with both surfaces, the less air interstitials there are.
The chosen components the resin and conductive filler could be blended for the composite material development and achieved required characteristics such as thermal conductivity and stability, good filler-matrix integration, flexibility, surface conformability and
tackiness as well as user friendly (non-toxic).
The samples were tested and used as interface material between surfaces and flux meter
to fit the air gaps created by the rough surfaces to avoid thermal insulation, and positive
result was obtained.
50
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