Thermodynamics and kinetics analysis of in-situ synthesizing AlN

Vol. 45 No.3SCIENCE IN CHINA (Series E)June 2002Thermodynamics and kinetics analysis of in-situsynthesizing AINZHENG Xinhe (郑新和)', WANG Qun (王群), ZHOU Meiling (周美玲)2& Li Chunguo (李春国)31. State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences,Beijing 100083, China;2. School of Material Sciences and Engineering, Beiing Polytechnic University, Beijing 100023, China;3. Huludao Zinc Factory, Huludao 125003, ChinaCorrespondence should be addressed to Zheng Xinhe (email: xhzheng@red.semi.ac.cn)Received September 21, 2001Abstract AIN powders were prepared by in-situ synthesis technique. It is a reaction of binarymolten AI-Mg alloys with highly pure nitrogen. It was confirmed through thermodynamics calculationthat Mg element in Al-Mg alloys can decrease oxygen content in the reacting system. Thus, nitrida-tion reaction can be performed to form AIN. Moreover, an analysis of kinetics shows that the nitrida-tion reaction of Al-Mg alloys can be accelerated and transferred rapidly with the increment of Mgcontent.Keywords: AI-Mg alloy, in-situ synthesis, AIN, Mg, thermo-dynamical, dynamics.Aluminum nitride (AIN) is a covalent-bond compound with the wurtzite crystal structure,which is hard to melt!!. AIN ceramics, due to its high thermal conductivity, good insulating ability,low dielectric constant, high mechanical strength, and thermal expansion coefficient close to thatof Si, is suitable for multiplayer layout, and is considered as a promising material for heat-releasedpanel, electronic packing and large scale integrated circuitl2- 65. In addition, AlN is widely used inthe fields of optoelectronics such as MODFET'6, solar -blind photodetectorl7, and ultraviolet lightemission diodes (LEDs), and laser diodes(LDs)8-10] when it is combined with II-V compoundsGaN to form AlGaN/GaN heterostructure.As a ceramic, the practical application of AIN powder is restricted due to its high price andhigh sintering cost. Therefore, to obtain high quality AIN ceramics with low cost the key tec h-nique is to prepare AIN powder with low cost and good sintering properties, as well as low oxy-gen content.At present, AIN powders are mainly prepared by carbothermal reduction of Al0O1,12]1 undernitrogen atmosphere. The introduction of oxygen impurities is unavoidable due to uncompletedsolid solid reaction3]. On the one hand, Al2O3 will be中国煤化工f the added car-bon is insufficient; on the other hand, the subsequent de:MYHCNMHGnahighcontentof oxygen if it is surplus. Additionally, the reaction must be carried out at high temperature (1700一1800C) for a long time, which consumes much energy.250SCIENCE IN CHINA (Series E)Vol.45However, the in-situ synthesis technique of AIN powders can avoid the introduction of oxy-gen impurities, and the reaction temperature and energy consumption are low. In addition, the rawmaterials are easily available and the cost is low. It means that AIN lumps can be firstly synthe-sized by a reaction between molten Al alloys (Al-Mg or Al-Mg-Y) and high-purity nitrogen underproper condition. Then, the lumps are pulverized by a mortar and a steel-ball muller. Becausethere are some oxygen impurities in the reaction system and larger chemical affinity between oxy-gen and Al or Mg than that between nitrogen and Al or Mg, how to decrease oxygen content is akey step. In this paper, we interpret how the Mg element decreases oxygen impurities in the reac-tion system. In addition, the effect of Mg content on nitridation reaction is dynamically analyzed.1 Experimental procedureAl-Mg and Al-Mg-Y alloys were melted in a mid-frequency induction furnace. The reactionwas carried out in homemade ultra-vacuum equipment. A high-property temperature controller(SR53 series) was used to acquire the required reaction temperature at around 1200C. The elec-tronic balancer and computer were utilized to obtain the mass change in the nitridation reaction.2 Thermodynamics calculation of nitridation reaciton2.1 Minimum oxygen part- pressure required by the nitridation reaction of Al alloysThe chemical affinity between oxygen and Al or Mg is larger than that between nitrogen andAl or Mg, Therefore, less oxygen in the reaction room will react with Al or Mg to form compoundAl2O3 or MgO. That is to say, the composition of the resuting products will change. Thus, min-mum oxygen content or the critical oxygen part- pressure must be considered and controlled inorder to make the nitridiation reaction perform fully.When temperature ranges from 1 100K to 1500K, the reactions proceed as follows: .4Al+3O2- +2Al2O3,4Al+2N2-→4AIN,6Mg+3O2-→6MgO,(i)6Mg+2N2- + 2Mg;N2.The following equation can be deduced from eq. (i):2A2O3+2N2-→4AIN+3O2.(1)Eq. (1) describes the stabilization effect which occurs: AIN can be synthesized in very low oxygencontent. Quantitatively, we may calculate the free enthalpy change of reaction (1) by△G" = 4A,G%IN - 20，Gio, = 2057400- 184.40()"4,(2)where△G° is the standard reaction enthalpy, AG° is the中国煤化工d, ant T is tem-perature in Kelvin.YHCNMHGSimilarly, we can obtain6MgO+2N2→2Mg,N2 +3O2No.3THERMODYNAMICS & KINETICS ANALYSIS OP IN-STTU SYNTHESIZING AIN25and0G2 = 20,GMg;N2 - 6A,Gigo = 2679795 -234.9T(J)4.(4)It should be noted that the above reactions are performed under a low-flow ambient and othergas pressures such as those of Mg steams and oxygen are very low or negligible. Therefore, wecan simplify the calculation process by considering that the part-pressure of nitrogen is equal tothe total pressure in the reaction system. According to chemical isothermal equation, we have△G{=-RTlnKp =-RT In-(Po,1P°)-≈-RTIn(P, IP0)(5)(PR, 1P°)2(P,1P°)△G2=-RTInKj =-RTln.(R,1P")2≈- RTIn(Po,/ P°),(6)whereR is 8.314J* K-1 ' mol', T is temperature in Kelvin, P° is 1.01325X 105 Pa, and P，andP，are gas pressures of oxygen and nitrogen, respectively. Thus, the oxygen part-pressure of Alor Mg element can be obtained.Al elementIn Pl, =18.919- 82487.371/T .(7)Mg elementInPo, = 20.947- 107441.065/T.(8)Using thermodynamics data taken from ref. [14], and T=1300K, we can calculate the critical oxy-gen part-pressure of Al and Mg element as 4.57X 10-20 Pa, 1.60X 10-27 Pa, respectively. The cur-veof InP, vs 1/T is plotted in fig. 1.Fig.1 shows that 6) the critical oxygen-35part-pressure to form AIN is lower than that to-40form Mg;N, that is to say, AIN must be syn--45-50thesized if Mg3N2 can be obtained; (i) the之-55critical oxygen part-pressure increases with-65-2temperature, which helps to realize the nitri--70dation reaction; (ii) the critical oxygen part-7.07.8.0.5 9.0x 10*pressure to form AIN is extremely low, indi-1/7\K)cating much larger chemical affinity of Al andFig.1. InP, vs 1/T for Al(1) and Mg(2), respectively.0 than that of Al and N.中国煤化工Note that oxygen content is only up to 104 Pa in,MHCNMHGlueisfarabovethe minimum oxygen content required by nitridation reaction; therefore, AIN cannot be prepared,say nothing of Mg3N2. However, XRD analysis shown in fig. 2 gives us convincing evidence that252SCIENCE IN CHINA (Series E)Vol.45yellow-green Mg3N2 was observed to cover the reaction producsls, implying that the expectedoxygen content of nitridation reaction is achieved. The XRD spectrum of the reaction productsAIN is shown in fig. 3.2.2 Decrease of oxygen content by Mg elementWhen Al-Mg alloys were melted at elevated temperature, the volatilized Mg made molten口Al含|30 405060708090 1000607(20(°)Fig. 2. XRD patterns from yellow-green MgN.Fig. 3. XRD spectrum of the AIN products.alloys activate everywhere. Thus, on one hand, the nitridation reaction of Al is easily achieved; onthe other hand, the Mg stream gas reacted more easily with oxygen so as to decrease the oxygencontent. It is well known that the Mg vapor pressure increases with temperaturll,14), as is c-scribed by the equation,lg(PMg /kPa)=-6.181x10'/T + 6.990,(9)where Pirg is the vapor pressure of Mg element. It is worth noting that the above equation standsfor the relationship of the pure Mg vapor pressure vs temperature. The Mg vapor pressure in Al-Mg alloys should be expressed asRg = RMg gAsyg,(10)where OxMg is activation extent of Mg in the Al-Mg alloys.Before molten Al-Mg alloys reacted with nitrogen, Mg violeutly volatized to reacte withoxygen to form MgO, following the reaction:Mg+ 1/202-→MgO.(11)And the enthalpy change of reaction is obtained as△G°=- 739170 + 211.66T(J).(12)From the four equations above, we can deduce the following equation:889060.66 llgPrg =42.75(13)Similarly, if T is 1300K, PMe and P, are calculated to be 5563.97 and 2.99X 10-28 Pa, respec-tively.中国煤化工Comparing Po, with the minimum oxygen contenMYHCNMHGnofAlandMg,it can be found that the P， in the reaction system is lower than the minimum oxygen contentNo.3THERMODYNAMICS & KINETICS ANALYSIS OP IN-STTU SYNTHESIZING AIN253required by the nitridation reaction at the same temperature. Thus, compounds AIN and Mg3N2can be synthesized. In other words, the theoretical prediction is in agreement with the experimen-tal results. However, it is worth noting that Mg3N2 is not introduced in the resulting AIN.3 Dynamics analysis of nitridation reactionIn fact, AIN is synthesized by the combination of Al atom and N atom. The mass changesobviously before and after the reaction. Therefore, we can apply thermo-gravity (TG) to performthe dynamics analysis. Here, the thermo-gravity analyzer connected to the computer was used tocollect the reaction data. Additionally, we applied the translation ratio of reaction (0), namely theratio of actual change of reaction to theoretical one, to substitute actual change of mass when thethermo-gravity analysis was utilized to depict the process of the nitridation reaction.3.1 TG curveFig. 4 shows TG curves for the nitridation reaction of Al-Mg alloys and pure Al. It can beseen that the nitridation reaction of Al-Mg alloys is achieved more easily than that of pure Al, andthe translation ratio is higher. Therefore, we may safely conclude that Mg helps to improve theAlN synthesis of Al and N.3.2 Effect of Mg content on the nitridation reaction of Al-Mg alloysThe change of translation ratio vs time at various Mg content is depicted in fig. 5.1.0Fixed temperature s(1200C )Temperature curveAl lloys0.8-。0.6-百0.4心心●AI 6%Mg. Al 11%MgPure Al0.2●A116%Mg0.0054080120160200240280320t/minFig.4. TG curve of the translation ratiovs time.Fig.5. Influence of the Mg content on the nitridationreaction of AI-Mg aloys.From these results, we noted that the nitridation reaction is strongly affected when Mg con-tent ranges from 6% to 16% in mass fraction. When the Mg content was 6% and temperature washeated to 1200C， the nitridation reaction was accelerated and the reaction ended after about 200min. The translation ratio a is 0.94. When the Mg content further increases, the reaction is moreeasily realized and the temperature decreases. When the Mg content is up to 16%, the reactiontemperature decreases to 900C and the reaction ended中国煤化工translation ratioa is up to 0.97. These results give a convincing proof tTYHC N M H Gonounced efeton the nitridation reaction, but the interpretation of sucn eects 1S unclear. However, it is alsoworth noting that the effect of the Mg content on the nitridation reaction is slight when the Mg254SCIENCE IN CHINA (Series E)Vol.45content is over 20% (not shown here).4 Conclusions(1) The thermodynamics calculation indicates that the Mg element in Al-Mg alloys can小crease oxygen content in reaction system to realize the nitridation reaction of Al-Mg alloys. Theappearance of yellow-green Mg;N2 gives convincing evidence that the theoretical calculation is ingood agreement with the experimental results.(2) The dynamics analysis shows that the Mg content can accelerate the nitridation reactionand improve the transformation ratio of nitridation reaction of Al-Mg alloys.Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.59802001).References1. Meng, W.. Group II Nitrides (ed. Edgar, J. H.), London: inspec, 1994, 22- 29.2. Li, J. Kawasaki, A., Watanabe, R., Hot isostatically pressed SiC-AIN powder mixtures: Effect of milling on solid-solutionformation and related properties, J. Am. Ceram. Soc, 1998, 81(6): 1445- -1452.3. Egashira, M.. Shimizu, Y, Takatsuki, S.. Chemical surface treatments of aluminium nitride powder suppressing its reac-tivity with water, J. Mater. Sci. Lt, 1994, 10: 994- 996.4. Slack, G., Tralnzili, R., Pohl, R., The intrinsic thermal conductivity of AIN, J. Phys. Chem. 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