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Fabrication of high thermal conductive Al–cBN ceramic sinters by high temperature high pressure method
Solid State Sciences 13 (2011) 1041e1046
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Fabrication of high thermal conductive AlecBN ceramic sinters by high temperature high pressure method P.F. Wang a, b, Zh. H. Li b, *, Y.M. Zhu b a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Procession Mechanics, Chinese Academy of Sciences (CAS), Xi’an, Shanxi 710119, PR China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Material Science and Engineering, Tianjin University, 92, Weijin road, Nankai district, Tianjin 300072, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2010 Received in revised form 24 December 2010 Accepted 29 January 2011 Available online 4 March 2011
AlecBN ceramic sinters were fabricated by sintering micro-powder mixture of Al and cBN under high temperature and high pressure condition. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electronic microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) elemental mapping analyses and laser flashing thermal conductivity measurements were performed to investigate the sintering properties and thermal conductivity of the AlecBN ceramic sinters. XRD analysis revealed these AlecBN ceramic sinters were composed of a large portion of cBN and of a small portion of AlN, and very little amount of AlB12 and hBN. Formation of boundary phase resulted in the rapid densification of the sinters, as well as the increase of their relative density with increasing Al additions. The AlecBN ceramic sinters have a maximum thermal conductivity of about 1.94 W/cm K at room temperature and a much higher value of about 2.04 W/cm K at 200 C. Their high thermal conductivity over that of AlN ehBN composites promise AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: High temperature high pressure sintering Cubic boron nitride Thermal conductivity Heat sink materials
1. Introduction Among III-V nitride compounds, cubic boron nitride (cBN) and AlN have attracted both scientific and technological interests in recent years, due to their fascinating mechanical and functional properties [1,2]. They hold much promise as wide band gap semiconductors for the use in optoelectronic and microelectronic devices operating under extreme conditions. Especially, cBN has a number of extraordinary properties, e.g., chemical inertness, high melting temperature, and high thermal conductivity. Its electronic properties, dominated by a wide band gap (6.1e6.4 eV) [3] and a relatively small dielectric constant, may have potential applications in ultraviolet optics, high power/frequency microelectronics, and heat-conducting substrates. Because of these fascinating properties, cBN has received a great deal of attention from experimentalists and theoreticians [4]. Despite the wide variety of possible multifunctional applications, practical application of cBN is limited. The production of individual single crystals of cBN with large size is extremely costly, and film technologies have not given reassuring results of producing large-
* Corresponding author. Tel./fax: þ86 22 27404260. E-mail address: [email protected] (Zh.H. Li). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.023
sized unique phase cBN films with requisite optical and electrophysical characteristics [5]. Meanwhile, a good alternative to cBN single crystals and thin films for functional applications can be sintered pure cBN ceramics or cBN compacts containing binders of metal or ceramics. Many works have provided useful information on sintering of cBN ceramics and compacts under high temperature and high pressure conditions [6e13]. Similar to cBN, AlN ceramics have been extensively investigated because of its relative high thermal conductivity (over traditional substrate materials, Al2O3 and BeO, etc.), excellent electric resistivity and a thermal expansion coefficient close to that of silicon, making AlN as heat sinks for Si substrate [14e16]. Usually, hBN is introduced into AlN matrix to improve the matrix’s machinability [17e20], which, however, inevitably result in the degradation of AlN matrix’s thermal conductivity. Even many works have been carried out to deal with coordination between the machinability and thermal conductivity of AlNehBN composites [21e25]. However, with regard to the thermal conductivity of AlNehBN composites, only a maximum of about 1.1 w 1.41 W/cm K [20, 22] was achieved. Dislike hBN, cBN had isotropic crystal structure, and it maintains attractive functional properties superior to AlN, especially much larger band gap, higher thermal conductivity (theoretically 12 W/cm K for the pure cBN single crystal, about 4.5e6.5 W/cm K
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
for hBN directly transitioned and long time sintered pure cBN polycrystals [26]) and lower dielectric constant. Meanwhile, little amount of binders always are introduced in order to reduce the sintering temperature of pure cBN polycrystals. Among which, high pressure sintering of cBN with Al binders are widely researched [7,9e13], and the researched AlNecBN composites exhibit outstanding mechanical properties. However, to date, the functional properties of AlNecBN composites, e.g. thermal conductivity and dielectric properties have rarely been reported. In this paper, AlecBN ceramic sinters with high density were fabricated by sintering the mixtures of cBN and Al micro-powders at high temperature and high pressure conditions, and the relations between microstructure, high relative density and thermal conductivity and Al additions were investigated. The high thermal conductivity property would give AlecBN ceramic sinters large potential applications as high efficiency heat sink materials for wide band gap semiconductors. 2. Experimental AlecBN ceramic sinters were prepared directly from mixed micro-powders of cBN (cBN-M990, particle size 1.5e3 mm, >99.5% purity, Henan Funik Ultrahard Material Co. Ltd, China) and aluminum micro-powder (an average particle size 20 mm, >99.5% purity, Tianjin Keweier Metal Material Co., Ltd, China) under HTHP conditions. The micro-powders were manually mixed in ethyl alcohol, using an agate mortar and pestle for 4 h, at a proportion of 2, 4, 6, 8 and 10 wt% Al in AlecBN system, respectively. The starting mixtures were degassed under vacuum of 4.0 103 Pa and temperature of 600 C for 2 h prior to sintering process. The mixed micro-powders were set in synthesis capsule and sintered at 1400 C under a pressure of about 5.0 GPa for 2 min, using a belttype high pressure graphite apparatus. The pressure was estimated by the oil pressure reading calibrated with the method of silver melting point at high pressure [8], and the temperature calibration was performed using a Pt/Pt-13%Rh thermocouple. DSC measurement of the AlecBN mixtures was carried out using a Netzsch Model 449C instrument in the temperature range of room temperature to 1500 C at a heating rate of 10 C/min, in nitrogen atmosphere. AlecBN ceramic sinters were disposed, polished to a flat surface, and finally cleaned ultrasonically in ethanol, acetone and distilled water, respectively, followed by a drying process in vacuum chamber (detail process referred to Ref. [8]). Archimedes method was used to measure the volume density of these specimens. A Philips XL30E scanning electron microscope (SEM) was used to examine the cross-section microstructure fractured AlecBN ceramic sinters, and XRD (Cu Ka radiation, l ¼ 1.54056 Å) was performed to analyze the phase compositions using an X-ray diffractometer (Rigaku D/Max 2500V/PC). Same samples shaped as cylinders with diameters of 12.7 0.1 mm and heights of 1.9 0.1 mm were coated with graphite for use in thermal diffusivity examination. Thermal diffusivity measurement of these samples was taken by the LFA 427 laser flashing thermal conductivity analyzer at room temperature in argon atmosphere with a flow speed of 100 ml/min. A single sample, with 10 wt% Al addition, was taken to research its temperature dependence of thermal conductivity within the range from room temperature to 500 C. 3. Results and discussion DSC curve of AlecBN mixing micro-powders is shown in Fig. 1. Because the influence of air on the fast reaction of Al and cBN under high temperature and high pressure was very tiny, therefore, the DSC measurement of the AlecBN mixtures was performed in
DSC /(uV/mg) ↓ exo 0.40 Peak: 661.5 °C Peak: 1462.0 °C
0.30
0.20 Peak: 1353.6 °C [1]
0.10 Peak: 1283.3 °C
0
-0.10 Peak: 1110.4 °C
-0.20 200
400
600
800 1000 Temperature /°C
1200
1400
Fig. 1. DSC curve of AlecBN mixing micro-powders in nitrogen atmosphere.
nitrogen atmosphere to avoid the effect of air. In Fig. 1, the endothermic peak is observed at about 661 C, which was attributed to Al melting’s thermal effect. Furthermore, the obscure one in the range of 700 w 900 C was attributed to the reaction of Al and cBN, and the peak at about 750 C corresponded to the reaction of Al with N2 to AlN. That of Al with cBN was mostly tense at about 1000 C. Finally, the weak peaks at about 1283 C and 1353 C corresponded to the transformation of cBN to gBN, but around 1462 C the obvious peak should be assigned to gBN/hBN structural change. It was regarded that the experimental high pressure condition resulted in the rapid distribution of molten Al simple substance between cBN grains and accelerated the diffusion of other atoms (B, N) through molten Al layer in relative low temperature range, which may lead to the rapid densification of AlecBN ceramics sinters. XRD patterns of the AlecBN ceramic sinters are given in Fig. 2, which reveals the sintered samples are composed of a large portion of cBN and of a small portion of AlN, also very little amount of AlB12 and hBN. With increasing Al additions, the peaks of AlN were getting tenser and those of cBN weaker and peaks of hBN were observed to be gradually disappearing as well. Because, formation of high hardness boundary phase (AlN, AlB12) under high pressure would to great extent impede cBN’s transformation into hBN. In addition, no trace of Al simple substance in these sinters revealed high reactivity between Al and cBN at high temperature and high pressure condition. Fig. 3 shows the SEM images of fractured AlecBN ceramic sinters with various Al additions. As it can be seen, with increasing
Intensity(a.u.)
1042
6000
cBN AlN hBN AlB12
4000
e d
2000
c b a
0 20
40
60 2θ (degree)
80
100
Fig. 2. XRD patterns of AlecBN ceramic sinters with different Al addition: (a) 2 wt%, (b) 4 wt%, (c) 6 wt%, (d) 8 wt%, (e) 10 wt%.
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
1043
98 16
96 12 94
8
92
4
AlN content/%
Relative density/%
Relative density AlN content
0
90 0
2
4 6 Al addition/wt%
8
10
Fig. 4. Change of relative density (-) and AlN content (,) with Al addition for AlecBN ceramic sinters.
Fig. 3. SEM images of fractured AlecBN ceramic sinters with various Al additions: (a) 2 wt%, (b) 4 wt%, (c) 6 wt%, (d) 8 wt%.
Al additions, the microstructure of AlecBN ceramic sinters is getting to be more and more compact. Here, the densification effect can be illustrated, as seen from Fig. 3 (a) to (d), by the gradual change of fracture model from intergranular crack to transgranular crack, due to the remarkable increase in grain boundary phase. This resulted in a strengthening effect in bonding cBN grains tightly. Meanwhile, it was also confirmed by the continuous increase in relative density and the increasing AlN content in the AlecBN ceramic sinters which was calculated from their XRD spectra, as given in Fig. 4. From this viewpoint, the Al (10 wt%)ecBN sample with highest relative density and largest AlN content was investigated with high resolution microstructure observation and EDS elemental distribution maps of B, N, Al, and O on the fractured surface of the sample, as presented in Fig. 5. Al element can be found mainly in and around the surface of cBN grains with a rather uniform distribution. Besides B, N and Al, the sample also contained a small amount of O elements, most likely introduced by raw materials. Approximate content of B, N, Al, O element was 43.91, 39.58, 15.63 and 0.76 wt%, respectively. Finally, Fig. 6 illustrates the dependence of thermal conductivity on Al addition and measuring temperature for AlecBN ceramic sinters. As a comparison, sum rule of empirical constants [27], DSC method [28] and laser flashing measuring method was used to obtain thermal capacity for calculation of the thermal conductivity, respectively. Thermal conductivity equals to the product of thermal capacity, thermal diffusivity and density of the samples. In Fig. 6(a), the thermal conductivities of the AlecBN ceramic sinters show a general increase with increasing Al additions, and the DSC and laser flashing method gives almost close values, but a little higher than those calculated from empirical constants. It should be noted, compared to the pure cBN ceramic samples in this work, only the Al (2 wt%)ecBN sample had a little lower thermal conductivity (about 1.43e1.58 W/cm K), indicating the existence of apparent interfacial thermal resistance in the AlNecBN composites. The existence of interfaces usually impeded the heat conduction by scattering the incident phonons and contributed to an interfacial thermal resistance. The maximum thermal conductivity of the Al (10 wt%)ecBN ceramic sinter was about 1.71e1.94 W/cm K. This value is much higher than that of AlNehBN composites, w1.1 W/cm K [20]. The temperature dependence of thermal conductivity for Al (10 wt%)e cBN sample is presented in Fig. 6 (b). The sample’s thermal conductivity increases tensely with temperatures before 200 C, reaches a maximum of about 1.9e2.04 W/cm K at 200 C, and then declines gradually to about 1.8 W/cm K with increasing
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P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
Fig. 5. SEM image of fractured Al (10 wt%)ecBN sample (a) and EDS elemental distribution maps of: (b) B, (c) N, (d) Al, and (e) O.
temperature. Moreover, among abovementioned three methods the laser flashing one gives highest value. As we know, the heat carriers in semiconductors and insulators are mainly phonons. Phonons may be scattered by three important mechanisms: phononephonon interaction, phonon-lattice disorder (e.g. point defects, impurities, dislocations, etc.) interaction and collision of a phonon with boundaries [29]. These phonon scatterings are the main thermal resistance in composites. In general, the mean free path of phonons is determined by these scattering centers and by the mean length of elastic waves excited at a given temperature inside crystals [30]. With regarded to AlNecBN composites’ thermal conductivity, it was closely related to the amount of grain boundary phase in the composites. With
growing Al additions, larger amount of grain boundary phase was produced. This redounded to the densification of the composites, and as a result reduced the scattering of phonons by grain boundaries, pores and defects. Note that the grain boundary phase was of relatively low amount and had very thin thickness [11,12]. In this case, the effect in promoting densification would overpass the thermal resistance effect in reducing the total thermal conductivity, which at last resulted in increase of the thermal conductivities. Those microstructures that most grains were densely packed and the grain boundaries were oriented randomly affected the thermal conductivity depending on the temperature. With regard to the temperature dependence mechanism of thermal conductivity, it was regarded that the total thermal conductivity (k) is the
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
Thermal conductivity( W/cm K)
a ·
Thermal conductivity(W/cm K)
b
1045
1.8
character as cBN single crystals, which was reported by Slack [35] and Makedon et al. [36]. It is worth noting that the ability to keep high thermal conductivity over AlNehBN composites at the temperature of 200e500 C makes the AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors, whose use would increase the latter’s much higher working temperature to a large extent.
1.7
4. Conclusions
2.0
Calculated DSC Laser flashing
1.9
1.6 1.5 1.4 0
2
4 6 8 Content of Al (wt%)
10
2.1 Calculated DSC Laser flashing
2.0
Ceramic sinters of AlecBN were prepared rapidly by sintering AlecBN mixtures at 1450 C under pressure of 5.0 GPa. These AlecBN ceramic sinters, mainly made of cBN and AlN, had high relative density and compact microstructure due to the formation of boundary phase. The dense microstructures and presence of low thermal resistant boundary phase made AlecBN ceramic sinters have much higher thermal conductivity over that of AlNehBN composites. This would promise the AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors to be used at much higher temperature than the traditional ones. Acknowledgments We would like to give our special thanks to Dr. X.Y. Huang (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the help with thermal conductivity measurement.
1.9 1.8
References
1.7 1.6 0
100
200 300 Temperature/°C
400
500
·
Fig. 6. Change of thermal conductivity (a) with Al addition for AlecBN ceramic sinters and (b) temperature dependence of thermal conductivity for Al (10 wt%)ecBN sample obtained by different methods.
sum of the contributions arising from the lattice waves (phonons) kph and a contribution ke from the charge carriers [31,32]. Although kph experienced an exponential increase below the critical temperature (Tc), the presence of charge carrier and grain boundaries scattering set a limit on its growth therefore kph diminished as temperature increased further after Tc. The maximum position depended on relative magnitudes of phonon scattering with carrier, defects, grain boundaries and phonon itself. At low temperatures (much lower than the Debye temperature), defects become effective phonon scatters and kph exhibits a power temperature behavior. At even lower temperatures, grain-boundary scattering dominated and the usual Debye cubic temperature behavior appeared. It was evident that ke vanished at low temperature and exhibited power temperature behavior above Tc [32]. As a sum, k was dominated by ke above Tc. Therefore, in this work k maintained a peak value near 200 C (see Fig. 6 (b)). After that, as the temperature continued to rise, the phonon mean free path kept decreasing, and the heat capacity basically did not change with increasing temperature [33,34], even now ke still grew bit by bit, but the quick decrease of kph overtaken the smooth increase of ke, as a result k decreased slowly with increasing temperatures. It is clearly observed that the thermal conductivity of AlecBN ceramic sinters displayed a similar temperature dependence
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[32] D. Varshney, K.K. Choudhary, M.S. Azad, R.K. Singh, Physica B. 385e386 (2006) 79e81. [33] M.A. Omar, Elementary Solid State Physics. Addison-Wesley Publishing Company Inc., London, 1975.
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Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Fabrication of high thermal conductive AlecBN ceramic sinters by high temperature high pressure method P.F. Wang a, b, Zh. H. Li b, *, Y.M. Zhu b a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Procession Mechanics, Chinese Academy of Sciences (CAS), Xi’an, Shanxi 710119, PR China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Material Science and Engineering, Tianjin University, 92, Weijin road, Nankai district, Tianjin 300072, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2010 Received in revised form 24 December 2010 Accepted 29 January 2011 Available online 4 March 2011
AlecBN ceramic sinters were fabricated by sintering micro-powder mixture of Al and cBN under high temperature and high pressure condition. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electronic microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) elemental mapping analyses and laser flashing thermal conductivity measurements were performed to investigate the sintering properties and thermal conductivity of the AlecBN ceramic sinters. XRD analysis revealed these AlecBN ceramic sinters were composed of a large portion of cBN and of a small portion of AlN, and very little amount of AlB12 and hBN. Formation of boundary phase resulted in the rapid densification of the sinters, as well as the increase of their relative density with increasing Al additions. The AlecBN ceramic sinters have a maximum thermal conductivity of about 1.94 W/cm K at room temperature and a much higher value of about 2.04 W/cm K at 200 C. Their high thermal conductivity over that of AlN ehBN composites promise AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: High temperature high pressure sintering Cubic boron nitride Thermal conductivity Heat sink materials
1. Introduction Among III-V nitride compounds, cubic boron nitride (cBN) and AlN have attracted both scientific and technological interests in recent years, due to their fascinating mechanical and functional properties [1,2]. They hold much promise as wide band gap semiconductors for the use in optoelectronic and microelectronic devices operating under extreme conditions. Especially, cBN has a number of extraordinary properties, e.g., chemical inertness, high melting temperature, and high thermal conductivity. Its electronic properties, dominated by a wide band gap (6.1e6.4 eV) [3] and a relatively small dielectric constant, may have potential applications in ultraviolet optics, high power/frequency microelectronics, and heat-conducting substrates. Because of these fascinating properties, cBN has received a great deal of attention from experimentalists and theoreticians [4]. Despite the wide variety of possible multifunctional applications, practical application of cBN is limited. The production of individual single crystals of cBN with large size is extremely costly, and film technologies have not given reassuring results of producing large-
* Corresponding author. Tel./fax: þ86 22 27404260. E-mail address: [email protected] (Zh.H. Li). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.023
sized unique phase cBN films with requisite optical and electrophysical characteristics [5]. Meanwhile, a good alternative to cBN single crystals and thin films for functional applications can be sintered pure cBN ceramics or cBN compacts containing binders of metal or ceramics. Many works have provided useful information on sintering of cBN ceramics and compacts under high temperature and high pressure conditions [6e13]. Similar to cBN, AlN ceramics have been extensively investigated because of its relative high thermal conductivity (over traditional substrate materials, Al2O3 and BeO, etc.), excellent electric resistivity and a thermal expansion coefficient close to that of silicon, making AlN as heat sinks for Si substrate [14e16]. Usually, hBN is introduced into AlN matrix to improve the matrix’s machinability [17e20], which, however, inevitably result in the degradation of AlN matrix’s thermal conductivity. Even many works have been carried out to deal with coordination between the machinability and thermal conductivity of AlNehBN composites [21e25]. However, with regard to the thermal conductivity of AlNehBN composites, only a maximum of about 1.1 w 1.41 W/cm K [20, 22] was achieved. Dislike hBN, cBN had isotropic crystal structure, and it maintains attractive functional properties superior to AlN, especially much larger band gap, higher thermal conductivity (theoretically 12 W/cm K for the pure cBN single crystal, about 4.5e6.5 W/cm K
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
for hBN directly transitioned and long time sintered pure cBN polycrystals [26]) and lower dielectric constant. Meanwhile, little amount of binders always are introduced in order to reduce the sintering temperature of pure cBN polycrystals. Among which, high pressure sintering of cBN with Al binders are widely researched [7,9e13], and the researched AlNecBN composites exhibit outstanding mechanical properties. However, to date, the functional properties of AlNecBN composites, e.g. thermal conductivity and dielectric properties have rarely been reported. In this paper, AlecBN ceramic sinters with high density were fabricated by sintering the mixtures of cBN and Al micro-powders at high temperature and high pressure conditions, and the relations between microstructure, high relative density and thermal conductivity and Al additions were investigated. The high thermal conductivity property would give AlecBN ceramic sinters large potential applications as high efficiency heat sink materials for wide band gap semiconductors. 2. Experimental AlecBN ceramic sinters were prepared directly from mixed micro-powders of cBN (cBN-M990, particle size 1.5e3 mm, >99.5% purity, Henan Funik Ultrahard Material Co. Ltd, China) and aluminum micro-powder (an average particle size 20 mm, >99.5% purity, Tianjin Keweier Metal Material Co., Ltd, China) under HTHP conditions. The micro-powders were manually mixed in ethyl alcohol, using an agate mortar and pestle for 4 h, at a proportion of 2, 4, 6, 8 and 10 wt% Al in AlecBN system, respectively. The starting mixtures were degassed under vacuum of 4.0 103 Pa and temperature of 600 C for 2 h prior to sintering process. The mixed micro-powders were set in synthesis capsule and sintered at 1400 C under a pressure of about 5.0 GPa for 2 min, using a belttype high pressure graphite apparatus. The pressure was estimated by the oil pressure reading calibrated with the method of silver melting point at high pressure [8], and the temperature calibration was performed using a Pt/Pt-13%Rh thermocouple. DSC measurement of the AlecBN mixtures was carried out using a Netzsch Model 449C instrument in the temperature range of room temperature to 1500 C at a heating rate of 10 C/min, in nitrogen atmosphere. AlecBN ceramic sinters were disposed, polished to a flat surface, and finally cleaned ultrasonically in ethanol, acetone and distilled water, respectively, followed by a drying process in vacuum chamber (detail process referred to Ref. [8]). Archimedes method was used to measure the volume density of these specimens. A Philips XL30E scanning electron microscope (SEM) was used to examine the cross-section microstructure fractured AlecBN ceramic sinters, and XRD (Cu Ka radiation, l ¼ 1.54056 Å) was performed to analyze the phase compositions using an X-ray diffractometer (Rigaku D/Max 2500V/PC). Same samples shaped as cylinders with diameters of 12.7 0.1 mm and heights of 1.9 0.1 mm were coated with graphite for use in thermal diffusivity examination. Thermal diffusivity measurement of these samples was taken by the LFA 427 laser flashing thermal conductivity analyzer at room temperature in argon atmosphere with a flow speed of 100 ml/min. A single sample, with 10 wt% Al addition, was taken to research its temperature dependence of thermal conductivity within the range from room temperature to 500 C. 3. Results and discussion DSC curve of AlecBN mixing micro-powders is shown in Fig. 1. Because the influence of air on the fast reaction of Al and cBN under high temperature and high pressure was very tiny, therefore, the DSC measurement of the AlecBN mixtures was performed in
DSC /(uV/mg) ↓ exo 0.40 Peak: 661.5 °C Peak: 1462.0 °C
0.30
0.20 Peak: 1353.6 °C [1]
0.10 Peak: 1283.3 °C
0
-0.10 Peak: 1110.4 °C
-0.20 200
400
600
800 1000 Temperature /°C
1200
1400
Fig. 1. DSC curve of AlecBN mixing micro-powders in nitrogen atmosphere.
nitrogen atmosphere to avoid the effect of air. In Fig. 1, the endothermic peak is observed at about 661 C, which was attributed to Al melting’s thermal effect. Furthermore, the obscure one in the range of 700 w 900 C was attributed to the reaction of Al and cBN, and the peak at about 750 C corresponded to the reaction of Al with N2 to AlN. That of Al with cBN was mostly tense at about 1000 C. Finally, the weak peaks at about 1283 C and 1353 C corresponded to the transformation of cBN to gBN, but around 1462 C the obvious peak should be assigned to gBN/hBN structural change. It was regarded that the experimental high pressure condition resulted in the rapid distribution of molten Al simple substance between cBN grains and accelerated the diffusion of other atoms (B, N) through molten Al layer in relative low temperature range, which may lead to the rapid densification of AlecBN ceramics sinters. XRD patterns of the AlecBN ceramic sinters are given in Fig. 2, which reveals the sintered samples are composed of a large portion of cBN and of a small portion of AlN, also very little amount of AlB12 and hBN. With increasing Al additions, the peaks of AlN were getting tenser and those of cBN weaker and peaks of hBN were observed to be gradually disappearing as well. Because, formation of high hardness boundary phase (AlN, AlB12) under high pressure would to great extent impede cBN’s transformation into hBN. In addition, no trace of Al simple substance in these sinters revealed high reactivity between Al and cBN at high temperature and high pressure condition. Fig. 3 shows the SEM images of fractured AlecBN ceramic sinters with various Al additions. As it can be seen, with increasing
Intensity(a.u.)
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cBN AlN hBN AlB12
4000
e d
2000
c b a
0 20
40
60 2θ (degree)
80
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Fig. 2. XRD patterns of AlecBN ceramic sinters with different Al addition: (a) 2 wt%, (b) 4 wt%, (c) 6 wt%, (d) 8 wt%, (e) 10 wt%.
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
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98 16
96 12 94
8
92
4
AlN content/%
Relative density/%
Relative density AlN content
0
90 0
2
4 6 Al addition/wt%
8
10
Fig. 4. Change of relative density (-) and AlN content (,) with Al addition for AlecBN ceramic sinters.
Fig. 3. SEM images of fractured AlecBN ceramic sinters with various Al additions: (a) 2 wt%, (b) 4 wt%, (c) 6 wt%, (d) 8 wt%.
Al additions, the microstructure of AlecBN ceramic sinters is getting to be more and more compact. Here, the densification effect can be illustrated, as seen from Fig. 3 (a) to (d), by the gradual change of fracture model from intergranular crack to transgranular crack, due to the remarkable increase in grain boundary phase. This resulted in a strengthening effect in bonding cBN grains tightly. Meanwhile, it was also confirmed by the continuous increase in relative density and the increasing AlN content in the AlecBN ceramic sinters which was calculated from their XRD spectra, as given in Fig. 4. From this viewpoint, the Al (10 wt%)ecBN sample with highest relative density and largest AlN content was investigated with high resolution microstructure observation and EDS elemental distribution maps of B, N, Al, and O on the fractured surface of the sample, as presented in Fig. 5. Al element can be found mainly in and around the surface of cBN grains with a rather uniform distribution. Besides B, N and Al, the sample also contained a small amount of O elements, most likely introduced by raw materials. Approximate content of B, N, Al, O element was 43.91, 39.58, 15.63 and 0.76 wt%, respectively. Finally, Fig. 6 illustrates the dependence of thermal conductivity on Al addition and measuring temperature for AlecBN ceramic sinters. As a comparison, sum rule of empirical constants [27], DSC method [28] and laser flashing measuring method was used to obtain thermal capacity for calculation of the thermal conductivity, respectively. Thermal conductivity equals to the product of thermal capacity, thermal diffusivity and density of the samples. In Fig. 6(a), the thermal conductivities of the AlecBN ceramic sinters show a general increase with increasing Al additions, and the DSC and laser flashing method gives almost close values, but a little higher than those calculated from empirical constants. It should be noted, compared to the pure cBN ceramic samples in this work, only the Al (2 wt%)ecBN sample had a little lower thermal conductivity (about 1.43e1.58 W/cm K), indicating the existence of apparent interfacial thermal resistance in the AlNecBN composites. The existence of interfaces usually impeded the heat conduction by scattering the incident phonons and contributed to an interfacial thermal resistance. The maximum thermal conductivity of the Al (10 wt%)ecBN ceramic sinter was about 1.71e1.94 W/cm K. This value is much higher than that of AlNehBN composites, w1.1 W/cm K [20]. The temperature dependence of thermal conductivity for Al (10 wt%)e cBN sample is presented in Fig. 6 (b). The sample’s thermal conductivity increases tensely with temperatures before 200 C, reaches a maximum of about 1.9e2.04 W/cm K at 200 C, and then declines gradually to about 1.8 W/cm K with increasing
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Fig. 5. SEM image of fractured Al (10 wt%)ecBN sample (a) and EDS elemental distribution maps of: (b) B, (c) N, (d) Al, and (e) O.
temperature. Moreover, among abovementioned three methods the laser flashing one gives highest value. As we know, the heat carriers in semiconductors and insulators are mainly phonons. Phonons may be scattered by three important mechanisms: phononephonon interaction, phonon-lattice disorder (e.g. point defects, impurities, dislocations, etc.) interaction and collision of a phonon with boundaries [29]. These phonon scatterings are the main thermal resistance in composites. In general, the mean free path of phonons is determined by these scattering centers and by the mean length of elastic waves excited at a given temperature inside crystals [30]. With regarded to AlNecBN composites’ thermal conductivity, it was closely related to the amount of grain boundary phase in the composites. With
growing Al additions, larger amount of grain boundary phase was produced. This redounded to the densification of the composites, and as a result reduced the scattering of phonons by grain boundaries, pores and defects. Note that the grain boundary phase was of relatively low amount and had very thin thickness [11,12]. In this case, the effect in promoting densification would overpass the thermal resistance effect in reducing the total thermal conductivity, which at last resulted in increase of the thermal conductivities. Those microstructures that most grains were densely packed and the grain boundaries were oriented randomly affected the thermal conductivity depending on the temperature. With regard to the temperature dependence mechanism of thermal conductivity, it was regarded that the total thermal conductivity (k) is the
P.F. Wang et al. / Solid State Sciences 13 (2011) 1041e1046
Thermal conductivity( W/cm K)
a ·
Thermal conductivity(W/cm K)
b
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character as cBN single crystals, which was reported by Slack [35] and Makedon et al. [36]. It is worth noting that the ability to keep high thermal conductivity over AlNehBN composites at the temperature of 200e500 C makes the AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors, whose use would increase the latter’s much higher working temperature to a large extent.
1.7
4. Conclusions
2.0
Calculated DSC Laser flashing
1.9
1.6 1.5 1.4 0
2
4 6 8 Content of Al (wt%)
10
2.1 Calculated DSC Laser flashing
2.0
Ceramic sinters of AlecBN were prepared rapidly by sintering AlecBN mixtures at 1450 C under pressure of 5.0 GPa. These AlecBN ceramic sinters, mainly made of cBN and AlN, had high relative density and compact microstructure due to the formation of boundary phase. The dense microstructures and presence of low thermal resistant boundary phase made AlecBN ceramic sinters have much higher thermal conductivity over that of AlNehBN composites. This would promise the AlecBN ceramic sinters favorite candidates as high efficiency heat sink materials for wide band gap semiconductors to be used at much higher temperature than the traditional ones. Acknowledgments We would like to give our special thanks to Dr. X.Y. Huang (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the help with thermal conductivity measurement.
1.9 1.8
References
1.7 1.6 0
100
200 300 Temperature/°C
400
500
·
Fig. 6. Change of thermal conductivity (a) with Al addition for AlecBN ceramic sinters and (b) temperature dependence of thermal conductivity for Al (10 wt%)ecBN sample obtained by different methods.
sum of the contributions arising from the lattice waves (phonons) kph and a contribution ke from the charge carriers [31,32]. Although kph experienced an exponential increase below the critical temperature (Tc), the presence of charge carrier and grain boundaries scattering set a limit on its growth therefore kph diminished as temperature increased further after Tc. The maximum position depended on relative magnitudes of phonon scattering with carrier, defects, grain boundaries and phonon itself. At low temperatures (much lower than the Debye temperature), defects become effective phonon scatters and kph exhibits a power temperature behavior. At even lower temperatures, grain-boundary scattering dominated and the usual Debye cubic temperature behavior appeared. It was evident that ke vanished at low temperature and exhibited power temperature behavior above Tc [32]. As a sum, k was dominated by ke above Tc. Therefore, in this work k maintained a peak value near 200 C (see Fig. 6 (b)). After that, as the temperature continued to rise, the phonon mean free path kept decreasing, and the heat capacity basically did not change with increasing temperature [33,34], even now ke still grew bit by bit, but the quick decrease of kph overtaken the smooth increase of ke, as a result k decreased slowly with increasing temperatures. It is clearly observed that the thermal conductivity of AlecBN ceramic sinters displayed a similar temperature dependence
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