Effect of surface oxidation on thermal shock resistance

Effect of surface oxidation on thermal shock resistance of the ZrB 2–SiC–ZrC ceramic

Zhanjun Wu a ,?,Zhi

Wang a ,Guodong Shi a ,Jin Sheng b

School of Aeronautics and Astronautics,Faculty of Vehicle Engineering and Mechanics,State Key Laboratory of Structural Analysis for Industrial Equipment,Dalian University of Technology,Dalian 116024,PR China b

Troop 91550of PLA,Dalian 116023,PR China

a r t i c l e i n f o Article history:

Received 17August 2010

Received in revised form 26May 2011Accepted 15June 2011

Available online 23June 2011Keywords:A.Ceramics B.Strength D.SEM

a b s t r a c t

The isothermal oxidation of the ZrB 2–SiC–ZrC ceramic was carried out in static air at a constant temper-ature of 1000±15°C,1200±15°C and 1400±15°C for 30min,https://www.360docs.net/doc/9210160081.html,pared with the original strength of 580MPa,the strength for the specimen oxidized at 1000°C,1200°C and 1400°C for 30min increased to 609MPa,656MPa and 660MPa,respectively,because the ?aws in the surface of the spec-imen were sealed by the oxide layer.The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching.The measured D T crit for the specimen oxidized at 1000°C,1200°C and 1400°C were 352°C,453°C and 623°C,respectively,which was obviously higher than 270°C for the unoxidized specimen.The improvement in the thermal shock resistance was attrib-uted to the formation the oxide layer on the surface of the specimen.The results here pointed to a prom-ising method for improving strength and thermal shock resistance of ZrB 2-based ceramics.

1.Introduction

ZrB 2-based ultrahigh temperature ceramics (UHTCs)with appropriate compositions are attractive for high-temperature structural applications because of their unique combination of high melting point [1],high strength and hardness [2],high thermal and electrical conductivity [3]and good chemical stability [4].Although these materials have many advantages,their intrinsic characteris-tics such as low fracture toughness (premature failure due to brittle fracture),poor thermal shock resistance are still obstacles for them to be used widely,especially for applications in the thermal shock conditions with high heat transfer and/or rapid environmental temperature changes,such as furnace elements,plasma arc electrodes,hypersonic aircraft,reusable launch vehicles,or rocket engines and thermal protection structures for leading edge parts on hypersonic reentry space vehicles [5–7].ZrB 2-based UHTCs,be-cause of inherently low fracture toughness,are susceptible to cata-strophic fracture caused by the thermal stresses under the thermal shock conditions.Such susceptibility of ceramic matrix composites to thermal stresses has been recognized for a long time.More than one hundred years ago the theoretical prediction equations for the thermal stresses arising from the temperature gradients were ?rstly derived by Duhamel,which was reported in the literature [8].Since that time,many reports have appeared which mainly con-sidered the measurement of the thermal shock resistance subjected to thermal stresses.Also there are many methods available for the improvement of the thermal shock resistance of the ceramic matrix composites [1].They can be broadly classi?ed into two groups:(1)improving strength and (2)decreasing thermal stresses.(1)On the basis of the thermal shock fracture theory,the basic equation to predict thermal shock resistance is well known and is a triaxial modi?cation of Hooke’s law to account for thermal strains during rapid cooling (Eq.(1))[9].The critical change in temperature,R ,can be expressed in terms of the strength of the material (r ),Young’s modulus (E ),Poisson’s ratio (v )and coef?cient of thermal expansion (a ).The critical temperature difference,R ,to represent thermal shock resistance predicts the maximum change in temper-ature that can occur without the initiation of cracks.

R ?

r e1àv TE a

e1T

According to Eq.(1),the increase in the strength is favorable to improve thermal shock resistance.In addition,the critical temper-ature difference,R ,to represent thermal shock resistance can be measured experimentally by quenching specimens from various elevated temperatures and determining the quenching tempera-ture that results in a reduction of strength for a given specimen geometry.The measured critical temperature difference,D T crit ,value is de?ned as 70%of the original strength,which was deter-mined using linear interpolation of the retained strength values as described in ASTM C1525-04[9].(2)It is also recognized that to obtain an improved thermal shock resistance,it is necessary to reduce thermal stress.There are two methods available for the reduction of the thermal stress.First,a kind of material with higher thermal conductivity,in general,possesses a better resistance to

Corresponding author.Tel./fax:+8641184706791.

E-mail address:wzdlut@https://www.360docs.net/doc/9210160081.html, (Z.Wu).

thermal shock because of its trend to create less interior tempera-ture gradient and to bring about smaller thermal stress[10].Sec-ond,the surface of material is coated with the thermal barrier layer,which is favorable to improve thermal shock resistance because the thermal stress was consumed by the breaking of the thermal barrier layer.

Furthermore,ZrB2-based UHTCs are brittle materials,the?aws inevitablely occurred on the surface of the materials during the machining process.When the components are loaded,the stress concentration occurs at?aw tip.The concentrated stress(r m) can be expressed in terms of the average strength of the material (r),?aw size(C)and curvature radius(f)of?aw tip as shown in Eq.(2)[11].

r m ?1t2

???

re2TThe curvature radius(f)of?aw tip is much smaller than?aw

size(C),which results in the fast fracture of the ceramic compo-nent at lower average stress that is signi?cantly lower than the strength of the ceramic component.The literature has con?rmed that the thermal shock resistance is susceptible to the?aws on the surface of the specimen[9].The previous investigations have reported that the?aws on the surface of the ceramic component was healed by the heat treatment in many ceramics so as to increase the strength of the ceramic component[12,13].

In the present work,the ZrB2–SiC–ZrC ceramic was oxidized at the different temperatures for30min.The oxidized specimen was water-quenched into the water of room temperature to evaluate the effect of the surface oxidation on the thermal shock resistance. The purpose of this paper is to report an attractive method for improving strength and thermal shock resistance of ZrB2-based UHTCs,which can be applied to aid materials engineering design for the development of quality assurance and characterization assessment of durability.

2.Experimental

The preparation procedure of a ZrB2–20vol.%SiC–6vol.%ZrC (ZrB2–SiC–ZrC)ceramic was described elsewhere[14].The?exural strength of the specimen before the oxidation was tested in three-point bending on3mm by4mm by36mm bars,using a30mm span and a crosshead speed of0.5mm minà1.Before testing,all specimens were ground and polished with diamond slurries down to a1l m?nish and the edges of all specimens were chamfered to minimize the effect of stress concentration due to machining ?aws.The specimen was supported on the zirconia crucible and the temperature of the specimen center was measured by multi-wavelength pyrometer with measurement range of1000–3000°C.The isothermal oxidation of the specimens was carried out in static air and the specimen was oxidized at a constant tem-perature of1000±15°C,1200±15°C and1400±15°C for 30min.The water quenching is the most popular thermal shock test method in which the specimens are heated to a desired temper-ature and then quenched into water bath of room temperature.The water quenching temperature differences were200,300,400,500, 600,700,800and900°C,respectively.The specimen quenched into water was immerged for5min and then was taken out to reduce the hydration of B2O3.The?exural strength of the oxidized speci-men before and after water quenching was tested in three-point bending on3mm by4mm by36mm bars,using a30mm span and a crosshead speed of0.5mm minà1.The microstructural obser-vations of specimen were carried out by scanning electron micros-copy(SEM,FEI Sirion,Holland)along with energy dispersive spectroscopy(EDS,EDAX Inc.)for chemical analysis.3.Results and discussion

3.1.Oxidation at1000°C

The surface microstructure of the ZrB2–SiC–ZrC ceramic before oxidation is shown in Fig.1A.It could be found by EDS analysis (not shown here)that the microstructure of the ZrB2–SiC–ZrC cera-mic was characterized by the grey ZrB2,dark SiC and light ZrC.The relative density of the ZrB2–SiC–ZrC ceramic was calculated to be >97.5%according to the rule of mixtures based on the densities of6.09,3.21,and6.44g/cm3for ZrB2,SiC,and ZrC,respectively [15].The surface of the specimen was oxidized when the specimen was exposed to high temperature air.For example,ZrC was oxi-dized to ZrO2and CO2at above380°C[16];ZrB2was oxidized to ZrO2and B2O3at above650°C[17];SiC was oxidized to SiO2and CO2at above900°C[18].The surface microstructure of the speci-men oxidized at1000°C for30min is inseted in Fig.1A.The sur-face of the specimen was covered with the smooth oxide layer. EDS analysis con?rmed that the oxide layer mainly contained O and a small quantity of Zr elements,but was free of Si,which is consistent with the presence of B2O3glass[18].Because the oxida-tion rate of SiC is much slower than that of ZrB2in this temperature regime,the SiC particles did not oxidize appreciably.Furthermore, the white spots that were con?rmed to be ZrO2phase covered by the B2O3glass were readily observed in the surface of the specimen, which was attributed to the convective transport of B2O3liquids[19]. Fig.1B shows the surface microstructure of the oxidized specimen quenched from temperature difference of200°C in the water of room temperature.The large numbers of ZrO2particle were exposured due to the hydrolyzation of the B2O3glass,and the hydrolyzation vestiges were readily observed in Fig.1B.Because of a negative standard Gibbs free energy of hydrolyzation reaction, B2O3would react spontaneously with the water according to Eq.(3),resulting in the hydrolyzation of the B2O3glass[20].

B2O3t3H2O!2H3BO3D G298?à28:84kJ molà1e3TThe surface microstructures of the oxidized specimen quenched at temperature difference of200°C and300°C were similar,as shown in Fig.1C.The temperature difference was more than 300°C,a lot of microcrack occurred on the surface of the specimen, as shown in Fig.1D.The formation of the microcracks on the sur-face of the specimen was attributed to the thermal stress during water quenching.The amount of the hydrolyzation of the B2O3 glass increased as the temperature difference increased because the increase in the temperature of the specimen was favorable to the hydrolyzation of the B2O3glass.

The original strength of580MPa is shown in Fig.2and the strength of the unoxidized and oxidized specimens after the water quenching with increasing temperature difference is also shown in Fig.2.It could be seen from Fig.2that the measured D T crit for the unoxidized specimen is270°C.When the temperature difference was more than300°C,the strength for the unoxidized specimen gradually decreased with increasing temperature difference up to 900°https://www.360docs.net/doc/9210160081.html,pared with the original strength of580MPa,the strength for the specimen oxidized at1000°C for30min increased to609MPa because the?aws in the surface of the specimen were completely sealed by the B2O3glass[12,13].Compared with the strength of609MPa for the oxidized specimen,the strength of the oxidized specimen after the water quenching did not show change at the temperature differences of200and300°C.The temperature difference was more than300°C,the strength of the oxidized specimen after the water quenching sharply decreased, and then retained at300MPa.The sharp reduction in the strength was attributed to the presence of the microcracks on the surface of the specimen because the ZrB2–SiC–ZrC ceramic is susceptible to

1502Z.Wu et al./Composites Science and Technology71(2011)1501–1506

the surface microcracks.The measured D T crit for the specimen oxidized at1000°C was352°C,which was signi?cantly higher than270°C for the unoxidized specimen.The increase in the critical temperature difference was attributed to the presence of the surface oxide layer(speci?c mechanisms are discussed below). Furthermore,the strength distribution of the oxidized specimen after the water quenching increased with increasing temperature difference,which was presumedly attributed to the increase in the size of critical?aws[9].

3.2.Oxidation at1200°C

Fig.3A shows the surface microstructure of the specimen oxi-dized at1200°C for30min.The white particles were con?rmed by EDS analysis to be ZrO2phase and the ZrO2particles

ered with the glass layer that was con?rmed by EDS

SiO2-rich.Although B2O3shows high volatility at

the SiO2-rich layer is expected to contain some B

heating at1200°C based on either incomplete volatility

B2O3or the continued production of B2O3beneath

[18].The surface microstructure of the oxidized

quenched at the temperature difference of200°

Fig.3A.The obvious microcracks that appeared in

were attributed to both the volume change and the

during water quenching.It was obvious to that the

not penetrate into the SiO2-rich layer.The temperature increased up to400°C,the obvious change in the

structures of the oxidized specimen after water

not observed compared with the surface microstructure temperature difference of200°C.However,a lot

were easily detected on the surface of the specimen,

Fig.3B,when the temperature difference was more

The microcrack density in the surface of the specimen decreased as the temperature difference increased,whereas the microcrack size increased.

Fig.4shows the strength of the unoxidized and oxidized spec-imen after the water quenching with increasing temperature dif-ference.The specimen was exposured to air at the temperature of1200°C for30min and the surface of the specimen was obvi-ously oxidized,which led to that the strength of the oxidized spec-imen increased to656MPa due to the formation of the SiO2-rich layer compared with the original strength of580MPa.At the temperature differences of200,300and400°C,the strength of the oxidized specimen after the water quenching did not change. The temperature difference was more than400°C,the strength of the oxidized specimen after the water quenching sharply de-creased,and the measured D T crit for the oxidized specimen as high

micrographs of the unoxidized specimen(A)and the oxidized specimen quenched at temperature difference of200(B),300(C)and400°C micrograph of the oxidized specimen before water quenching.

Strength of the unoxidized and oxidized specimens as a function of thermal

temperature difference.

as453°C was obviously higher than D T crit of270°C for the unox-idized specimen and D T crit of352°C for the specimen oxidized at 1000°C.The increase in the critical temperature difference was attributed to the formation of the SiO2-rich layer that has better sealing capability than does B2O3layer.Furthermore,the strength distribution of the oxidized specimen after the water quenching increased with increasing temperature difference,which was pre-sumedly attributed to the increase in the size of critical?aws[9].

3.3.Oxidation at1400°C

The specimen was oxidized at1400°C for30min and the oxi-dized specimen was quenched at the different temperature differ-ences.Fig.5shows the surface micrographs of the oxidized specimen quenched at the different temperature differences and the surface micrograph of the oxidized specimen before the water quenching was inseted in Fig.5A.It could be seen from the inset in Fig.5A that the surface of the oxidized specimen was covered with the SiO2-rich layer and no ZrO2particles were observed due to because the oxidation of SiC is much quicker than that of ZrB2at the temperature of>1350°https://www.360docs.net/doc/9210160081.html,pared with the surface micro-structure of the oxidized specimen before the water quenching, the surface microstructures of the oxidized specimen quenched at temperature difference of200and300°C did not show change. When the temperature difference was400°C,a lot of microcracks appeared on the surface of the specimen and it was obvious to that the microcracks did not penetrate into the surface oxide layer.The surface micrograph of the oxidized specimen quenched at temper-ature difference of500°C is shown in Fig.5B.In contrast to the surface microstructure of the specimen quenched

difference of400°C,the surface oxide layer was

the microcracks and the desquamation of the oxide

ily discovered.Furthermore,the microcrack density

of the specimen did not increase as the temperature increased from400°C to500°C.Fig.5C shows the

graph of the oxidized specimen quenched at temperature

ence of600°C.It could be seen from Fig.5B and

microcrack density in the surface of the specimen

imen as a function of thermal shock temperature https://www.360docs.net/doc/9210160081.html,-pared with the unoxidized strength of580MPa,the strength of the oxidized specimen increased to be660MPa due to the formation of the SiO2-rich layer.The?exural strength for the specimens oxi-dized at1200and1400were statistically identical at656and 660MPa,which revealed that the?aws on the surface of the spec-imen were completely sealed by the SiO2-rich layer.The strength of the oxidized specimen quenched at the temperature differences of200,300,400and500°C were equivalent to the strength for the oxidized specimen before the water quenching,which was attrib-uted that the microcracks did not penetrate into the surface of the substrate,although penetrate the surface oxide layer at tem-perature difference of500°https://www.360docs.net/doc/9210160081.html,pared with the strength of the oxidized specimen before the water quenching,the strength of the oxidized specimen quenched at the temperature difference of 600°C was reduced slightly to551MPa,which indicated that the surface of the substrate was penetrated by the microcracks and the substrate was slightly damaged.The temperature difference was more than600°C,the strength of the oxidized specimen after the water quenching sharply decreased,and the measured D T crit for the oxidized specimen as high as623°C was essentially higher than that for the unoxidized specimen and specimen oxidized at 1000°C and1200°C.The increase in the critical temperature dif-ference was attributed to the presence of the thick oxide layer. 3.4.Improvement mechanism of thermal shock resistance

The specimen was oxidized at1000°C,the surface of the spec-imen is mainly ZrO2and B2O3phases,no SiO2phase was observed

micrographs of the oxidized specimen before(A)and after((B)500°C)the water quenching;the inset in(A)was surface micrograph of the temperature difference of200°C.

Strength of the unoxidized and oxidized specimens as a function of

temperature difference.

because the oxidation rate of SiC phase was signi?cantly lower than ZrB2phase in this temperature regime.The specimen was oxi-dized at1200°C,the surface of the specimen is mainly ZrO2and SiO2phases,and borosilicate that was resulted from the reaction of a small amount of non-volatile B2O3with SiO2phases.The spec-imen was oxidized at1400°C,the surface of the specimen is mainly SiO2-rich phases,and borosilicate that was resulted from the reaction of a small amount of non-volatile B2O3with SiO2 phases.The improvement of thermal shock resistance was attrib-uted to the formation of oxide layers.The formation of oxide layers resulted in(I)the healing of the surface?aws,(II)the increase in the?exural strength,(III)the appearance of the compressive stress zone beneath the surface oxide layers,(IV)the decrease in the

oxide layers was favorable to inhibit the crack initiation and prop-agation during the transient water quenching prior to?nal failure, which was bene?cial to resistance to thermal shock because the thermal stress was counteracted partially due to the appearance of the compressive stress.(IV)Compared with the high thermal conductivity(80–130W mà1Kà1)[21]of the ZrB2–SiC–ZrC cera-mic,the surface oxide layers of the low thermal conductivity (2–4W mà1Kà1)[22,23]could act as the thermal barrier layer of the specimen substrate.During the water quenching,the large temperature difference appeared between the surface of the oxide layers and the interior of the specimen substrate,as illustrated in Fig.7.When the surface temperature of the oxide layers was reduced to the temperature of the water bath,the surface of the specimen substrate was still retained at higher temperature due to the low thermal conductivity of the oxide layers.The B

Surface of oxide layer

Surface of substrate

Residual strength of the unoxidized and oxidized specimen as a function

shock temperature difference.

Z.Wu et al./Composites Science and Technology71(2011)1501–15061505

temperature difference between the surface and the interior of the specimen substrate was reduced due to the presence of the oxide layers,which would result in the decrease in the thermal stress. (V)Furthermore,the thermal stress was consumed by the breaking of the oxide layers during the water quenching,thus protecting the specimen substrate and enhancing the thermal shock resistance of the specimen substrate.

4.Conclusions

In the present work,the isothermal oxidation of the ZrB2–SiC–ZrC ceramic was carried out in static air at a constant temperature of1000±15°C,1200±15°C and1400±15°C for 30min,https://www.360docs.net/doc/9210160081.html,pared with the original strength of 580MPa,the strength for the specimen oxidized at1000°C, 1200°C and1400°C for30min were increased to be609MPa, 656MPa and660MPa,respectively,because the?aws on the sur-face of the specimen were sealed by the oxide layer.The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching.The measured D T crit for the specimen oxidized at1000°C,1200°C and1400°C were 352°C,453°C and623°C,respectively,which was obviously higher than270°C for the unoxidized specimen.The improvement of ther-mal shock resistance was attributed to the formation of oxide lay-ers.The formation of oxide layers resulted in(I)the healing of the surface?aws,(II)the increase in the?exural strength,(III)the appearance of the compressive stress zone beneath the surface oxide layers,(IV)the decrease in the thermal stress and(V)con-sumption of the thermal stress.The?ve aspects were favorable to improve the thermal shock resistance of ZrB2–SiC–ZrC ceramic. The results here pointed to a promising method for improving strength and thermal shock resistance of ZrB2-based ceramics.

Acknowledgements

This work was supported by the China Postdoctoral Science Foundation Funded Project(20100481220),and Fundamental Research Funds for the Central Universities(3014-852001and DUT10ZDG05)and the National Natural Science Foundation of China(51002019and91016024).

References

[1]Zhang XH,Wang Z,Sun X,Han WB,Hong CQ.Effect of graphite?ake on the

mechanical properties of hot pressed ZrB2–SiC ceramics.Mater Lett 2008;62:4360–2.

[2]Zhang XH,Hu P,Han JC,Meng SH.Ablation behavior of ZrB2–SiC ultra high

temperature ceramics under simulated atmospheric re-entry conditions.

Compos Sci Technol2008;68:1718–26.

[3]Mishra SK,Rupa PKP,Das SK,Shcherbakov V.Effect of titanium diluent on the

fabrication of Al2O3–ZrB2composite by SHS dynamic https://www.360docs.net/doc/9210160081.html,pos Sci Technol2007;67:1734–9.

[4]Han JC,Hu P,Zhang XH,Meng SH,Han WB.Oxidation-resistant ZrB2–SiC

composites at2200°https://www.360docs.net/doc/9210160081.html,pos Sci Technol2008;68:799–806.

[5]Monteverde F.Progress in the fabrication of ultra-high-temperature ceramics:

‘‘in situ’’synthesis,microstructure and properties of a reactive hot-pressed HfB2–SiC https://www.360docs.net/doc/9210160081.html,pos Sci Technol2005;65:1869–79.

[6]Chen CM,Zhang LT,Zhou WC,Hao ZZ,Jiang YJ,Yang SL.Microstructure,

mechanical performance and oxidation mechanism of boride in situ https://www.360docs.net/doc/9210160081.html,pos Sci Technol2001;61:971–5.

[7]Mishra SK,Das SK,Sherbacov V.Fabrication of Al2O3–ZrB2in situ composite by

SHS dynamic compaction:a novel https://www.360docs.net/doc/9210160081.html,pos Sci Technol 2007;67:2447–53.

[8]Kingery WD.Factors affecting thermal stress resistance of ceramic materials.J

Am Ceram Soc1955;38(1):3–15.

[9]Zimmermann JW,Hilmas GE,Fahrenholtz WG.Thermal shock resistance of

ZrB2and ZrB2–30%SiC.Mater Chem Phys2008;112:140–5.

[10]Wang Z,Hong CQ,Zhang XH,Sun X,Han JC.Microstructure and thermal shock

behavior of ZrB2–SiC–G composite.Mater Chem Phys2009;113:338–41. [11]Grif?th AA.Crack propagation studies in brittle materials.J Mater Sci

1920;8:1527–33.

[12]Chu MC,Cho SJ,Park HM,Yoon KJ,Ryu H.Crack-healing in reaction-bonded

silicon carbide.Mater Lett2004;58:1313–6.

[13]Kim HS,Kim MK,Kang SB,Ahn SH,Nam KW.Bending strength and crack-

healing behavior of Al2O3/SiC composites ceramics.Mater Sci Eng A2008;483–484:672–5.

[14]Qu Q,Han JC,Han WB,Zhang XH,Hong CQ.In situ synthesis mechanism and

characterization of ZrB2–ZrC–SiC ultra high-temperature ceramics.Mater Chem Phys2008;110:216–21.

[15]Zhang XH,Qu Q,Han JC,Han WB,Hong CQ.Microstructural features and

mechanical properties of ZrB2–ZrC–SiC composites fabricated by hot pressing and reactive hot pressing.Scr Mater2008;59:753–6.

[16]Shimada S.A thermoanalytical study on the oxidation of ZrC and HfC powders

with formation of carbon.Solid State Ionics2002;149:319–26.

[17]Guo WM,Zhang GJ,Kan YM,Wang PL.Oxidation of ZrB2powder in the

temperature range of650–800°C.J Alloys Compd2009;471:502–6.

[18]Rezaie A,Fahrenholtz WG,Hilmas GE.Evolution of structure during the

oxidation of zirconium diboride–silicon carbide in air up to1500°C.J Eur Ceram Soc2007;27:2495–501.

[19]Karlsdottir SN,Halloran JW.Oxidation of ZrB2–SiC:in?uence of SiC content on

solid and liquid oxide phase formation.J Am Ceram Soc2009;92(2):481–6.

[20]Hu ZB,Li HJ,Fu QG,Xue H,Sun GL.Fabrication and tribological properties of

B2O3as friction reducing coatings for carbon–carbon composites.New Carbon Mater2007;22(2):131–4.

[21]Zhang XH,Xu L,Han WB,Weng L,Han JC.Microstructure and properties of

silicon carbide whisker reinforced zirconium diboride ultra-high temperature ceramics.Solid State Sci2009;11:156–61.

[22]Orain S,Scudeller Y,Brousse T.Thermal conductivity of ZrO2thin?lms.Int J

Therm Sci2000;39:537–43.

[23]Callard S,Tallarida G,Borghesi A,Zanotti L.Thermal conductivity of SiO2?lms

by scanning thermal microscopy.J Non-Cryst Solids1999;245:203–9.

Technology71(2011)1501–1506