effect of microstructure and physical parameters of hollow microsphere on insulation performance

8/12/2019 Effect of microstructure and physical parameters of hollow microsphere on insulation performance

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Effect of microstructure and physical parameters of hollow glass microsphere on

insulation performance

Bing Li a,b, Jing Yuan a,b, Zhenguo An a,b, Jingjie Zhang a,a Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, Chinab Graduate University of Chinese Academy of Science, Beijing 100049, China

a b s t r a c ta r t i c l e i n f o

Article history:

Received 4 January 2011Accepted 15 March 2011

Available online 21 March 2011

Keywords:

Hollow glass microsphere

Microstructure

Thermal properties

True density

Stacking coefcient

Hollow glass microsphere (HGM) is a special type of inorganic functional powder with wide applications. HGM

canbe applied inthe insulationareaas llersowingto thehollow structurethatis notconductive tothe transfer of

heat. The mechanism of heat transfer in HGM is analyzed and the thermal conductivity is proved to be the main

heat transfer. Thena simplemodel comprised of continuous solid phase and independent gas phase is proposed

to study the methods for decreasing the thermal conductivity of the lled system through the methods of

reducingthe density of HGMor increasingthe stackingcoefcientof HGM. Thisworkprovidesactual guidancefor

thedesignand controlledpreparation of compositematerials with hollow glass microspheres as functionalller.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Hollow glass microsphere (HGM) is nely dispersed, free-owing

inorganic powder, exerting important prospects in aerospace [1],deep-sea exploration [2], and hydrogen storage [3,4] and so on. Besides,

the hollow core endows HGM excellent thermal insulating property,

which makes HGM a promising candidate in insulation elds[5,6].

Hollow glass microsphere can be used as an insulating material owing

to its low thermal conductivity, a key factor for evaluating the thermal

insulation of materials. But up to date, little work has been seen for the

research of thermal conductivity of HGM independent of any matrixes or

adhesives. In order to makethe HGM better applied in the insulation area,

it is urgentto carry out relevant study to understandthe effectof structure

and physical parameters on the insulation performance.

In this work, the results of the effect of the microstructure and

properties of HGM on the thermal conductivity have been studied and

the heat transfer process of HGM has been analyzed in detail, aiming to

nd out the relation between the density and thermal conductivity, and

the relation between stacking coefcient and thermal conductivity for

the HGM, laying an exceptional foundation for the actual applications in

heat insulating.

2. Experiments

The ve samples of HGMselected were prepared by our laboratory

via soft chemical method (True density t=0.25, 0.32, 0.40, 0.46, and

0.60 g/cm3, stacking coefcient=0.540.56), for studying the relation

between thermal conductivity and density. In order to have a clearer

understanding about the effect of stacking coefcient, HGM screened

by 250-mesh standard sieve (=0.45 g/cm3

, particle diameter070m) and oating microsphere of ash ash screened by 120-

mesh standard sieve (=0.80 g/cm3, particle diameter 85200m)

were chosen to study by using the method of xing the quality of

oating microspheres and then continually adding small-size HGM

into the system. All the microspheres were dried at 120 C for an hour

before the experiments.

As the insulation performance of microsphere is closely related to

particle size, we used the LS 13 320 laser diffraction particle size

analyzer (Beckman Coulter) to obtain the results of the statistical

distribution of particle size by measuring the light scattering pattern of

particles of the samples. The thermal conductivity was measured by

QTM-500 rapid thermal conductivity meter. The temperature was kept

at 23 C during the whole experiment.

3. Results and discussion

3.1. Structural characteristics

It is well known that thebehavior of the HGM is strongly dependent

on its microstructure. Typical SEM images of HGM involving full view

and cross section are observed by scanning electron microscopy (SEM,

HITACHI S-4300). Fig. 1(a) shows that most of theHGM are of spherical

appearance with ne sphericity, possessing different size for several or

tens of microns. Fig. 1(b)showsthe clear hollowcore andhomogeneous

wall thickness.

Materials Letters 65 (2011) 19921994

Corresponding author. Tel./fax: +86 10 82543691.

E-mail addresses:[email protected](B. Li),[email protected](J. Zhang).

0167-577X/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2011.03.062

Contents lists available at ScienceDirect

Materials Letters

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
http://dx.doi.org/10.1016/j.matlet.2011.03.062http://dx.doi.org/10.1016/j.matlet.2011.03.062http://dx.doi.org/10.1016/j.matlet.2011.03.062mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.matlet.2011.03.062http://www.sciencedirect.com/science/journal/0167577Xhttp://www.sciencedirect.com/science/journal/0167577Xhttp://dx.doi.org/10.1016/j.matlet.2011.03.062mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.matlet.2011.03.062


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3.2. Mechanism of heat transfer

Thebasicmechanisms of heat transfer aregenerallyconsidered to be

conduction,convectionand radiation. Conductionis the mostsignicant

means of heat transfer withina solid or between solid objects in contact.

Fluids, especially gasses are less conductive, and Skochdopole [7]

concluded that the natural convection would not occur when bubble

diameter was less than 4 mm for porous materials, so convective heat

transfer can be neglected since the diameter of HGM is only 0100m.

Thermal radiation refers to the transfer of heat energy through empty

space by electromagnetic waves, it is usually signicant only at high

temperature compared to other mechanisms of heat transfer. In

conclusion, conduction is the main mechanism of heat transfer and

the other two can be neglected for HGM. There are several factors

affecting conduction including density, heat transfer path, and contact

points and so on.

3.3. Methods for reducing thermal conductivity of system

Fig. 2(a) shows the simple system containing continuous solid

phase and independent gas phase. Thus,

ss + gg; it is known that gs; Soggss; and ss:

Wherein, thermal conductivity of the system, sthermal

conductivity of the solid, gthermal conductivity of the gas, s

volume percentage of the solid, gvolume percentage of the gas sand g are related to essence of materials (here, assuming they are

constant). So can be reduced by increasing g and decreasing sthrough expanding the ratio of the internal diameter and the external

diameter (the size of microspheres of raw system is constant) in

method I, and increasing the stacking coefcient in method II as well.

3.3.1. Relationship between and tThe theoretically true density of HGM tcan be calculated as t=

[1(d/D)3] 0, wherein, d is the average internal diameter of

microspheres, D is the average external diameter of microsphere, and0is the glass density of shell part excluding hollow part. It has been

veried in the articles [8,9] that the glass density can be calculated

according to glass compositions and there are tiny differences in the

compositionsof thesamples,leading to similar0 ofthem. So wecansee

from the above formula that twill vary signicantly as d/D changes,

which means that mass will decrease per unit volume when d/D

increases, thus leading to a decline in thermal conductivity of system.

Fig. 3shows that the thermal conductivity of HGM increases as the

true density of HGM rises at 23 C in the real state. Besides, the ve

samples essentially have the similar stacking coefcient, which means

that they have the similar space utilization. So it can be inferred that

there's a close relationship between thermal conductivity and true

density. Therefore, using low-density HGM decreases thermal con-

ductivity of both HGM and system for applying in the insulation area.

3.3.2. Relationship between and N

The thermal conductivity of the simple system will decline when

the stacking coefcient of HGM increases just as method II. The mode

of classic stacking theory[10]is sphere just like HGM and it has been

thought that the large particles constitute the framework, the middle

grainsll in the gap within the large particles and ne powderlls in

the gapbetween large particlesand middle grainsfor the ideal state of

close packing. When the ratio of diameter of large particles and ne

particlesis less than 3, it is not effective for matching the particlessizeand increasing free packing density of complexes compared to that of

just large particles, because the gap size within large particles at this

time is smaller than the diameter ofne particles and it is difcult for

ne particles to ll the gap, leading to ineffective gradation matching.

So HGM screened by 250-mesh standard sieve (=0.45 g/cm3) and

oating microsphere screened by 120-mesh standard sieve

(=0.80 g/cm3) were used for exploring binary system of accumu-

lation. The ratio of the average diameters of the large and small sized

HGM is about 4:1; Fig. 4 shows the relation between weight percentage

of large size oating microsphere and thermal conductivity, and also

the relation between weight percentage of large size oating

microsphere and stacking coefcient. It can be seen that the stacking

coefcient increasedat rst, then it decreased with the increase of small

size HGM, curve a reached a peak when the weight percentage of largesize oating microsphere is about 0.7, which is similar with curve b. It

Fig. 1.SEM image (a) the front view of HGM and (b) the cross section of HGM.

Gas Gas

SolidSolid

Gas

Solid

(a)(b) (c)

I II

Fig. 2.Method I for increasing the volume of hollow part of HGM; method II for increasing the amount of HGM.

1993B. Li et al. / Materials Letters 65 (2011) 19921994
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proves that the close packing of HGM can be articially controlled and

the thermal conductivity of HGM, together with the volume of gas will

both increase as samples stack close.

4. Conclusions

Hollowglass microsphere with hollow structurecan be used in the

insulation areas owing to itslow thermal conductivity and conduction

heat transfer is the most importantmeans for transferring heat within

the hollow glass microspheres according to the mechanism of heat

transfer of HGM. In view of the simple system containing continuous

solid phase andindependentgas phase, two main methods areusually

used to reduce the thermal conductivity of the system: increase the

ratio of internal diameter and external diameter of HGM and increase

the stacking coefcient of HGM, which also means to reduce the true

density of HGM and increase the amount of HGM. The thermal

conductivity of HGM and the system will be reduced by using the

former method, and the thermal conductivity of the system will be

reduced but the thermal conductivity of HGM will be increased byusing thelatter method. So thethermal conductivity of thesystem can

be reduced by the combined use of the above two methods, which

provide effective guidance for hollow glass microspheres being used

as llers to reduce the thermal conductivity of the system in practice.

Acknowledgments

This work has been supported by the National Natural Science

Foundation of China (projects No. 04B7131801) and the State High

Technology Development Program 863 (2006AA09Z209).

References

[1] Geleil AS, Hall MM, Shelby JE. J NonCryst Solids 2006;352:6205.[2] Khimiya. Handbook of llers for polymeric composite materials; 1981 [Russian

translation] Moscow.[3] Rapp DB, Shelby JE. J NonCryst Solids 2004;349:2549.[4] Brow Richard K, Schmitt Melodie L. J Eur Ceram Soc 2009;29:1193201.[5] Allen MS, Baumgartner RG, Fesmire JE, Augustynowicz SD. Cryogenic engineering

conference; Sep. 2226, 2003. p. 18.[6] Allen M.S., Willen G.S., Mohling R.A. US Patent No. 6,858,280. 2005.[7] Skochdopole RE. Eng Prog 1961;57:558.

[8] Huggins Maurice L, Sun Kuan-Han. J Amer Ceram Soc 1943;26:411.[9] Scholze H. Glas. natur, struktur and eigenschaften, glass. Nature, structure and

properties. Berlin: Springer; 1977.[10] Westman AR, Hugill HR. J Amer Ceram Soc 1930:10.

Fig. 3.Relationship between thermal conductivity and true density. Fig. 4.Relationship between weight percentage of large size oating microspheres W

and thermal conductivity, and stacking coefcient N.

1994 B. Li et al. / Materials Letters 65 (2011) 19921994
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effect of microstructure and physical parameters of hollow microsphere on insulation performance

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