THERMAL BEHAVIOUR OF GRAIN BOUNDARIES IN ALUMINIUM NITRIDE CERAMICS

Transcription

1 THERMAL BEHAVIOUR OF GRAIN BOUNDARIES IN ALUMINIUM NITRIDE CERAMICS J. Sachet, J. Laval, F. Lepoutre, A. Boccara To cite this version: J. Sachet, J. Laval, F. Lepoutre, A. Boccara. THERMAL BEHAVIOUR OF GRAIN BOUNDARIES IN ALUMINIUM NITRIDE CERAMICS. Journal de Physique Colloques, 1990, 51 (C1), pp.c C < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1990 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 COLLOQUE DE PHYSIQUE COllOque Cl, suppl6ment au nol, Tome 51, janvier 1990 THERMAL BEHAVIOUR OF GRAIN BOUNDARIES IN ALUMINIUM NITRIDE CERAMICS J.P. SACHET*-**, J.Y. LAVAL**, F. LEPOUTRE*** and A.C. BOCCARA""" *centre des Materiaux P.M. Fourt, ENSMP, BP. 87, F Evry Cedex, rzance Laboratoire des Microstructures CNRS-ESPCI, 10 Rue Vauquelin, F :?$is Cedex 05, France Laboratoire dloptique Physique CNRS-ESPCI, 10 Rue Vauquelin, F Paris Cedex 05, France Resume - Nous avons mis au point une technique de photoreflection basbe sur les variations du coefficient de reflection optique induites par un faisceau laser. Cette technique a permis de mettre en evidence l'importance des phases intergranulaires sur la dissipation thermique dans les ceramiques B base de nitrure d'aluminium. On trouve une bonne correlation entre l'experience et le calcul adapt6 aux diffbrents types d'interfaces. Abstract - A photorefletance technique based on the detection of the variations of the optical reflection coefficient was developed. This technique pinpoints the influence of the intergranular phases on the heat dissipation in AlN-based ceramics. There exists a good correlation between experimental and theoretical data which were adjusted for different interfaces. 1 - INTRODUCTION Single crystals of Aluminium nitride are characterized by a high thermal conductivity of 320 W/m0K which is closed to the theoretical value/l/ and remarkable electrical properties. It is why an important effort has been carried out on the thermal conductivity of polycrystalline aluminium nitride which is one of the hopeful candidates for substrates in semi-conductor devices. Unfortunately. in such substrates the thermal properties are reduced by the presence of impurities such as oxygen and transition metal ions in the starting powder, and defects such as dislocations, porosity and finally grain b0undaries.a~ a matter of fact, these grain boundaries can contain a second phase. Since it is a compound having a strong covalent bond and a high melting point, A1N is difficult to sinter. Consequently polycrystalline A1N is prepared by adding a variable amount of Y,03 to form a dense sintered product with intergranular phases such as Y,Al,O,, and Y,A1,0,. The purpose of this investigation is to determine the effect of grain boundaries with or without second phases and defects on the heat dissipation. The detection of these thermal barriers necessitates a laser induced thermal wave detection technique. A correlation between experimental results and calculated data, obtained by a theoretical model to provide a quantitative measurement of thermal barriers is reported. 2 - EXPERIMENTAL METHODS Sample preparation High density AlN-Y203 samples were prepared by the following procedure : 2 and 10 wt% Y203 -added A1N powder, which contains 2.5 wt% 02, was mixed in butyl alcohol ; after drying, the mixed powder was uniaxially pressed at lt/cm2 ; A1N green bodies were pressureless sintered in a BN crucible between 1800 C and 1970'C in nitrogen atmosphere for one hour. Then specimens were polished with alumina powder for microstructural observations and photothermal experiments Photothermal experiment The principle of a photothermal experiment is to detect the very small periodic Article published online by EDP Sciences and available at

3 Cl-618 COLLOQUE DE PHYSIQUE variations of temperature induced by the optical absorption of a modulated pump beam incident on the sample. This small periodic temperature acts in the sample as a thermal wave propagating in the sample. Such a wave is able to detect defects i.e. thermal barriers which perturb the homogeneous heat diffusion. Furthermore this wave is strongly damped and the characteristic damping length P can be controlled by the modulation frequency, f, of the pump : - where a is the thermal diffusivity of the sample. In a good heat diffuser (a 10-4m2/s) p will be of the order of some pro for f equal to some MHz. If in addition the pump beam diameter is also of the order of 1 W, we will be able to thermally characterize the sample at a scale of 1 pro. CIR 1 dr R, R, dt The measured modulated reflectance is - = - - AT (2) where AT is the variation of the temperature of the sample surface from Toand l/ro dr, the temperature coefficient of reflectivity.the whole signal collected by the~~hotomultiplicator contains two optical informations which must be substracted by using a simultaneous acquisition through a computer : the variation of the optical reflection of the He-Ne laser between grains and grain boundaries and the Ar+-ion laser power which alters the local absorption of the material. Finally, since the absorption of the material is very low, it is necessary to make a complementary gold coating (5 nm thick). 3 - RESULTS AND DISCUSSION The principle of the reflectance technique as it is seen, is based on the periodic variations of the complex refractive index of the sample from the absorbed intensity which is modulated by the pump beam. However these variations can result from non-thermal phenomena. For instance, in materials with low energy gap such as semiconductors, there are contributions to the photoreflectance signals arising from the presence of free carriers which are generated by the pump beam at the sample surface. The aluminium nitride band gap is too high (Eg 6.3 ev) to allow for direct carriers photogeneration. However since the laboratory-made substrates ( A1N-2), contain a noticeable amount of oxygen due to a substitution process in the aluminium nitride lattice /3/. it is possible, by means of indirect transitions to create photogenerated carriers. To these considerations must be added the grain size, the second phase distribution, structural defects, porosity, etc...which complicate the interpretation. It is why analyses were carried out on other materials to obtain comparative informations : a second phase-free AIN (AlN-l), with a smaller grain size (W 4 pm) and a very small oxygen content (< 100 ppm) and a zirconia with a large grain size (W 50 pm) and without intergranular phase but with poor thermal properties contrary to the two others. Experimental results have been compared with a theoretical model based on a method proposed by MC Donald and a1 /h/. Assume an infinite medium with a grain boundary (fig.1) in the X-z plane (y = 0) characterized by a thermal resistance R,. If a unit point source is on the y axis the temperature may be written for y = 0, as : W with m: = p2 + j - w = 2nf and G the temperature far from the barrier OL, Thus the value of R,can be deduced from the measurements of T. This measurement of R, can be compared with a value calculated from a model described in fig. 2. Suppose a grain with a thermal resistivity lg/kg ( lg : mean grain size and kgthe thermal conductivity of a single crystal). we have :

4 c interface Fig. 1 : Schematic diagram for photothermal experiment across a grain boundary. Fig. 2 : Thermal resistivity modeli at a grain boundary. CC-.....>...,... " : : : /.. : : : : : ; amplitude '" phase.m Fig. 4 : Amplitude and phase thermal scans at the vicinity of a grain boundary in zirconia a) experimental data b) theoretical data Fig. 3 : Optical micrograph of zirconia showing a set of grain boundaries studied Fig. 5 : Bright field TEM in A~N-I by the photoreflectance technique. ( oxygen content < 100 ppm).

5 COLLOQUE DE PHYSIQUE with ktg : macroscopic heat conductivity ; kg was directly measured on our samples. found in ref /l/ while ktg was The calc,ulated thermal resistivities of the 3 samples are then deduced from relation (3) and summarized in table 1. TABLE Thermal behaviour of zirconia Fig. 3 shows the analyzed microstructure. The experimental data of the photoreflectance technique are reported in fig. 4a. These results indicate that there is a good correlation with theoretical calculation (fig. 4b) : the amplitude is not reproduced by the theory as well as the phase, but the result which must be emphasized, is the large value of RT(10-5m2'Cw-') obtained in very good agreement with the data of table 1. It is this large value which explains the very small heat conductivity kvg of the sample despite of the very large grain size Thermal behaviour of AlN-based materials Experimental and theoretical results show that it is necessary to take into account a microstructural parameter in order to explain the thermal behaviour differences between the two types of A1N. This parameter is here the intergranular phase. TEM characterizations do not reveal any second phase in A1N-1 (fig. 5) while this phase is very important when a high Y203 amount is added (fig. 6b) for the A1N-2. These intergranular phases correspond essentially to Y3A15 0,, and Y4 A1, O9 phases which are localized at the triple junctions when only 2wt% Y203 is added. For 10wt% Y20g these phases are surrounding the grains. The macroscopic thermal conductivity and electrical resistivity increase with the Y2O3 content as well /5/. The second parameter we have to consider is the oxygen content and its distribution since it may have a great effect on heat dissipation. Slack and a1 /6/ have studied its influence and shown that thermal conductivity decreases when oxygen increases in the A1N lattice. The oxygen distribution within grains is obtainedby cathodoluminescence /7.8/ for AlN-2. For 10wt% Y203 the radiative recombinations occur at the periphery of grains (fig. 7). The photothermal experiment shows off more significant variations at the periphery of grains with higher oxygen concentration than in their center, namely in the amplitude mode the signal is 4 times higher at the periphery. For the AlN-1 the oxygen content is too low within grains to produce radiative recombinations ; consequently variations in amplitude and phase mode are not visible. From data calculated in table 1 for AlN-1 R, = 10-9m2"Cw-1, it is seen that the experimental line scan is in good agreement with the calculations between grains which present high optical contrast (fig. 8). For grains showing little contrast, there are neither phase nor magnitude variations. These results imply that the phonon scattering is higher for highly misorientated grains (which present a high contrast). For AIN-2 a thermal resistivity R," 10-~ m' 'Cw-l has been found, which corresponds to a thermal barrier with a hardly noticeable second phase ; this value implies a similar thermal behaviour than for AlN-1 but with a higher phase change (10 degrees at 2 MHz instead of 2 degrees for AlN-1). One note that at the center of the grain boundaries, in the theoretical calculation, the amplitude is vanishing. In order to take into account the beam spot size, it would be necessary to do a convolution whereas the probe beam is considered as a point for the calculation.

6 Fig. 6a: Bright field TPI of 98 wt% *IN - Zwt% Yz O3. ~ig. 6b: Bright field TEM of 90 wt% AIN - 10wt;Y Y20J. Fig. 7a: Cathodoluminescence of grains in 98wt% A1N - 2wt% Y203. Fig. 7b: Cathodoluminescence of grains in gowt% A1N - lowt% Y2O MHZ a 1 '-6m a) experimental data amplitude Xt-l phase "l-t b) theoretical data Fig. 8 : Comparison of experimental and theoretical thermal data in AlN-1.

7 Cl-622 COLLOQUE DE PHYSIQUE On a grain boundary with an intergranular phase, the experimental results indicate a higher variation in amplitude and phase mode. The thermal characteristics of the second phase are very different from the matrix with ks = 10 w/m'k and as= m2/s ; these data imply a R, about 10-5m2*Cw-1similar to the one obtained in zirconia. The calculation indicates a phase change of about 40 degrees which agrees with the experimental results. Variations of 100 degrees can be obtained for lower thermal characteristics or larger second phases Discussion The investigation of these 3 materials, with very different thermal properties, proves that the photoreflectance signal is mainly due to a thermal effect. The larger are the thermal properties changes from part to part of the material, the greater are the variations of temperature phase changes. This behaviour can be attributed in the case of A1N-2 to the intergranular phase. This second phase produces a high phonon scattering, while the misorientation between grains involves a smaller effect which agrees with Slack's observations 161. However this effect is more important for the A1N which contains a second phase because of the amount of oxygen which is higher in this material and most probably because of the possible thin-second phase which is hard to reveal with the optical microscope. Consequently the amount of second phase can increase the thermal resistivity and then modify the experimental results. This behaviour may be connected with the oxygen distribution observed in cathodoluminescence which is more important at the grain's periphery for high Y203 contents. This effect is due to the trapping of oxygen by Y 0 which leads to the 2? formation of the intergranular phase and which induces higher variations in magnitude and phase of the temperatuw near grain boundaries. Thus it is necessary to add a sufficient Y203amount to promote the limitation of aluminium vacancies in A1N lattice during the sintering step which is carried out in N, atmosphere. The limitation will induce an augmentation of the thermal conductivity by increasing the mean free path of phonons and also improve the electrical resistivity. For zirconia, the thermal effects in amplitude can be concealed by porosity and also by the oxygen distribution inside grains, but the thermal change is visible in the phase mode. IV - CONCLUSION The modulated reflectance technique has allowed thermal barriers to be shown up in materials with different thermal properties. The amount of the intergranular phase and the oxygen content are the most important parameters we must consider.to obtain a good thermal conductivity. The intergranular phase increases the thermal resistivity and the misorientation between grains also decreases the heat sink but in lower proportions. Moreover this behaviour is enhanced by the oxygen content : consequently it is necessary to control accurately the chemical composition of the starting powders in order to limit the oxygen and impurities content. This condition will lead to optimizing the Y203 amount to promote a second phase and reduce local thermal barriers. Moreover, the reduction of A1 vacancies near the grain boundaries will obviously contribute to improve the electrical resistivity. REFERENCES /l/ G.A. Slack, "Nonmetallic crystals with high thermal conductivity", J. Phys. Chem. Solids, 1973, Vol. 34, pp /2/ P.K. Kuo and al. Canadian J. of Physics, 64. n"9. (1986). pp /3/ Yasuri Kurokawa and Al. J. Am. Ceram. Soc.. Vo1.71 n' 7, (1988). pp /4/ F. Alan MC Donald, Grover C. Wetsel, Jr. and Georges E. Jamieson. Can. J. Phys (1986). /5/ J.P. sachet, J. Y. Lava1, D. Broussaud, Silicates industriels, (1989) /7-8, /6/ G.A. Slack. M.P. Borom and J.W. Szymaszek, "Thermal Conductivity of Commercial Aluminium Nitride". Ceramic Bulletin. Vol. 51. n' 11, (1972). /7/ R.A. Youngman. D.A. Chernoff. Ceramic Bulletin. Vol. 66, n08, (1987). p /8/ J. Pacesova, L. Jastrabik Czech. J. Phys. B29, (1979). pp This research has been supported by the French Government (Ministere de la Recherche et de la Technologie, convention ).