Thermionic work function of low index planes of rhenium1 J. ROBICHAUD AND F. E. GIROUARD Dc;ptrr.le~tretzt tir plry.sic~rte-t)rtrtlrc;~ii~r~it~~~t~.s, U~riot,r.silt; tir, Motzt,io~c. Motic.~otz (N. -B.), Co~icrtir~ EiA SEY Received Junc 19. 1979' The therpionic e~issionconstan&>f a rheniu9-spherical single crystal have been determined for the 1101 I), { 1010). 121 121, (21101. and {2111) ojentations. Examination of the crystal surface revealed plateau-like structure in the [000 I]. (10 1 I]. [I0 101, and (1013] directions. Lej constantes d'emlssron thermoelectriq~re d'un n?_onocrzst_al sphcr~que de__rhenium_s_ont rapportees pour les orlentations cristallographiq~res(101 I I. { 1010). (2112). 121 10) et {21 I I } L'examen de la surface du-cristal a :ivele Line structure superl~c~elle de plateaux dans le\ directions [OOOI], [I01 I]. [1010] et [I0131 Cdn J Phya .58.43 (1980) Introduction After growth the crystal may be resistively heated The propel-ties of materials are very after separating the stranded wires into two groups. n~uch dependent on the electron work function. The material used was 0.25 mm wire 4N stock from ~h~ observed values of this parameter are in turn Alfa Inorganics. The crystal used for the electron influenced by the surface distribution of crystal emission measurements had a diameter of 1.18 mm facets and even the experimental method. It is and Was shown to be of monocrystallinity by ob- therefore important that the surface preparation serving the symmetry of the emission pattern and and the surface structure be known in order to compare experimental results. In this respect re- This sample was in a 10cm diameter sults have been presented for most metals (1) but diode similar to the one described previously (I I). little work has been done with metals having the The only modification to the diode was the removal hexagonal structure. This experi- of the ribbon normally used to electron bombard merit deals with the emission constants from hcp the crystal. After vacuum processing and seal-off a rhenium. Using surface ionization, its average 15 L/s ion pump maintained a pressure in the work function is quoted with values from 5.17 to lo-'' Torr range in the system. The crystal could 5.43 eV (2-4) whereas the thermionic emission re- be heated resistively to 2570 K. After growth the SU~~S range from 4.86 to 4.96 eV (5-7). A difference crystal was annealed at approximately 2000 K for between the two methods is expected since the first Over 200 h. Thereafter- the sample was flashed to is weighed more toward the high work function 2400 K for 1 min between each experimental run. patch values whereas the thermionic emission re- These lasted less than & h. That such a heat treat- suits are weighed toward the low work function ment should leave a clean sulface has been deter- values. Of the published results only a few are from mined by a number of authors (12- 14). Data based single crystals and most used the field emission on Auger spectroscopy (13) indicate that heating method (8, 9). In the present work, electron emis- the crystal above 1800 K is sufficient to ~ r o d u c e a sion constants are reported for five low index di- clean (0001) su~face. Gorodetskii et nl. (12) showed rections from a spherical rhenium crystal along that heating an oxygen covered (0001) sulface with photomicrographs of the principal surface above 1970 K restores the work function to its clean features observed upon recrystallization. surface value but heating to 2270 K was found necessary to obtain the clean LEED pattern. Experimental Method The current density from the different crystallo- The rhenium single crystals were grown by the graphic directions was measured using a photo- "hanging-drop method" developed by Webster metric method. The angular resolution of the pho- (10) and modified by LeBlanc et 01. (1). It consists tometer was 5". This compares to the 2 to 3" sub- of melting the tip of a multi-strand wire by electron tended by the smaller plateaus shown in Figs. 2 and bombardment in a vacuum of lop6Torr or better. 3. Experimental Results and Analysis 'Research partially supported by the Natural Sciences and Engineering Research Council of Canada. The emission data obtained from the projection ~~evisioi received August 22. 1979. diode for a particular crystallographic' diiection 0008-4204/80/010043-O5$01 .OO/O @I980 National Research Council of Canada/Conseil national de recherches du Canada Can. J. Phys. Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/04/13 For personal use only.