The peculiarities of applying cathodes made of reversible getter hydride-forming alloys in the gasdischarge devices with crossed ExH fields have been revealed. It has been shown that the availability of such electrodes allows to realize a correlation between the intensity of working gas releasing and the discharge parameters. The possibility for controlling erosion processes and heat regimes for the cathode materials has been shown to exist by initiating a controlled hydrogen desorption from the electrodes

Borisko, V.N.

Rudaya, I.A.

Sereda, I.N.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ 2000. №1.Серия: Плазменная электроника и новые методы ускорения (2), с. 82-85.82UDK 533.9PECULIARITIES OF APPLYING REVERSIBLE HYDROGEN GET-TERS AS MATERIALS FOR CATHODES IN GAS DISCHARGEDEVICESV. N. Borisko, I. A. Rudaya, I. N. SeredaKharkov National University. Physical technical department.61108, 31 Kurchatov av., Kharkov,UkraineThe peculiarities of applying cathodes made of reversible getter hydride-forming alloys in the gas-discharge devices with crossed ExH fields have been revealed. It has been shown that the availability ofsuch electrodes allows to realize a correlation between the intensity of working gas releasing and the dis-charge parameters. The possibility for controlling erosion processes and heat regimes for the cathode ma-terials has been shown to exist by initiating a controlled hydrogen desorption from the electrodes.In systems of gas feeding for various plasma andenergy physical devices using hydrogen isotopes as aworking gas, the gas is generally injected into a dis-charge chamber from a tank through  Pd or Ni accu-mulators which clean hydrogen from various gaseousimpurities [1]. Recently, in the above systems, solid-state gas generators based on binary hydrides of suchmetals as Ti, Zr, Sc, Er, U, etc. have been widely used[2]. First of all, the advantages of those generators are ahigh purity of the gas injected (99,99 – 99,999%),along with the safety and compactness in storage. How-ever, intermetallic compounds of the ST-707 and ST-172 type based on Zr-V, Zr-V-Fe, capable to absorbhydrogen reversibly, which were developed in Italy by“SAES Getters” Company [3, 4], allow to design uni-versal gas supplying systems that combine the functionsof not only a working-gas generator but a getter as well.At room temperatures, such intermetallic compoundsexhibit the equilibrium pressure not exceeding 1 – 10Pa, the dynamics of sorption-desorption and hydrogencapacity being improved. The decomposition of thehydride phases of the above materials allows to providefor filling hydrogen isotopes in the range of workingtemperatures 400 – 900 K, and the gas itself releases uniformly. The feature pecu-liar to those compounds is a decrease in the ionizationpotential of hydrogen generated by 0,3 – 0,5 eV causedby the recombination of hydrogen atoms at the metal-hydride surface following by the desorption of themolecules in the excited and thermodynamically non-equilibrium state [5, 6].In recent years, the possibility for applying suchmaterials as electrodes in gas discharge devices hasbeen investigated [2, 7]. It has been found that theavailability of the metal-hydride electrodes saturatedwith hydrogen decreases the combustion voltage in theself-maintained glow discharge which results fromincreasing in the hydrogen ionization cross-section by a0 200 400 600 800 103004005006007008009001000110021U (V)H (Oe)200 300 400 55010015020025030021I (mA)U (V)Fig.1a. The ignition curves of reflecting  dis-charge.1 – cathode from metal-hydride.2 – cathode from stainless steel.Fig.1b. Current–voltage characteristics of mag-netron discharge.1 – cathode from stainless steel.2 – cathode from metal-hydride.ab83factor of 1,3 to 1,5 as compared to common molecularhydrogen from a tank. It has been also shown by mass-spectrography methods that in this case, the propertiesof the discharge gas phase change: the shares of  H+and H+3 enhance relative to the content of the molecularions H+2. However, the plasma parameters and theprocesses in plasma have not been studied. Besides, theinvestigations were only performed in the glowdischarge, and some of the experimental resultsobtained were not simply interpreted and are of acontradictory character. Here, we summarize theexperimental data presented on the influence of themetal-hydride cathode saturated with hydrogen on thegas-discharge characteristics, as well as the influence ofthe discharge plasma on the gas-emission properties ofthat electrode. The investigations were carried out inthe discharges with crossed electrical and magneticfields: the Penning and planar magnetron discharges.The discharge cathodes were made of the getterhydride-contained alloy Zr55V40Fe5  modified with 3mass percent B2O3 . For reference measurements, thecathodes from molybdenum or stainless steel wereused. The two types of the discharge were investigatedin the hydrogen medium, both in self-consistent inpressure regime [8, 9], in which case the increase indischarge current due to the power introduced into thedischarge results in the increase in the hydrogenpressure inside the chamber, and the decrease in theabove current lowers the pressure; and in the flow ofhydrogen. Studies on the conditions for the existence ofthe discharge of both types in cases of applying thecathodes made of metal-hydrides or stainless steel,revealed that, unlike the findings of the previousauthors, while the external parameters being the same,the ignition of the discharge with metal-hydridecathodes was worse than that with stainless steel ones,and the discharge voltage exceeded a similar value forthe reference discharge (Fig. 1). The reasons are asfollows. The cross-section for the process ofdissociative adhesion of the low-energy electrons to theoscillatory excited molecules of the working gas isinversely proportional to the electron energy andincreases also with increasing the oscillation quantumnumber of molecules [10]. Near the cathode, electronshave a minimal energy which fact is connected withturning the trajectories of the oscillating electrons in thereflecting discharge, as well as with the emission of thelow-energy secondary electrons resulting from the ionicbombardment of the cathode. On the other hand, themetal-hydride cathode saturated with hydrogendesorbes the molecules which are already in theoscillatory excited state. Therefore, these processesnear the cathode lead to the significant increase in thecross-section for generating the negative ions and, as aconsequence, to the deterioration of the ignition andincrease in the discharge-supporting voltage. To verifythe above assumption, a possibility for creating asource of the negative hydrogen ions which could usethe reflective discharge with a metal-hydride cathodefor a plasma-forming stage has been investigated [11].The comparative analysis performed on the negativehydrogen ions yield in case of using the cathodes madeof a hydride-forming material and stainless steelshowed that the usage of the above intermetalliccompounds allowed to enhance significantly the currentin the H-beam. The mass-spectrometry of the neutralplasma component in the magnetron discharge burningin hydrogen atmosphere which is desorbed from themetal-hydride cathode showed that the insertion of sucha compound into the cathode of a gas-discharge devicechanged markedly the composition of not only the mainplasma gas component but also the impurity one, i. e. inthis case, various plasma-chemical reactions involvinghydrogen are stimulated [9]. The changes in the com-position of the gas-phase impurities are not connectedwith the emission of the hydrogen atoms from the0 20 40 60 80 10002468101214P P (Pa)I (mA)400450500550600650700T (K0 )T50 100 150 200 2506070809010021P (Pa)I (mA)Fig.2a. Magnetron discharge. Dependence ofhydrogen pressure in gas discharge devices.1 – cathode from metal-hydride.2 – cathode from stainless steel.Fig.2b. Reflecting discharge. Dependence ofhydrogen pressure (P) and metal-hydride cath-ode temperature (T) on the discharge currentvalue.a b84metal-hydride cathode surface, i. e. such a cathode alsofeatures the catalytic activity in reactions accompaniedwith the hydrogen transmission.Fig. 2 displays the dependence’s of the working-gas pressure on the discharge current for dischargesoperating in the autostable in pressure regimes. Forcomparison, in Fig. 2a, curve 2, the change in thehydrogen pressure in the discharge chamber with astainless steel cathode is plotted. From Fig. 2 it can beseen that in a magnetron discharge, the presence of ametal-hydride cathode leads to the significant change inpressure in the discharge chamber, and the dependenceitself is linear. For the reflecting discharge, when one ofthe cathodes was made of the metal-hydride saturatedwith hydrogen and the other was made of stainlesssteel, the above dependence is non-linear at lowdischarge currents (up to 50mA) and near - linear at Id >50mA. The reason is that the mechanism of theinteraction between cathodes and discharge gas phase ischanged. Within the low discharge current range, thedispersion of the cathode material made of stainlesssteel by high-energy particles and the absorption ofhydrogen by the resultant film are crucial. However, asthe discharge current and, consequently, thetemperature of the cathodes enhance, the role of thehydrogen desorption from the metal-hydride cathode isgrowing (Fig. 2b) The plasma parameters for thedischarges of both types were determined by the probetechnique. In the planar magnetron discharge, themeasurements were performed at the height of 0,5cmunder the anode section in the points positioned underthe area of plasma beam location.The electron temperatures in the discharges withthe cathodes of both types appeared to be the same: Te~ 2eV. However, the dependence’s of the plasma den-sity on the discharge current significantly differed (Fig.3).In case of discharge with the metal-hydride cath-ode, the plasma density was monotonously growing asthe discharge current increased, whereas for the refer-ence discharge, the tendency for saturation was ob-served.The presence of the metal-hydride cathode causesthe gas density within the near-cathode region to bedetermined by the dynamical equilibrium betweensorption and desorption processes which depends onthe temperature and hydrogen-sorbing characteristics ofthe hydride-forming material. Therefore, in this regime,increasing the hydrogen pressure, as the dischargecurrent is rised, is accompanied by increasing the gasdensity in the discharge chamber. The resultant gain inthe rate of collisions between electronsand neutral particles leads both to the intensive ioniza-tion processes and to the enhancement of the diffusiveelectron flow from the area of the plasma beam local-ization. For this reason, the increase in the dischargecurrent with the metal-hydride cathode is accompaniedby the rise in the plasma density beyond the electro-magnetic trap. The investigation on the spatial distribu-tion of the plasma parameters within interelectrodeclearances in the Penning discharge forcathodes made of stainless steel and hydrogen-saturatedmetal-hydride showed that both the space potentialsand the electron temperature did not depend on thecathode material. However, within the near-cathodeareas, the plasma densities are observed to differ by afactor of 2 - 3. In the area of the metal-hydride cathode,the plasma density is slightly lower which fact seems tobe connected with the processes of recharging positiveions at the molecules of hydrogen desorbed from thatcathode.The explorations on the dispersing of the metal-hydride cathodes in the regime of hydrogen desorptionwere carried out. The cathodes were irradiated withargon and hydrogen ions with energies (0,5 - 5) keVand current densities (10 - 100) mA/cm2 . The analysisrevealed the significant decrease in the erosion as com-pared to the dispersion of the same materials with nodesorption. As can be seen from Fig. 2, the hydrogendesorption from the metal-hydride cathodes has aunique functional dependence on the discharge current.In other words, the bombardment of the discharge ofthe metal-hydride cathode with high-energy plasmaparticles is an effective tool for controlling the gas-emission regime of its operating. As a result, close tothe cathode surface, the protective gas target is formedwhich reduces markedly the erosion of the material dueto the dissipation on it of the particle-beam energy.In the Penning discharge, the temperature of thecathodes was controlled by thermocouples, one of theelectrodes being made of the metal-hydride saturatedwith hydrogen and the other being made of stainlesssteel. The increase in the discharge current was ob-served to slow down the growth in the temperature ofthe metal-hydride cathode (Fig. 2b, curve T). Theanalysis of the thermal balance showed that the majorportion (up to 70%) of the heat energy released at the0 100 2000510 21n x 10 9   ( cm  -3  )I (mA)Fig.3. Dependence of plasma density on thedischarge current value.1 – control discharge.2 – discharge with metal-hydride cathode.85cathode was expended on the desorption of gaseoushydrogen. Such a softening of the thermal regime dur-ing the operation of the electrodes made using reversi-ble hydrogen sorbents with high rates and large thermaleffects of sorption-desorption was achieved due to boththe heat decomposition of the hydride phases and en-ergy dissipation of the particle beams on the gas targetwhich was formed with the hydrogen desorbed.Fig.4.   Dependence of metal-hydride temperature(1) and difference of temperatures between cath-odes (2 – metal-hydride – stainless steel, 3 – metal-hydride – metal-hydride) on the exposition time.The dependence of the difference between the op-erating temperatures for the cathodes on the expositiontime for the bombarding the hydrogen particles with theenergy 2,5 kV and current density 5 mA/cm2  plotted inFig. 4 shows that the temperature of the metal-hydridecathode is significantly lower than that of the stainlesssteel one, and in case of using two metal-hydride cath-odes, a minor temperature difference is observed onlyduring the first 15 minutes of the discharge operation,then it completely disappears.As a result of the studies performed, it has beenshown that applying the reversible sorbents of hydro-gen at a low pressure with high rates and large thermaleffects of sorption-desorption allows to reduce signifi-cantly the rates of their dispersion by the plasma ions aswell as to lower the heat load for those materials. Theabove effect is achieved due to creating the protectivegas target as a result of both thermal decomposition ofthe metal-hydride and ionic stimulated desorption. Theinfluence of the plasma beams upon those cathodesstimulates the hydrogen desorption from the electrodeimmediately into the operation zone of the plasma-forming stage which fact provides a return couplingbetween the intensity of the gas emission and parame-ters of the gas discharge.The application of the reversible hydrogen gettersas cathodes for the plasma sources of the negative ionspermits to enhance significantly the negative ion yieldowing to the desorption of the molecules in the oscil-latory excited state from those materials.References1 G. V. Grekova., A.V. Kobelev, G.G. Lokshin Vo-dorodnij natekatel dlya moshnih gazorazryadnih pri-borov // Pribori i tehnika experimenta. 1984, vol. 1,P. 219 – 220.2. A.V. Bastiev, M.A. Obolenskij, V.V. Solovej. Akti-vaciya vodoroda i vodorodosoderjaschih energono-sitelej. Kiev: “Naukova dumka”, 1993, 162 p.3. P. Della Porta, C. Pizani. Non-evaporable getteringpumps. // SAES Getters, Techical Reports, vol. 8-16,Milano (Italy), 1969.4. A. Barosi, T. A. Giorgi. A non-evaporable getter forlow temperatures. // Vacuum, 1973, vol. 23, № 1,P. 15 – 19.5. S.B. Valuiskaya, M.V. Lotockij, L.P. Scripal i dr.Issledovanie processa “aktivacii” vodoroda metal-logidridami. II Massspectrometricheskoe opredeleniepotenciala i secheniya ionizacii vodoroda // VANT.Ser. Yadernie tehnika i tehnologiya. 1989, vol. 1,p. 58–61.6. Yu.F. Shmal’ko, Ye.V. Klochko, N.V. Lototsky.Influence of isotopic effect on the shift of the ioniza-tion potential of hydrogen desorbed from metal hy-dride surface // Int. J. Hydrogen energy, 1996,vol. 21, p. 1057–1059.7. S.A. Galchinskaya, V.V. Dorohov, N.D. Lazarev idr. Issledovanie processa “aktivizirovaniya” vo-doroda metallogidridami. I. Mass-spectrometricheskijanaliz plasmi tleyushego razryada // VANT. 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Peculiarities of applying reversible hydrogen getters as materials for athodes in gas discharge devices

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Peculiarities of applying reversible hydrogen getters as materials for athodes in gas discharge devices

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