| The material of 110-oriented monocrystal tungsten which was achieved by the technique of chemical vapor deposition on the substrate of monocrystal molybdenum alloy is one of the most promising materials which have been currently utilized for the emitter of thermionic energy converter. The proportion of {110} plane to the emitter surface has a significant effect both on the power and the work time of the thermionic energy converter.This kind of material, which was provided by Lutch Academe of Russia, was studied in this paper. And the SEM, TEM, EDS and Electron diffraction techniques were applied to reveal the microstructure of it.The SEM samples were prepared by the methods of the chemical and electrochemical deep corrosive which were developed in this paper, and the argon ion corrosive method was also used to prepare the SEM samples. The TEM samples were prepared by the method of welding and ion-beam thinning. Low melting point Ag-Cu alloy were used as solder.The results show: not only the substrate and its layer are single crystals, but also the axial crystal orientation of the layer and its substrate are [111] orientation; the lattice constant of tungsten layer is almost same as that of molybdenum alloy, both are about 3.14 A; the atom of tungsten layer was located at the epitaxial lattice of its substrate; the thickness of the inter-diffused layer is about 10 m and the heavy inter-diffused layer is about 3 m; Additionally, it is found that carbon and oxygen are enriched in inter-diffused layer. And the diffusivity of molybdenum in its layer is deeper than that of tungsten in its substrate.The structure of substrate, layer and their inter-diffused layer are all composed of sub grain; and the sub grain's dimension of tungsten layer is larger than that of the inter-diffused layer; the cross section of monocrystal tungsten layer takes on sandwich structure, that is probably due to the differences of their density, component, orientation and state of stress, and there are a lot of micro cavity in the monocrystal tungsten layer after corroded; it's very easy to crack about on the original interface between substrate and its layer.The sub grain's dimension of tungsten layer that is about hundreds of nanometre observed by the method of TEM is smaller than that of observed by the method of SEM,the orientation of those sub grains is same as each others; and there are a few oxide or carbide whose dimension is also about hundreds of nanometer in the CVD tungsten layer.The cylindrical surface of CVD monocrystal tungsten coating that its axial direction is [111] orientation can be gained high proportion {110} planes after etched; the morphology of {110} planes on the cylindrical surface is very regularly, that it can be equally divided into six area by six {110} planes; the angle between each {110} plans is 120 ; In each area, the neighbor's {110} planes take on steps form, and the width of {110} steps in each area appear a recurrent change: the width of {110} steps is the widest in first that is the bright area , than become narrower gradually, after get across a transitional area, the width of {110} steps become wider and wider again.The quality of etching is not only related to the voltages and the electrical currents, but also related to the surface state of machining and the shape of cathode. The 1 etching voltage is a idea etching voltage.The results gained in this paper are useful to the fabrications of CVD thermionic energy converter's tungsten emitter and the processing of etching {110} plans.
|
PDF Full Text Request