APFIM AND FEM STUDY OF Mo-La ALLOY WIRE

Transcription

1 APFIM AND FEM STUDY OF Mo-La ALLOY WIRE Zhao-Xiang Zhang, Jin-Tang Jiao, De-Qing Hu To cite this version: Zhao-Xiang Zhang, Jin-Tang Jiao, De-Qing Hu. APFIM AND FEM STUDY OF Mo- La ALLOY WIRE. Journal de Physique Colloques, 1987, 48 (C6), pp.c6-283-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 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 JOURNAL DE PHYSIQUE Colloque C6, suppliment au noll, Tome 48, novembre 1987 APFIM AND FEM STUDY OF Mo-La ALLOY WIRE Zhao-xiang Zhang, Jin-tang Jiao and De-qing Hu Department of Radio-Electronics, Peking University, Beijing, China Abstract:Some experimental results are given of Mo-4w&La alloy wire by FEM and APFIM. The carburization of the tip may improve the function of field emission of the sample. In recent years, more and more attention has been paid to Mo-La alloy wire as a new type of cathode material. It can be used either as a kind of thermionic cathode material to substitute for W-Th ~irel'.~ or as a field emitter, because of its smaller work function, longer lifetimes, lower operating temperature and higher emission current density than W-Th wire. This novel material is made of substrate Mo, reducing agent MoaC (or C) and the source of the activated La, Lato,. When La201 is reduced by MoaC, active La is produced, meanwhile the La atoms diffuse to the sur- face through the grain boundaries of Mo so that emission regions with low work function are formed. We have studied Mo-4wt%La alloy wire by means of FEM and APFIM. The work functions at different temperatures are measured by the Fowler-Nordheim method. The temperature dependence of the work function is shown in Figure 1 for non carburized samples. The tip was heated for one minute for every measurement. It can be seen from the curve :hat a minimum field emission work function appears at about T= 1.4x10 K. The fact that the work function decreases with increasing temperature may be attributed to more La diffusing out through the grain boundaries, and the minimum work function can be found at a spe- cial temperature of Th.If the temperature is enhanced further, the de- crement of La on the surface results in the increase of the work func- tion because of fast desorption of La. When the temperature is high Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE enough, the La atoms on the surface desorb completely, and the work function becomes the same as the clean Mo. Keeping the temperature at To, we continued heating the tip for about one hour. As the result, the change of the work function about 2.0ev can be obtained. It shows that the work function is much less than the value of 4.3ev for pure Mo and3.3ev for pure La. ~ig.l~+-~ curve of a non carburized sample Keeping the emission cnrrent of lua, we also observed the field emission patterns at the different heating temperatures. The pattern of clean Mo can be observed after high temperature degassing. With the increase of heating temperature, the bright spots initially gathered on (111),{I221 and {116)planes, then thay disappeared gradually near the (1221 planes, while on the (1161 and I111) planes the bright spots still remained. The bright spots of (111 )planes are brighter than that 6f (116 1 planes. When the work functionmmes to the minimum, only bright spots of the {Ill) planes can be seen. This phenomenon indicates that the amount of La on the surface is most adequate. If the time of heating the tip is prolonged, it may be observed that with the decrea- sing of the work function the bright regions of {Ill) planes become bigger, and the brightness of their centre is dim, as shown in Fig.2. It seems that the La atoms of the {lll}planes form small platforms and

4 make the centre of the patterns a little darker. The above variation of the work function and the field emission patterns with temperature may be interpreted as the interaction of grain boundary diffusion, surface diffusion and thermal desorption, and their dynamic equilibrium resilt- ing in the lowest work function. At the equilibrium state the amount of La distributed on every plane remains constant. The field emission pattern performance of samples can be improved obviously if the tip is treated with carbon, and activated under higher temperature than the operating temperature. In that case carbon can move into the substrate and form MolC thin film to enhance the re- duction in the chemical reaction. Fig.3 shows the temperature dependen- ce of the work function for the carburized tip. We may conclude that carburization is an efficient approach to improve the performance of Fig.3~+-~ curve of carburized sample the sample, not only due to the wider temperature range for the lower work function, as shown in Fig.3, but also the shorter heating time (about half an hour) to reach the lowest work function. And a good emission stability can also expected for such alloy wire. The field ion image was observed as shown in Fig.4 under the imaging gas He. From Fig.4 we can see the dark ring structure of sub- strate Mo at the (011) plane, while the irregular bright spots are distributed near the area of (111) and I1161 planes. Sometimes under a certain direct voltage some new i,rregular bright spots could be seen unceasingly, then disappeared in a little while. It seems that these bright spots result from the diffusing La atoms on the surface, and that the dark ring structure around the (011) plane indicates that La atoms can not stay on the (011) plane. This analogous distribution of the FIM image can also be found in FEM patterns.

5 Fig.4 FIM image of Mo-La We also used atom-probe to analyze the Mo-4wt%La alloy wire. When field evaporation emerged, the spectrum was measured which mostly consisted of MO+~ and ~0'~ ions, and no ~a+' and ~ a ions + ~ was detected. However, somefimes the La" and Lai2 as well as its compound ions could be detected and the spectrum distribution was very wide when the tip was fractured in the course of the field evaporation. This phenomenon tells us that substrate Mo and La don't from interstitial solid solution alloy, and that La atoms seem to gather in the grain bound- aries and weaken the alloy wire. In addition, the tip is easy to be fractured by the stress-induced by the strong electrical field. The authors would like to thank Mr. Xiao Gui-li, Mr.Chen Zhi-he, and Mrs. Li Shi-li for numerous help. The work was supported by the Chinese Natural Scientific Foundation. Raference: [I] D.M.Goebe1, other authors, Rev.Sci. Instrum., 51(1980) 1468