PHYSICAL REVIEW B VOLUME 46, NUMBER 6 1 AUGUST 1992-II Structure and low-temperature thermal conductivity of pyrolytic boron nitride L. Duclaux, * B. Nysten, and J-P. Issi Unite de Physico-Chimie et de Physique des Materiaux, Uni versite Catholique de Louvain, place Croix du Sud, 1, B-1348 Louvain-la-1Veuve, Belgium A. %. Moore Union Carbide Coatings Service Corporation, 12900 Snow Road, Parma, Ohio 44130 (Received 11 February 1992) The microstructure and morphology of three samples of pyrolytic boron nitride deposited at different temperatures have been characterized with use of x-ray diffraction and thermal-conductivity measure- ments. The x-ray analysis allowed the determination of the mean interlayer spacing, the out-of-plane coherence length, and the crystallites preferred orientation. It revealed the presence of three distinct morphologies. The thermal conductivity was measured as a function of temperature in the range 1.5 & T &300 K. The temperature variations of the thermal conductivity were fitted by using a model previously developed for the analysis of the thermal conductivity of graphites and carbons. This fit al- lowed the determination of the in-plane coherence length. It also allowed the analysis of point-defect concentration and of the interlayer shear modulus. It is shown that low-temperature thermal- conductivity measurements may be used to complement x-ray diffraction data for the microstructural characterization of materials. I. INTRODUCTION It has been previously shown how low-temperature thermal-conductivity measurements can be used as a tool for the microstructural characterization of layered ma- terials such as graphites, ' carbon fibers, ' or pyrolytic boron nitride (PBN). Particularly, this method allows the determination of the in- and out-of-plane coherence lengths. ' While x-ray determination of crystallite sizes is limited to low values (approximately ( 100 nm), thermal-conductivity measurements present the advan- tage that they have no resolution limit. These measure- ments are thus a very useful complementary tool to x-ray diffraction for the microstructural characterization of materials. Advantages of thermal-conductivity measure- ments are also that extra information about the concen- tration of point defects and about mechanical parameters of the material may be obtained. ' Pyrolytic boron nitride presents a layered structure similar to that of graphite except that the stacking of the layers is AAA. .. (Fig. 1). Along the c axis, a boron atom lies above a nitrogen atom alternatively. The in- plane distance between B and N atoms is 0. 154 nm, and the interplanar distance co is 0. 333 nm. Sichel et al. ' have also shown that boron nitride presents an anoma- lous difference between the temperature dependence of the lattice thermal conductivity ( ~ T ) and the specific heat which varies as T below 10 K and at T at higi temperatures. In fact, pyrolytic boron nitride presents a structure and a thermal conductivity behavior very simi- 1ar to that of graphite. Moore and Strong have shown that pyrolytic boron nitride prepared at or near normal conditions can exhibit three distinct types of. structure and morphology. The I I I I I I ~ Boron Q Nitrogen FIG. 1. Structure of crystalline hexagonal boron nitride. The layer stacking is AAA. . . with an interlayer spacing of 0. 333 nm. The unit cell is represented by means of dot-dashed lines. The in-plane lattice constant ao is equal to 0. 2504 nm, and the out-of-plane constant co is equal to 0. 6661 nm (Ref. 5). first is a turbostratic structure with an interlayer spacing d higher than 0. 340 nm. The second is more crystalline (d=0. 334 — 0. 335 nm). The third one presents an inter- layer spacing similar to that of the second type, but con- sists of twinned columnar microcrystallites. Most sam- ples contain more than one type of structure. We have investigated samples of each structure and morphology prepared as described later. These samples were first characterized by specific-gravity measurements, x-ray diffraction, and x-ray fluorescence. More specifically, x-ray diffraction allowed the determination of the interplanar distance d, the c-axis stacking length L„ 46 3362 1992 The American Physical Society