In-situ formation of Aluminium Nitride composite powder Chapter In-situ formation of Aluminium Nitride composite powder 3.1 Introduction AlN has been known as a very important ceramic material in many applications due to its high thermal conductivity, high electrical resistivity, lower thermal expansion coefficient than alumina ceramics, low dielectric constant, good thermal shock resistance and good corrosion resistance [1-6]. AlN can be considered to be an ideal substrate or packaging material for semiconductors [7] and also an attractive material for making LEDs in the ultraviolet region, electron-emitting devices and hightemperature electronic devices due to its wide energy band gap (~6.2 eV) [8]. Commercial AlN powder is usually produced by carbothermal reduction of Al2O3 or Al(OH)3 and direct nitridation of aluminium [1,9-11]. In the carbothermal method, the starting material is reduced by carbon and reacted with N2 at high temperatures (1300°C). In the direct nitridation method, Al metal powder reacts with N2 or NH3 at high temperatures (1150°C). It is often prepared by methods such as DC arcdischarge [12], sublimation recrystallization [13,14], sodium flux [15], volatilizationcondensation of AlN powder [16], chemical vapor deposition [17] and combustion synthesis or self-propagating high-temperature synthesis [18, 19]. MM technique has been mainly used for preparation of materials which are difficult to be prepared by the ordinary melting methods, for example, oxide dispersion strengthened alloys and amorphous or supersaturated solid solution alloys with 46 In-situ formation of Aluminium Nitride composite powder remarkably extended solid solubilities. This method has been extended to use for low temperature synthesis of AlN by direct nitridation of Al metal in nitrogen and dry ammonia atmosphere under pressure of 600 kPa through mechanochemical reaction [20,21]. Since the formation of AlN from Al metal occurs by an exothermic reaction, it proceeds spontaneously at higher temperatures due to the heat generated by the nitridation [22]. (a) (b) Figure 3.1 (a) Morphology of Al powder and (b) structure of pyrazine. In this study, in-situ formed AlN composite powder is obtained by milling the mixture of elemental Al with pyrazine (C4H4N2) which shows a ring-type organic material as shown in Figs. 3.1(a) and (b). Due to the high concentration of nitrogen in parazine, relatively fast direct nitriding reaction compared to NH3 gas can be maintained during the milling process. During the ball milling process, the mechanical energies transferred to the powder mixture in the presence of highly reactive transition metal surfaces accelerates the decomposition of pyrazine into simple HxC-, HxN- or CNradicals and side chains [23]. The dissociated nitrogen (N) from pyrazine molecules is 47 In-situ formation of Aluminium Nitride composite powder expected to react with highly reactive Al metal surfaces especially when Al is in nanocrystalline structure after certain period of MM. 3.2 Experimental Al powder (Alfa Aesar, -325 mesh, 99.5% purity) and pyrazine (H4C4N2) (Alfa Aesar, 99+% purity) were used as the starting materials in this study. 14g Al powder mixed with pyrazine in the amount to satisfy the stoichiometric ratio of Al:N=1:1 was loaded into 250 ml stainless steel vial together with 20 carbon steel balls in a 99.9% pure argon atmosphere in an AMBRUAN glove box. A Retsch PM100 Planetary Ball Mill was employed for MM at 300 rpm. The weight ratio of Al powder to ball was 1:20. For blended powder mixture, Al and pyrazine were thoroughly mixed in an agate mortar with an agate pestle. After every 20h of milling, a small quantity of powder was withdrawn and annealed at 500 and 1250°C in a tubular furnace for one hour in purified argon flow. The blended powder mixture was also annealed for the comparison study. An X-ray diffraction (XRD) measurement using Cu Kα radiation operating at 40 kV and 30 mA was carried out for structural examination of MMed powders and MMed powders after annealing at 500°C and 1250°C (designated hereafter as xxh-MMed, xxh-MMed-500 and xxhMMed-1250 samples respectively, where xx is milling hours and the blended powder mixture is indicated as 0h). The microstructures of the samples were examined using a Quanta 200F field emission scanning electron microscope (FESEM) and Jeol 2010F TEM. Shimadzu DTG-60/60H was employed for the simultaneous measurements of thermogravimetry and differential thermal analysis. 48 In-situ formation of Aluminium Nitride composite powder 3.3 Results and discussion 3.3.1 Structural evolution Al2O3 Al Intensity (a.u.) 1250°C 500°C 30 40 50 60 70 80 2 (d egree) Figure 3.2 X-ray diffraction patterns of 0h-MMed Al-Pyrazine mixture after annealing at 500°C and 1250°C for 1h. Fig. 3.2 shows the X-ray diffraction patterns of 0h-MMed Al-pyrazine mixture annealed at 500 and 1250°C for one hour in purified argon gas flow. Only Al peaks are detected in the 0h-MMed-500 sample indicating no reaction between Al and pyrazine. For the 0h-MMed-1250 sample, significant oxide formation is detected from XRD patterns with high intensity alumina (Al2O3) peaks. Although annealing was carried out in high purity argon gas, oxidation could not be fully prevented. Since the melting temperature of pyrazine is 54°C which is far below the melting temperature of Al, it evaporated at much lower temperature than that of reaction between Al and N. Hence, AlN could not be formed. The XRD patterns of the MMed aluminium-pyrazine mixtures at different milling durations are shown in Fig. 3.3. All the diffraction peaks from the 20h- and 30h-MMed samples are from the pure Al implying no formation of compound between N/carbon 49 In-situ formation of Aluminium Nitride composite powder (C) atoms and Al. The DTA traces of MMed powder for different milling durations in Fig. 3.4 show endothermic peaks at 668°C due to Al melting from the powders milled for 20h and 30h. It is possible that the mechanical energy supplied to the milling process was not high enough to break the bonds between C and N in the pyrazine molecules and/or to overcome the diffusion barrier for N atoms to diffuse into the Al particles. AlN Al2O3 Al 100h 80h Intensity (a.u.) 60h 40h 30h 20h 30 40 50 60 70 80 2 (d egree) Figure 3.3 X-ray diffraction patterns of MMed Al-Pyrazine powder at different milling durations. As milling progressed, N atoms gradually diffused into highly defective Al particles. After 40h of milling, AlN started to form as clearly shown in Fig. 3.3. Although weak 50 In-situ formation of Aluminium Nitride composite powder Al peaks are detected, the amount of residual unreacted Al may not be large enough to produce any endothermic peak corresponding to the Al melting in DTA traces (Fig. 3.4). It can be seen that the AlN peaks are broad, which corresponds to the characteristics of nanocrystallites with the presence of interfacial components composed of Al, H and N atoms [20]. The average crystalline sizes estimated using Scherrer’s formula based on the theory of broadening of XRD diffraction peaks are 10, 9, and nm for 40, 60, 80 and 100h-MMed powders respectively. 20h 30h 40h Exothermic DTA (mW/mg) 60h 80h 100h -1 -2 -3 00 14 00 12 00 10 80 60 40 20 -4 T em perature (°C ) Figure 3.4 DTA traces of the Al-Pyrazine powders MMed for different milling durations. Due to low melting point and asymmetric structure of pyrazine, it is unstable under thermal treatment and/or mechanical activation [23]. Pyrazine therefore decomposed to some extent after 40h of milling and a certain amount of released N reacted with Al with the evidence of weak and broad AlN peaks. The proportion of AlN in the powder increased with further milling. Higher and distinct AlN peaks indicating higher 51 In-situ formation of Aluminium Nitride composite powder crystallinity of AlN particles could be observed in the 100h-MMed sample. Disappearance of endothermic peaks at 668°C in DTA traces corresponding to Al melting in the 60, 80 and 100h-MMed samples confirms the complete transformation of Al to AlN and Al2O3. Al2O3 oxide peaks were detected in the MMed powders except in 20h- and 30h-MMed powder samples. The sources of Al2O3 formation in the AlN powder could have originated from the oxide layers on the Al raw material [24], possible oxidation during milling and powder handling due to its highly pyrophoric nature. In the dry-milling process, under continuous impact and shear stress during MM, Al2O3 passivation layer on the as-received Al particles was fractured creating new surfaces to react with free N dissociated from pyrazine ring structure. Due to the strong affinity between Al and N, the N atoms were adsorbed on the newly created Al surfaces and then incorporated in the interfaces by the pressure welding of the Al powder followed by diffusing into the Al matrices through grain boundaries, dislocations and other defects [25]. Milling induced dislocation and vacancy density in the MMed Al enhanced the favorable situation for direct chemical nitridation following the decomposition of pyrazine molecules. During mechanochemical activation, increased diffusion of nitrogen was attributed to development of nanostructural or amorphous phase [26]. The exothermic behavior owing to the extensive heat of reaction enhances the reaction rate [27]. 52 In-situ formation of Aluminium Nitride composite powder AlN Al2O3 AlN Al4C3 Al Al2O3 FeAl2 100h 100h 80h 80h 30 40h 40 50 60  (deg ree ) (a) 70 Intensity (a.u.) Intensity (a.u.) 60h 60h 40h 30h 30h 20h 20h 80 30 40 50 60 70 80 2 (d eg ree) (b) Figure 3.5 X-ray diffraction patterns of MMed Al-Pyrazine powder at different milling durations annealed at (a) 500°C and (b) 1250°C for 1h. Figs. 3.5(a) and (b) show the XRD patterns of MMed-500 and MMed-1250 samples respectively. The XRD patterns of MMed-500 samples are very much similar to those of as-milled samples indicating the negligible effect of annealing at 500°C. No AlN structure could be detected in 20h and 30h-MMed-500 samples. After annealing at 1250°C, AlN was found to form in both 20h and 30h-MMed powders. In addition to AlN formation, oxide and carbide were detected from Al2O3 and Al4C3 diffraction peaks. Al4C3 is the only intermediate compound in Al-C binary phase diagram [28]. A weak peak which might correspond to FeAl2 was observed in all MMed-1250 samples. 53 In-situ formation of Aluminium Nitride composite powder Al and Fe contamination from milling media formed FeAl2 intermetallic during milling. Compared to the powder annealed at 500°C, oxidation was more pronounced and Fe contamination from milling media was clearly identified. Average crystalline sizes of AlN after annealing at 1250°C were estimated to be 14, 13, 6, 9, 10 and 10nm for 20, 30, 40, 60, 80 and 100h-MMed samples respectively indicating no significant grain growth during annealing. Table 3.1 Contents of C, H, N, Al and Fe under different conditions Sample C (wt.%) H (wt. %) N (wt. %) Al (wt. %) Fe (wt. %) 100h-MMed-1250 28.83 0.50 17.86 43.98 1.08 80h-MMed-1250 28.16 [...]... 101 (19 93) 131 9- 132 3 WA Kaczmarek, BW Ninham, I Onyszkiewicz, J Mater Sci 30 (1995) 55145521 V Rosenband, A Gany, J Mater Process Technol 147 (2004) 197–2 03 M Miki, T Yamasaki, Y Ogino, Mater Trans JIM 34 (19 93) 952-959 Y Ogino, S Murayama, T Yamasaki, J Less-Common Met 168 (1991) 221- 235 67 In- situ formation of Aluminium Nitride composite powder 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45... growth, causing the very small ~0.2µm diameter side branches to occur [ 13, 38] 63 In- situ formation of Aluminium Nitride composite powder (a) (b) (c) (d) (e) (f) Figure 3. 12 Various morphologies of AlN whiskers in (a,b,c) 30 h- and (d,e,f) 20hMMed-1250 samples 64 In- situ formation of Aluminium Nitride composite powder Whiskers which had a structure as chains of roundish beads of c.a 4µm in diameter were... created Al surfaces and then incorporated in the interfaces formed by the pressure welding of the Al powders Due to the inhomogeneous distribution of decomposed N molecules in the Al layered structure and the distribution of stress field resulted from milling, various morphologies of AlN whisker were detected in the 20h- and 30 hMMed-1250 samples as shown in Fig 3. 12 According to Wagner and Ellis [41],... samples 59 In- situ formation of Aluminium Nitride composite powder (a) (b) (c) (d) (e) (f) Figure 3. 9 Morphologies of (a) 20h-, (b) 30 h-, (c) 40h-, (d) 60h-, (e) 80h- and (f) 100hMMed-1250 samples 60 In- situ formation of Aluminium Nitride composite powder AlN particles and whiskers as shown in Fig 3. 9 Figs 3. 10 (a) and (b) show a TEM image with SAED pattern (insert) and HRTEM image of lattice pattern from... existence implying complete transformation of Al to AlN and Al2O3 after 60h-MM With prolonged milling duration, the grain size becomes smaller and a large volume fraction of the atoms resides in the grain boundaries resulting in higher reactivity and diffusivity [39 ] Therefore, most of the Al is transformed into AlN, with only a few whiskers being produced from the very small amount of residual Al in the samples... the whiskers in 20h- and 30 h-MMed-1250 62 In- situ formation of Aluminium Nitride composite powder samples, without droplets at the tips, whiskers in bead-shaped and chain-like structure with kinks and wavy shape are detected as shown in Figs 3. 9(c), (d), (e) and (f) AlN whiskers with such structures have been reported by Jung et al [40] During milling, the dissociated N atoms from pyrazine were absorbed... the AlN particles are poly-crystalline with an interplanar spacing of 2.74 Å corresponding to the (100) planes of the AlN crystal grown along the [100] direction d100=2.74Å (a) (b) Figure 3. 10 (a) TEM image of 100h-MMed-1250 sample with SAED pattern (insert) and (b) HRTEM image of lattice pattern from the same sample It is interesting to find that the formation of AlN whiskers was most abundant in the... tip of AlN whisker (Fig 3. 9a) indicates that the growth mechanism is VLS mechanism [37 , 38 ] High percentage of Fe at the droplet from EDS (energy dispersive spectrum) analysis (Fig 3. 11) confirmed the role of Fe as a catalyst Counts Al KeV Figure 3. 11 The energy dispersive spectrum of droplet at the tip of AlN whisker From DTA traces (Fig 3. 4) and XRD patterns (Fig 3. 5), there was no indication of Al... presence of a tiny liquid droplet can act as a preferred site for whisker growth from the vapor From Al-Fe binary phase diagram, at the processing temperature of 1250°C, the FeAl2 would have melted and the AlN vapor dissolved into the FeAl2 droplets at high saturation pressure [40] Process of precipitation of AlN begins leading to the growth of AlN whiskers when the dissolved AlN reaches a certain supersaturation.. .In- situ formation of Aluminium Nitride composite powder Al2O3 crystal which appears to grow in a direction angled at ~30 ° with normal to the (104) plane d104=2.559Å (a) (b) Figure 3. 6 TEM images of (a) alumina whisker and SAED pattern (insert) and (b) HRTEM image of lattice pattern from the same whisker (growth direction along black arrow) Fig 3. 7 shows the morphologies of 0h-MMed-1250 sample In . 2 .33 13. 49 43. 73 0. 73 30 h-MMed-500 17.92 1.56 7.80 41.08 0. 53 20h-MMed-500 15.97 1 .37 7 .34 50.55 0.65 From the results of chemical analysis as tabulated in Table 3. 1, a certain amount of. Figure 3. 1 (a) Morphology of Al powder and (b) structure of pyrazine. In this study, in- situ formed AlN composite powder is obtained by milling the mixture of elemental Al with pyrazine (C 4 H 4 N 2 ). certain amount of released N reacted with Al with the evidence of weak and broad AlN peaks. The proportion of AlN in the powder increased with further milling. Higher and distinct AlN peaks indicating