PULSE--PLASMA SPRAY-DEPOSITION OF

LANTHANUM HEXABOI~IDE

L. R. S]~aginyan, V. N. Chernyaev, A. A. Kondrashin, and V. I. Bessaraba

UDC 621.793.42

The question of production of thin-fi lm lanthanum hexaboride emit ters has a l ready received the atten- tion of many invest igators . To overcome the difficulties encountered in the application of thin LaB 6 films and study of their p roper t ies , various special techniques have been developed [1-4] . In [ 5] a method is proposed for the application of fi lms by means of p u l s e - p l a s m a acce le ra to r s (PPAs) . The advantages of this method include fast ra tes of deposition, high degrees of ionization and coefficients of utilization of mater ia ls being evaporated, p rec i se control over the weight of condensing mater ia ls , and puri ty of the technological p rocess . Condensates produced in this way have a homogeneous composit ion and uniform thickness, and adhere s trongly to various types of substrate . The p rocess has not found industrial application because of the presence of a l iquid-drop phase in the p lasma s t r eam and uneven cathode erosion during the spray-deposi t ion of films. A new design of P P A with a device for separat ing the ionized l iquid-drop par t f rom the vapor par t of the p lasma s t r eam is descrfbed in [6].

In the presen t work arc d ischarge was excited by pulse- type injection of p lasma f rom a m i c r o a r c into the space between the e lec t rodes by means of a mechanical ignitor electrode through which a 10-# F capaci tor d ischarged i tself onto the cathode at a voltage of 200 V. After an a rc was struck, the ignitor electrode was moved away f rom the cathode. At the point of contact intense evaporation of the mater ia l of the cathode took place, initiating the main d ischarge arc . This method of ignition ensures uniform cathode erosion during p ro - longed operation. The cathode was a 60 - ram-d iamete r water -cooled disk made f rom a p res sed lanthanum hexaboride powder.

Lanthanum hexaboride was deposited on conducting and dielectr ic subst ra tes - tungsten, tantalum, Sitall ( P y r o c e r a m type mic rocrys ta l l ine g lass ) , si l icon single c rys ta l s with the (111) orientation, and NaC1 (001) single c rys ta l s . In addition to being purified by a s tandard procedure [4], immediately before sp ray-depos i - tion it was degassed by annealing for 1 h at t empera tures of 400-600~ in a vacuum corresponding to 1- 10 -6 tor r , and contaminants were removed f rom the subst ra te surfaces . The rate of deposition of lanthanum hexa- boride was 0.02-0.03 t~m/sec at d ischarge cur ren t s of 0.3-0.8 kA. The thickness of the resul tant condensates was 2-3 t~m. After spray-depos i t ion the s t ruc ture was stabil ized and the mater ia l of the l ayers homogenized by 2-h annealing at t empera tu res exceeding by 50~ the film deposition tempera tures .

An e lec t ron-di f f rac t ion photograph of a LaB 6 film applied to a NaC1 (001) single c rys ta l at 400~ is shown in Fig. 1. The vacuum condensate had a polycrystal l ine , f ine-grained s t ructure . Calculations of the in- te rp lanar spacings di and latt ice constant a indicated that the films had crys ta l l ized in the CaB G s t ructure with d i and a corresponding to lanthanum hexaboride. No significant effect of deposition t empera tu re (in the range 400-600~ on the mic ro s t ruc tu r e of the condensates was detected. However, calculations of electron

Fig. I. Electron-diffraction photograph of film produced by pulse-plasma spray- deposition of lanthanum hexaboride on sili- con (111) substrate.

Moscow Aviation Technological Institute. Transla ted f rom Poroshkovaya Metallurgiya, No. 9 (225), pp. 88-91, September, 1981. Original a r t ic le submitted, af ter revision, January 31, 1981.

0038-5735/81/2009-0659507.50 �9 1982 Plenum t=~blishing Corporat ion 659

/8

Fig. 2

IgUT z

6,5 V~/G~ 1/deg K

Fig. 3

Fig. 2. Reflection spec t ra of lanthanum hexaboride films applied to Sitall (1) and Si (111) (2) subs t ra tes at t empera - ture of 400~ (thicknesses 0.3 and 1.5/~m, respect ive ly) .

Fig. 3. Richardson curves for thin-fi lm lanthanum hexa- boride cathodes: 1, 2) specimens 2 and 4, respect ively.

TABLE 1. P rocess ing and Physica l Charac te r i s t i c s of Thin Lanthanum Hexaboride Fi lms Produced by P u l s e - P lasma Spray-Deposi t ion (dishcarge �9 1 kA, pulse duration 0.7 msec)

Specimen Substrate Thickness, Deposition ] No. of Work function No. material pm temp., ~C [ pulses (,0.053, eV

1 Ta 1,0 400 10 000 2,55 2 Ta 2,0 600 20 000 2,30 3 W 1,0 400 10 000 2,40 4 W 2,0 600 20 000 2,10 5 W 2,0 400 20 000 2,20

diffraction photographs of f i lms spray-depos i ted under cer ta in conditions with the separat ion of the p lasma s t r eam components pointed to the p resence of a LaB 4 phase in them. This was probably a resul t of differences in the degree of dissociat ion of the compound in the l iquid-drop and vapor phases, which manifested i tself dur - ing the synthesis of the boride in the condensate.

Reflection spec t ra of deposited specimens in the frequency range 14,000-50,000 cm -1 were studied with the aid of a reflect ing at tachment with a beam incidence angle of ~ 17 ~ (Fig. 2). In this frequency range the spec t rum of lanthanum hexaboride has an edge of p lasma light ref lect ion by f ree charge c a r r i e r s (at a f r e - quency of 16,300-16,500 cm-1). In the fi lms this edge was usually somewhat diffuse compared with that ex- hibited by crys ta l l ine LaB o which was due to the effect of c rys ta l s t ruc tu re imperfect ions on the kinetic proper t ies of the c a r r i e r s , and occasional ly displaced in one or the other direct ion depending on the direct ion of deviation of the composi t ion of the compound f rom stoichiometr ic . In the spec t rum of a film of small thick- ness (0 .3#m) a very diffuse ref lect ion minimum lay in the frequency range 22,000-24,000 cm -1 (Fig. 2, curve 1). A s imi la r t ransformat ion of the p lasma minimum has been observed by one of the authors of this paper in reflection spec t ra of s ingle-phase polycrys ta l l ine LaB 6 films produced by l a se r evaporation of lanthanum hexaboride. Such a displacement and dis tort ion of the ref lect ion minimum is probably attr ibutable to some features of pulse spray-depos i t ion p roces se s which br ing about deviations of the composit ion of the mater ia l f rom s to ichiometr ic and s t ra ins in the lattice. The dis turbances a re too small to a l te r the c rys ta l latt ice con- stant of the mater ia l of the film but sufficiently large to affect its optical cha rac te r i s t i c s . In the case of ap- preciable deviations f rom stoichiometry, as mentioned above, the appearance of a LaB 4 phase was detected in our work by e lectron diffraction. A thicker film (1 .5~m) exhibited the usual optical p roper t i es charac te r i s t i c of lanthanum hexaboride, in par t icu lar , a fair ly sharp reflect ion minimum at about 16,000 cm -t {Fig. 2, curve 2). This was evidence that the composit ion of the l ayer was close to s toichiometr ic , while the shape of the curve indicated that the c rys ta l latt ice was free f rom distort ions. In such a specimen a t ransi t ion in te r l ayer may be assumed to exist at the f i lm/subs t ra t e interface.

LaB 6 vacuum condensates applied to tungsten and tantalum foils exhibited ve ry interest ing thermionic

660

emiss ion cha rac te r i s t i c s . They were studied under diode conditions by the method descr ibed in [4]. The cathode temperal~re , which was var ied f rom 900 to 1500~ was measured with a thermocouple and checked with a pyromete r . The anodic voltage was var ied in the range 0-300 V. Typical R i e h a r d s o n - D e s c h m a n curves - plots of log ( I / T 2) vs l /T , where I is cu r ren t density and T is absolute cathode tempera ture - ob- tained for LaB~ films produced under various conditions of condensation are shown in Fig. 3. Some p roce s - sing pa rame te r s of specimens and their work functions �9 a re given in Table 1. ~ was found to depend on the nature of the subst ra te mater ia l : For cathodes on tungsten it was on average 0.20 eV lower than for cathodes on tantalum (with fi lms deposited under identical conditions). With r i se in deposition t empera tu re and in- c r ea se in lanthanum hexaboride thickness, �9 dec reased (for both W and Ta).

The data obtained are in agreement with the resul ts of determinat ions of �9 of lanthanum hexaboride coatings on tantalum repor ted in [1, 3] - 2.61 and 2.5 eV, respect ively (in our work �9 on Ta at 600~ and a l ayer thickness of I ttm was 2.44 eV). These values of work function for thin-f i lm cathodes a re slightly lower than those cha rac te r i s t i c of bulk bor ide cathodes.

I.

2.

3.

4.

5.

6.

LITERATURE CITED

C. Oshima, S. Horiuchi, and S. Kawai, "Thin film cathodes of lanthanum hexaboride (LAB6)," Proceed- ings of the Sixth International Vacuum Congress, Kyoto (1974), pp. 281-284. J. G. Wolfe, "Thermionic emission of boron and lanthanum coated boron filaments," J. Appl. Phys., 4__55, 3840-3843 (1974). J. G. Ociepa and S. Mr~)z, "The adsorption of lanthanum hexaboride on tantalum," J. Appl. Phys., 29, 241-244 (1979). V. I. Bessaraba, A. M. Vasil'ev, A. V. Kovalev, et al., "Thermionic emission properties of thin lan- thanum hexaboride films on tantalum," Poroshk. Metall., No. 6, 75-78 (1980). V. N. Chernyaev and A. A. Kondrashin, "Investigation of the deposition of films of various materials by the pulse plasma evaporation method," Elektron. Tekh., Mater., No. 5, 116-123 (1980). V. N. Chernyaev, A. Yu. Tyanginskii, and A. A. Kondrashin, "Pulse generator of metal plasma for the deposition of films," Elketron. Tekh., Tekhnol., Organizatsiya Proizv. Oborudovanie, No. 3, 8-11 (1980).

TITANIUM NITRIDE POWDERS SYNTHESIZED

UNDER CONDITIONS OF REPEATED DEFORMATION

ACTIONS

V. S. Polishchuk, I. I. Timofeeva, A. A. Rogozinskaya, and N. I. Sedrenok

UDC 621.762

Transi t ion metal ni tr ides find extensive application in various new branches of engineering. The p r o - duction of nitr ide par ts involves severa l operat ions, including the manufacture of powders, their eomminu- tion, and the shaping of par ts . Of these, the mos t important is the prepara t ion of t ransi t ion metal nitride pow- ders , since the quality of resul tant par t s depends to a large extent on the degree of completeness of the ni- triding p rocess and of uniformity of impregnat ion of par t ic les with nitrogen. However, the comminution of powders, which inc reases their activity by inducing in them s t ruc tura l changes, also plays an important pa r t [1].

It has been established that disperse materials can react with the atmosphere in which they undergo eomminution. This phenomenon forms the basis of a process in which nitrides are produced by subjecting metal powders to nitriding during their comminution. In this connection, it is of particular interest to study the structure forming during the comminution of nitrides, since the internal condition of a material is in fact characterized most fully by its structure.

Institute c,f Materials Science, Academy of Sciences of the Ukrainian SSR. Transla ted f rom Poroshkov- aya Metallurgiya, No. 9 (225), pp. 92-96, September, 1981. Original ar t ic le submitted, af ter revision, Novem- ber 30, 1980.

0038-5735/81/2009-0661507.50 �9 Plenum Publishing Corporat ion 66t