Microstructure Of Tungsten Single Crystal Obtained By Chemical Transport Method

Thermionic energy converter emitter material required to operate at a high temperature for a long time more than 1500°C environment, tungsten single crystal achieved by chemical transport method(CVTD) of W-WClx-Cl system meet this requirement. Although various fields of tungsten single crystal by CVTD have substantial development, microstructure by CVTD has been rarely reported.In this paper, tungsten single crystal layers are achieved at different substrate temperature by CVTD and growth mechanism is analyzed based on macroscopic morphology. Etch-pit method, conventional high resolution X-ray diffraction technique and synchrotron X-ray diffraction technique are used to study microscopic growth habits of monocrystal and dislocation in the single crystal is qualitatively and quantitatively analyzed. Abnormal morphology on the surface of the single crystal coatings is detected by scanning electron microscope(SEM) and electronic backscatter diffraction(EBSD). Meanwhile, reason of abnormal morphology is revealed. Details of the study are as follows:1. Tungsten single crystal coatings prepared by CVTD have obvious hexahedral or dodecahedron morphology containing alternating rough {110} and flat {112}. The differences of growth habits are caused by different motilities of tungsten atoms on the two crystal planes.2. Under conditions of a total pressure of 15.77 Pa and substrate temperature between 1300 and 1400°C, dislocation density of tungsten single crystal coatings prepared by CVTD are of 106 magnitude, which is based on etch-pit method. The dislocation density of {110} faces is lower than the {112} faces and dislocation density is decreased as temperature increased.3. Tungsten single crystal coatings prepared by CVTD may have the following abnormal morphologies:(1) Single hillock, hillock groups and banding hillocks with thorn on the top are observed on the surface of the monocrystal layers. Hillocks with different morphology are due to dislocations of substrate surface with different arrangements and energy.(2) Tungsten single crystal layers have bulges with different sizes and shapes, and locating on hillocks. They produced during cooling process. Cooling process was started too early, which lead to coating surface adsorb residual reactants in deposit chamber. Residual reactants on substrate surface become nucleation center of heterogeneous and abnormal growth happens.(3) Cluster structure containing a large number of grain boundaries is observed on the surface, which is caused by impurities dropped on substrate during surface treatment process or scratches produced during measurement.