International Journal on Theoretical and Applied Research in Mechanical Engineering (IJTARME)
_______________________________________________________________________________________________
An Assessment of Effective Thermal Conductivity of Epoxy-Tio
2Composites
1G.E.Patil, 2M.A.Bhandare, 3D.B.Dukale, 4V.D.Sapkale
1,2,3&4
Department of Mechanical Engineering,
1,2,3&4 KJEI’s Trinity Academy of Engineering, Pune-48,
E-mail : [email protected]1, [email protected]2, [email protected]3, [email protected]4
Abstract - The development of microelectronic devices like PCB, electronic packaging etc. requires high thermal conductivity materials for fabrication. This provoke the needs of development of Particulate filled polymer matrix composites (PMCs) that possess high thermal conductivity.
This project research provide the optimal solution to solve this problem. An analytical model is proposed in this research work to estimate the effective thermal conductivity of polymer composite filled with cubical particulate. By using the mathematical correlation developed by this model effective thermal conductivity is calculated by proposed model and the model developed by previous author in this regard for volume fraction of 0 vol.%, 13.4 vol.% and 26.8 vol.% of cubical TiO2
particulate. As per ASTM E-1530, UnithermTM 2022 tester is used to measure the effective thermal conductivity (keff) of epoxy-TiO2 composite for these volume fraction also.
These values of effective thermal conductivity (keff) obtained from the experiment are compared with these model developed in previous literature and the proposed model. It is found that the value of effective thermal conductivity calculated by proposed model is very near to the experimental value of effective thermal conductivity.
Thus the approach
Keywords- PMCs, particulates, effective thermal conductivity, volume fraction.
I. INTRODUCTION
Power density of the electronic device is quite high which causes the generation of heat within the electronic component. Thus it is required to carry away the heat from the electronic components and prevent the device by maintaining the temperature lower than the critical values [1]. Traditionally heat sink which were metal based and having high cost were used to dissipate the heat generated within the component which are not acceptable nowadays because of thermal cracking [2].
Also the weight of these devices is also the main consideration. Low weight electronic devices are the main requirement in today’s world so as to carry them from one place to other place. These requirements led to increase in the demand of high performance micro- electronic devices enforcing for the requirement of the advancement in the technologies regarding electronic packaging. The wide application of electronic and electrical technologies and their device requires the better micro - electronic packaging. Micro-electronic packaging is the multi-discipline subject in the field of electronic engineering. It considers so many issues like
cost, mechanical properties, heat transfer characteristics, reliability etc. [3].
Polymers and ceramics are generally in demand for packaging materials because of their better mechanical, electrical and thermal properties [4]. Generally available polymer material for Packaging are polypropylene (PP), polyethylene (PE), Polyamide (PA),epoxy polyimide, However, common polymers for packaging such as polyester, polyethylene (PE), acrylonitrile-butadiene- styrene (ABS) etc.
They have less thermal conductivities and high coefficient of thermal expansion (CTE) thus they cannot provide effective heat flow which led to thermal failure.
It is always demanded in the world of advancement to replace existing materials by advanced material having better properties to achieve the new requirements. The use of larger particle and surface treated filler resulted in composite materials with enhanced thermal conductivity. Carbon based filler material also used as filler material [5, 6] but they couldn’t give satisfactory results in the case of thermal conductivity. Ceramic filler materials like SiC, Al2O3, Si3N4 etc. also increases the thermal conductivity but improvement in the dielectric constant is also found of composite material which is quite dangerous for microelectronic devices.
Incorporation of metallic powder also increases the thermal conductivity of composite [7-9] but it is found that weight of the composite material may also increases more than required. Also with the use of metallic powder electrical conductivity also increases which is not required sometimes. Thus polymer composite material came into existence to take care of such issues.
For fabrication of such particulate filled polymer composite the characteristics like lo w material density, low coefficient of thermal expansion, low electrical conductivity, higher thermal conductivity etc. are taken into consideration [11].
Plenty of literatures are provided previously by so many authors for the estimation of thermal conductivity (keff) of composite material through theoretical models and on the basis of experiments. Rules of mixture give the formula for estimation of thermal conductivity for series and parallel heat conduction. Maxwell equation gave an idea for so many researchers to derive the expression for keff of spherical particulates in a continual medium [15].
Lewis and Nielsen [16] proposed a semi-theoretical
model by using the Halpin -Tsai equation of two phase mixture in which they also considered the shape and orientation of particle. The particulate reinforcement is assumed to be isotropic. Agrawal et al. derived some of the mathematical correlation for hybrid polymer composite filled with two different particulate filler material to calculate the effective thermal conductivity derived on the basis of minimum thermal resistance.
In this work TiO2 filled epoxy composite is fabricated and effective thermal conductivity is measured experimentally. It provides an analytical model for the estimation of effective thermal conductivity for cuboid TiO2 particulates reinforced in epoxy poly mer. The theoretical calculated values are compared with the experimental results obtained. Thus this work provides the model which gives the most approximate value of thermal conductivity to the experimental result. Thus that model is considered to be the model applicable for other polymer composite filled with the particulates only.
II. OBJECTIVE FOR PRESENT RESEARCH
The objectives for following work are as follows a) To develop an analytical model for estimating effective thermal conductivity of polymer composites reinforced with particulate fillers.
b) To validate the proposed mathematical correlation through experimental measurement of thermal conductivity of polymer co mposites fabricated with different filler concentration.
c) To compare the effective thermal conductivity for plenty of previous literature’s model and proposed model with the experimental measurement at volume fraction 13.4vol.% and 26.8 vol.%. To validate the proposed correlation.
III. MATERIAL AND METHODS EMPLOYED
A. Matrix material (Epoxy)
Epoxy are most widely used polymer matrix due to its easily availability, better mechanical and thermal properties over other polymer, good adhesion properties to different fibers, chemical resistance and better performance at higher temperature. Epoxy LY 556 resin is used as a matrix material which generally chemically belongs to epoxide family. The common name of this material is Bisphenol-A-Dig lycidyl-Ether. Epoxy is chosen because it has low density 1.1(g m/cm3) and it is most common used thermoset polymer and chemical resistant. It is used with its corresponding hardener HY 951. It has low thermal conductivity (0. 363W/ mK).
Table I : Properties of Epoxy
Properties Values
Density (gm/cc) 1.6
Thermal conductivity (W/mK) 0.363
B. Filler material (TiO2)
Micro-sized TiO2 is used as a particulate filler material for the fabrication of thermal conductive PMCs in this work. It is found in the nature as anatase, brookite, and rutile in the form of oxide of titanium. Ilmenite is the main source of TiO2. Ilmenite ore is abundant form of TiO2 and next is Rutile. The anatase and brookite are the metastable phases and by them, they can be converted into rutile.
c. Composite fabrication
Polymer matrix (epoxy) Composite filled with TiO2 sample is fabricated by hand lay-up technique. It is the quite old but very simple technique.
IV. THEORY AND MODELING FOR EFFECTIVE THERMAL CONDUCTIVITY
Depending upon the position of particulate within the polymer composites, the model developed
A. Heat transfer element for analytical proposed model
The effective value of polymer composite’s thermal conductivity is calculated by assuming that the heat is flowing in the direction of resistance connecting in series. Fig.1. presents a heat transfer element for the estimation of thermal conductivity of polymer composite filled cubical particulates. In this heat transfer element it is considered that the cubical particulate filler material are uniformly distributed and located within the cubical polymer matrix as face-centered cubic arrangement. The reason to choose face-centered cubic arrangement is that in the FCC arrangement the particulates are efficiently closely packed in the lattice.
B. Development of Analytical model
Depending upon the position of particulate within the polymer composites, the model developed would be varying. In the figure given below shows a 3-D view of heat transfer element of polymer composite filled with particulate. In the given figure a heat transfer model is taken out. The particulate are assumed to be cubical and the whole element is divided into section. In each section Ri represents the corresponding thermal resistance where i denotes the number of the corresponding layer. Thus in this model the thermal resistors are connected in series to the direction of heat flow.
Fig. 1. A heat transfer element of polymer composite filled with cubical particulates in FCC arrangement
Fig. 2. Side view of heat transfer for number of cube = 4 According to Fourier’s law of heat conduction, the amount of heat transfer (
q
) through the heat transfer element is written as equation:2 eff
a T q k
a
(1)Or
eff
a
q k T
(2)And the thermal resistance of this heat transfer element is
1
total
k a
effR
Heat transfer through the matrix and particulate in the whole matrix can be expressed by Fourier’s law heat conduction as
For matrix
m m m
q k A dT
dy
(3) For Particulatep p p
q k A dT
dy
(4)Where km and kp represents the thermal conductivities of the matrix and particulate respectively and Am and Ap are the corresponding cross sectional area of the matrix and particulate respectively.
Thus the total heat transfer (qt) through the whole heat transfer element is
'
t m p m m p p
q q q k A
dT
k AdT
k AdT
dy dy dy
A represents the heat transfer elements cross sectional area, thus
' m p
m p
A A
k k k
A A
(6)By the use of this geometry, the formula for estimation of thermal resistance is derived
1
2 2
1
1 8 ( p m) m
r
r k k k a
R
By observing the geometry it is concluded that R1= R3= R4= R6
1 2 2
4 2 m a r R k a Also R2= R4
Where R1, R2, R3, R4, R5, R6 represents the thermal resistance of corresponding layer
Thus total thermal resistance is
1 2 3 4 5 6
total
R R R R R R R (7)
1 1
2 2 2
1
4 4
8 ( )
total
p m m m
r a r
R r k k k a k a
(8) Now For No. of cube (particulate) within the matrix = 2
Fig. 3. Side view of heat transfer for number of cube = 2 Again by observing the geometry it is concluded that R1= R3
1 2 2
2 2 m a r R k a
1
2 2
1
1 8 ( p m) m
r
r k k k a
R
Thus Total thermal resistance is
1 2 3
total
R R R R
1 1
2 2 2
1
2 2
8 ( )
total
p m m m
r a r
R r k k k a k a
(9)
And finally the effective thermal conductivity is
2
1
eff total
total total
k k a
R a R a
1
eff
total
k R a
(10)
V. RESULT AND DISCUSSION
A. Calculation of effective thermal conductivity of Epoxy - TiO2 composites
Effective thermal conductivity of Epoxy -TiO2 composites is calculated by the equation provided above for different loading of volume fraction of particulate.
These values are calculated for volume fraction of 0%, 13.4% and 26.8% . The values obtained are 0.363W/mK, 0.8269 W/mK and 1.406 W/mK respectively for these volume fractions .
Fig. 4. Effect of volume fraction on Thermal conductivity
Fig. 5. Variation of Thermal conductivity with different models
Fig. 6. Comparison with theoretical thermal conductivity
VI. CONCLUSION
Incorporation of Particulate fillers affects the thermal conductivity of Matrix poly mer quite reasonably.
Increase in the 26.19% area of particulate TiO2 material results in the imp roving the thermal conductivity of the polymer composite around 1.5 times. This is due to the decrease in the contact distance between the particulate.
Incorporation of 13.4% volume fraction of TiO2 results in 2.73 times increase in the thermal conductivity of PM Cs.
Incorporation of about 6.7vol.% of glass microsphere and
micro -sized TiO2 particulates results in improving the effective thermal conductivity of the composite by more than 2 times. While the thermal conductivity of neat epoxy is 0.363W/mK that of the composite is estimated to be about 0.7920 W/mK
This work shows that hybrid filler composites can be teller made as per the required thermal conductivity. By keeping the matrix and primary filler (Glass micro-sphere) constant and by choosing suitable secondary filler, thus composite of any required thermal conductivity value can be obtained which is required for a particular application.
With enhanced thermal conductivity, these composites possibly be used in microelectronic application such as encapsulation, electronic packaging PCB etc.
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