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2214. Infrared detection and mixing in heavily doped Schottky-barrier diodes
Classified abstracts 2206-2214
range 297 to 430°C two phases are seen to form sequentially. The growth of the first phase, Ni2Si, is parabolic with time and obeys the rate constant expression RN'2Si ~ lO_Z exp (
1.3 e V )
k-----T-- cm~s-~"
The second phase, NiSi, grows linearly with time and its rate constant follows the relationship 1.4 eV'~ RNisi~103exp --~}cms-1. The simultaneous presence of both diffusion and reaction-rate-limiting mechanisms implied by the parabolic and linear growth laws respectively has not, so far, been reported for metal silieide formation. An investigation of the electrical characteristics of Schottky barrier diodes which were metallized and heat-treated simultaneously with the larger area samples used for backscattering analysis shows little change in the barrier height of about 0.7 eV despite the sequential formation of the two silicide phases. D J Coe and E H Rhoderiek, J Pkys D: ApplPkys, 9 (6), 1976, 965-972. 31. S P U T T E R I N G 31 2206. Highly oriented Z n O films by rf sputtering of hemispherical electrode system. (USA) Highly oriented ZnO films (c-axis orientation) were fabricated on glass and fused-quartz substrates by an rf-sputtering system with a hemispherical electrode configuration. The sputtered films show a high longitudinal mode electromechanical coupling factor, kt --0.230.24. Initial results suggest that the hemispherical electrode configuration may provide improved reproducibility over planar sputtering systems in the preparation of ZnO films for acoustic device applications. (Japan) K Ohji, JApplPhys, 47 (4), 1976, 1726-1728. 31 2207. Nb-Nb thin-film Josephson junctions. (USA) We report a simple technique for the construction of thin-film Josephson tunnel junctions in which both electrodes are of sputtered Nb. A thin (8-20 A.) layer of Cu evaporated over the first oxidized Nb film prevents the formation of supershorts between the Nb films. The junctions are extremely rugged, and do not require protection against condensation of water vapour. A number of junctions have been stored at room temperature for 6 months and cycled to liquidhelium temperatures 50 times without significant deterioration. Critical currents in the range 1 /xA < Ic < 10 m A may be achieved with an accuracy of :520 %. The dependence of junction characteristics on temperature, oxidation time, and Cu thickness is described. The junctions are particularly suited to the fabrication of practical devices such as SQUID's. G Hawkins and J Clarke, JApplPhys, 47 (4), 1976, 1616-1619. 31 2208. Amorphous GexH~_x bolometers. (USA) Selected materials from the amorphous Ge~Hx_~ system are evaluated at 300 K as thermistor bolometers operating at photon energies between approximately 0.2 and 2.5 eV. The devices are deposited on glass and sapphire substrates by rf sputtering of polycrystalline Ge in an argon-hydrogen atmosphere. Their current-voltage characteristics, their response times, the frequency and spectral dependence of their responsitivities, and the frequency dependence of the noise in a 3-Hz bandwidth are all measured. Response times between 1 and 10 msec and D,~* (0.633, 13, 1)'s of between 5 × 106 and 5 × 107 W -1 Hz 1~2 crn are obtained, using a calibrated H e - N e laser (,~ = 0.633 /~m) as an excitation source. (At this wavelength, all nonreflected incident light is absorbed.) T D Moustakas and G A N Connell, J Appl Phys, 47 (4), 1976, 1322-
1326. 33. N U C L E O N I C S 33 2209. Modification of a Van de Graaff ion source to accelerate metal elements. (USA) Large currents of metallic ions can be extracted from a radio-frequency ion source by fabricating a section of the exit channel from the desired metal. H Oona and W S Bickel, Rev Scient Instrum, 47 (4), 1976, 517-518. 496
33 2210. Mechanical response and thermal coupling of metallic targets to high-intensity 1.06-/~ laser radiation. (USA) Mechanical response and thermal coupling measurements are reported for aluminium and titanium targets exposed to high-intensity 1.06-tz laser radiation. Measurements are made in air and vacuum for pulse lengths from 1 to 100 tzsec, providing incident fluences of between 106 and l0 s W/cm 2. Total momentum delivered to the target and time-resolved pressure developed over the target surface were measured at irradiances spanning the threshold for laser-supported detonation (LSD) wave ignition. The slope of the impulse/energy ratio shows a marked discontinuity at LSD threshold intensity. Peak target surface pressure is found to increase as the 2/3 power of the beam intensity in agreement with the hydrodynamic model of LSD wave propagation. Thermal coupling coefficients e for A1 and Ti drop continuously from ~0.3 to 0.07 over the intensity range examined. This behaviour is consistent with the presence of an optically absorbing plasma at the target surface. The decrease in e is attributed to an increase in the plasma propagation velocity with intensity. L R Hettche et al, JApplPhys, 47 (4), 1976, 1415 1421. 33 2211. Plastic bubbles and tamper (pR) measurements for laser-driven fusion experiments. (USA) A new diagnostic technique is described which makes use of largediameter ( ~ 5 cm) thin-walled ( ~ 10 /zm) plastic bubbles to collect neutron-activated tamper material in a laser-driven fusion experiment. The plastic bubble and radioactive material are dissolved in a solvent and counted in a t~erenkov scheme to determine the average tamper ~pR) at peak compression conditions. The bubble energy flow and other plasma diagnostic characteristics are also briefly discussed. F J Mayer and W B Rensei, J Appl Phys, 47 (4), 1976, 1491-1495. 33 2212. Spread of energy losses of fast helium and nitrogen ions traversing metal films. (USSR) The spread of energy losses of helium and nitrogen ions with an initial energy of 330 keV/nucleon is measured at their passage through AI, Cu, Ag and Au films of various thickness. Experimental results are discussed. A A Bednyakov et al, Zh Eksper Teor FiG 68 (6), 1975, 2067 2074
(in Russian). 34. ELECTRONICS A N D H I G H A L T I T U D E PHYSICS A N D TECHNOLOGY 34 2213. Observations of collective ion acceleration with drifting intense relativistic electron beams. (USA) In this paper we report several observations related to the acceleration of positive ions when an intense relativistic electron beam is injected into a low-pressure neutral gas. In addition to varying the type and pressure of the neutral gas, we have varied a number of geometrical parameters including the radius of both the beam and the guide tube, the length of the guide tube (endplate effects), and dielectric constant of the guide tube. We find that ion acceleration occurs only if the peak electron-beam current Io exceeds the space-charge-limiting current, Izo, for the cavity. Maximum average ion energies are obtained when Io/Izo ~> 3 : the ion (proton) energy spectrum is rather broad with the spectrum peak occurring at ~ Ee, and the maximum energy cutoff at approximately 3 E,, where Ee is the electron-beam kinetic energy. R B Miller and D C Straw, J Appl Phys, 47 (5), 1976, 1897-1908. 34 2214. Infrared detection and mixing in heavily doped Schottky-barrier diodes. (USA) Transit-time and series-resistance effects in heavily doped Schottkybarrier diodes operating in the thermionic mode are evaluated by applying the laws of vacuum tube electronics to the space-charge region and by incorporating the treatment of the series-resistance effect into the discussion. It is assumed that the field distribution in the space-charge region is linear, that collisions in the space-charge region can be neglected, and that the signals are so small that the equations of motion can be linearized. The case of a uniform field distribution is also briefly dealt with; the results are not significantly different from the linear field case. At a frequency equal to three times the plasma frequency of the bulk semiconductor the detection and mixing efficiency is about 0.25 times the low-frequency value. The
Classified abstracts 2215-2225 effect of the high-frequency conductance of the space-charge region turns out to be quite small when the series-resistance effect is taken into account; the performance is then determined by a single transittime factor Ig(j~o) 12 and a series-resistance factor that is independent of transit time. The effect of tunnelling on the device performance is ,discussed briefly. Satisfactory detection and mixing performance seems feasible for the 10M0-/~m wavelength range. A van der Ziel, J Appl Phys, 47 (5), 1976, 2059-2068. 34 2215. Electron gun for generation of snbnanosecond electron packets at very high repetition rate. (USA) In order to create and study trapped particle modes in a plasma column, we have designed an electron gun which delivers packets with a duration shorter than 1 nsec at energies of about 100 eV, and with a repetition frequency of several hundred megahertz. This gun is made with parts of a classical disc-sealed microwave triode. This paper describes the gun itself and the pulse generator used to modulate it. Some results concerning trapped particle modes are shown in conclusion. (France) M Weinfeld and A Bouchoule, Rev Scient Instrum, 47 (4), 1976, 412417. 34 2216. Pulsed microsecond high-energy electron beam accelerator. (USA) Electron beams delivering up to 60 kA at 1.5 MV with pulse durations of 0.5-1.5/~sec were obtained by connecting a low-inductance Marx generator directly across a vacuum diode. Beams with up to 44 kJ and conversion efficiencies up to 51% from Marx energy into electron beam energy were obtained. T H Martin and R S Clark, Rev Scient Instrum, 47 (4), 1976, 460M63. 34 2217. On programmable control of electron-beam welding with space distributed weld. (USSR) Methods for the displacement of the electron beam focused to the surface of welded objects are discussed. Advantages of changing the focusing distance by changes in the focusing current are given. Expenditures necessary for control of focusing distance at welding of space distributed weld are analysed. E N Banya et al, Vestn Kiev Politekh Inst Set Automat Elektropriborostr, No 11, 1974, 137-139 (in Russian). 34 2218. Evaluation of resolution of a three-electrode spherical analyser for investigation of elastically reflected slow electrons. (USSR) Resolution of a three-electrode spherical analyser is calculated on the account of the defocusing action of entrance aperture, defocusing of electron beam at motion in the retarding field between the grid and the specimen and of the scattering action of grid cells. O M Artamonov et al, Uchen Zap LGU, No 370, 1974, 136-140 (in
Russian). 34 2219. Measurement of diffusion length of minority carriers with the aid of an electron beam. (USSR) A method is described for the measurement of small values of diffusion length is the order of tenth microns of minority charge carriers in semiconductors and for the investigation of local changes in diffusion length. The method is based on application of a fine electron beam in vacuum. V I Kononov et al, Coil of Works on Semicon Materials, Dev and Applic, Voronezh, 1973, 182-184 (in Russian). 35 2220. Grain size and film thickness of Nb3 Sn formed by solid-state diffusion in the range 650-800°C. (USA) A simple etching and replication technique for the determination of grain size in thin films of NbaSn has been established. The resulting data compares well with previous reports of grain size as a function of reaction temperature and of the pinning force FPmax as a function of grain size. The possibility of an optimum grain size for Fpmax is indicated. B J Shaw, JApplPhys, 47 (5), 1976, 2143-2145. 35 2221. Inhomogeneities in superconducting niobium surfaces. (USA) Inhomogeneities in superconducting Nb have considerable influence on ac losses in cables; they also give bad rf results. To study inhomogeneities near Nb surfaces, the magnetic induction profile B(x) and hence the critical current density.~(x) were determined as a function of distance x from the surface in samples which were mach-
ined, annealed, and chemically polished or electro-polished and anodized. With identical samples the magnetization and 18-kHz penetration depths were also measured as a function of dc magnetic field, applied parallel to the surface. These measurements on Nb samples, which were all exposed to air, indicate that two superconducting phases exist near the surface. One of them has an Hc2 much higher (50%) and the other somewhat higher (10%) than He2 in the bulk. In addition, strong pinning exists near the surface as evidenced by Jc higher than 106 A/cm z. These effects were found, although to a somewhat lesser extent, even after uhv annealing. The high He2 values and strong pinning near the surface, even above the bulk Hc 2, can be explained by the presence of inhomogeneities there. (Germany) A D Gupta ct al, J Appl Phys, 47 (5), 1976, 2146-2153. 35 2222. Narrow Bloch walls in the RCod-type rare-earth-cobalt compounds. (USA) For compounds of the type YCodxNis _ 5x the saturation moment and Curie temperature were determined. For large Co concentrations the rigid-band approach applies. The magnetic splitting of the 3d band collapses near x = 0.2. Magnetic isotherms were studied at 4.2 K as a function of concentration. The form of the virgin magnetization curve, the presence of large intrinsic coercive forces, and the observation of pronounced thermomagnetic history effects suggest the presence of narrow Bloch walls of only a few interatomic distances in these compounds. It is argued that the presence of narrow Bloch wails is the primary reason for the high coercive forces observed usually in solid pieces of materials of the type RCodxCu5_ 5x. (Netherlands) K H J Buschow and M Brouha, J Appl Phys, 47 (4), 1976, 1653-1656. 35 2223. The role of grain-boundary structure in shock-induced spallation of molybdenum. (USA) Grain boundaries and grain-boundary ledge structures are shown to have a controlling influence on intergranular spallation in molybdenum resulting from shock-wave rarefraction interactions. The structure of grain boundaries and their relationship to spall fracture are examined in detail utilizing the techniques of transmission and scanning electron microscopy, and are shown to be conducive to microcrack nucleation, coalescence, and propagation along the boundary planes. Grain boundaries possessing a high degree of complex structure such as dislocations and ledges are shown to vary significantly in effective dislocation density from the grain matrices, and this phenomenon coupled with the ability of ledges to nucleate cracks appears to account for intergranular spallation in molybdenum shock loaded to pressures of 150 and 250 kbar. L E Murr, J Appl Phys, 47 (4), 1976, 1364--1369. 35 2224. Sharpening of metal tips by heat treatment in vacuum. (GB) It is well known that a metal tip blunts if it is heated in vacuum. According to our hypothesis, a sharpening may occur in the case of an evaporation of the tip material. To test this, molybdenum tips were heated in vacuum. A typical result: at 2400 K, the tip radius decreases from 8 /zm to 0.3 /~m thus confirming the hypothesis. Measured and calculated final radii agree roughly. The pure material evaporation can be replaced by a surface reaction followed by the evaporation of the reaction products. This is shown on tungsten tips heated (1750 K) in the presence of oxygen (0.5 m torr). The evaporation of tungsten oxides results in a sharpening to a radius to 0.05 tzm. Under special conditions, in connection with the formation of solid drops, radii down to 0.01 t~m are obtained. (France) Vu Thien Binh et al, J Phys E: Scient Instrum, 9 (5), 1976, 377-381
(in French). 35 2225. Interaction of metal layers with polycrystalline Si. (USA) Solid-phase reactions of metal films deposited on 0.5-/zm-thick polycrystalline layers of Si grown by chemical vapour deposition at 640°C were investigated by MeV 4He backscattering spectrometry, glancing angle X-ray diffraction, and SEM observations. For the metals A1, Ag, and Au, which form simple eutectics, heat treatment at temperatures below the eutectic results in erosion of the poly-Si layer and growth of Si crystallites in the metal film. Crystallite formation is observed at T ~ 550°C for Ag, T >~ 400°C for AI, and T ~> 200°C for Au films. For the metals Pd, Ni, and Cr, heat treatment results in 497
range 297 to 430°C two phases are seen to form sequentially. The growth of the first phase, Ni2Si, is parabolic with time and obeys the rate constant expression RN'2Si ~ lO_Z exp (
1.3 e V )
k-----T-- cm~s-~"
The second phase, NiSi, grows linearly with time and its rate constant follows the relationship 1.4 eV'~ RNisi~103exp --~}cms-1. The simultaneous presence of both diffusion and reaction-rate-limiting mechanisms implied by the parabolic and linear growth laws respectively has not, so far, been reported for metal silieide formation. An investigation of the electrical characteristics of Schottky barrier diodes which were metallized and heat-treated simultaneously with the larger area samples used for backscattering analysis shows little change in the barrier height of about 0.7 eV despite the sequential formation of the two silicide phases. D J Coe and E H Rhoderiek, J Pkys D: ApplPkys, 9 (6), 1976, 965-972. 31. S P U T T E R I N G 31 2206. Highly oriented Z n O films by rf sputtering of hemispherical electrode system. (USA) Highly oriented ZnO films (c-axis orientation) were fabricated on glass and fused-quartz substrates by an rf-sputtering system with a hemispherical electrode configuration. The sputtered films show a high longitudinal mode electromechanical coupling factor, kt --0.230.24. Initial results suggest that the hemispherical electrode configuration may provide improved reproducibility over planar sputtering systems in the preparation of ZnO films for acoustic device applications. (Japan) K Ohji, JApplPhys, 47 (4), 1976, 1726-1728. 31 2207. Nb-Nb thin-film Josephson junctions. (USA) We report a simple technique for the construction of thin-film Josephson tunnel junctions in which both electrodes are of sputtered Nb. A thin (8-20 A.) layer of Cu evaporated over the first oxidized Nb film prevents the formation of supershorts between the Nb films. The junctions are extremely rugged, and do not require protection against condensation of water vapour. A number of junctions have been stored at room temperature for 6 months and cycled to liquidhelium temperatures 50 times without significant deterioration. Critical currents in the range 1 /xA < Ic < 10 m A may be achieved with an accuracy of :520 %. The dependence of junction characteristics on temperature, oxidation time, and Cu thickness is described. The junctions are particularly suited to the fabrication of practical devices such as SQUID's. G Hawkins and J Clarke, JApplPhys, 47 (4), 1976, 1616-1619. 31 2208. Amorphous GexH~_x bolometers. (USA) Selected materials from the amorphous Ge~Hx_~ system are evaluated at 300 K as thermistor bolometers operating at photon energies between approximately 0.2 and 2.5 eV. The devices are deposited on glass and sapphire substrates by rf sputtering of polycrystalline Ge in an argon-hydrogen atmosphere. Their current-voltage characteristics, their response times, the frequency and spectral dependence of their responsitivities, and the frequency dependence of the noise in a 3-Hz bandwidth are all measured. Response times between 1 and 10 msec and D,~* (0.633, 13, 1)'s of between 5 × 106 and 5 × 107 W -1 Hz 1~2 crn are obtained, using a calibrated H e - N e laser (,~ = 0.633 /~m) as an excitation source. (At this wavelength, all nonreflected incident light is absorbed.) T D Moustakas and G A N Connell, J Appl Phys, 47 (4), 1976, 1322-
1326. 33. N U C L E O N I C S 33 2209. Modification of a Van de Graaff ion source to accelerate metal elements. (USA) Large currents of metallic ions can be extracted from a radio-frequency ion source by fabricating a section of the exit channel from the desired metal. H Oona and W S Bickel, Rev Scient Instrum, 47 (4), 1976, 517-518. 496
33 2210. Mechanical response and thermal coupling of metallic targets to high-intensity 1.06-/~ laser radiation. (USA) Mechanical response and thermal coupling measurements are reported for aluminium and titanium targets exposed to high-intensity 1.06-tz laser radiation. Measurements are made in air and vacuum for pulse lengths from 1 to 100 tzsec, providing incident fluences of between 106 and l0 s W/cm 2. Total momentum delivered to the target and time-resolved pressure developed over the target surface were measured at irradiances spanning the threshold for laser-supported detonation (LSD) wave ignition. The slope of the impulse/energy ratio shows a marked discontinuity at LSD threshold intensity. Peak target surface pressure is found to increase as the 2/3 power of the beam intensity in agreement with the hydrodynamic model of LSD wave propagation. Thermal coupling coefficients e for A1 and Ti drop continuously from ~0.3 to 0.07 over the intensity range examined. This behaviour is consistent with the presence of an optically absorbing plasma at the target surface. The decrease in e is attributed to an increase in the plasma propagation velocity with intensity. L R Hettche et al, JApplPhys, 47 (4), 1976, 1415 1421. 33 2211. Plastic bubbles and tamper (pR) measurements for laser-driven fusion experiments. (USA) A new diagnostic technique is described which makes use of largediameter ( ~ 5 cm) thin-walled ( ~ 10 /zm) plastic bubbles to collect neutron-activated tamper material in a laser-driven fusion experiment. The plastic bubble and radioactive material are dissolved in a solvent and counted in a t~erenkov scheme to determine the average tamper ~pR) at peak compression conditions. The bubble energy flow and other plasma diagnostic characteristics are also briefly discussed. F J Mayer and W B Rensei, J Appl Phys, 47 (4), 1976, 1491-1495. 33 2212. Spread of energy losses of fast helium and nitrogen ions traversing metal films. (USSR) The spread of energy losses of helium and nitrogen ions with an initial energy of 330 keV/nucleon is measured at their passage through AI, Cu, Ag and Au films of various thickness. Experimental results are discussed. A A Bednyakov et al, Zh Eksper Teor FiG 68 (6), 1975, 2067 2074
(in Russian). 34. ELECTRONICS A N D H I G H A L T I T U D E PHYSICS A N D TECHNOLOGY 34 2213. Observations of collective ion acceleration with drifting intense relativistic electron beams. (USA) In this paper we report several observations related to the acceleration of positive ions when an intense relativistic electron beam is injected into a low-pressure neutral gas. In addition to varying the type and pressure of the neutral gas, we have varied a number of geometrical parameters including the radius of both the beam and the guide tube, the length of the guide tube (endplate effects), and dielectric constant of the guide tube. We find that ion acceleration occurs only if the peak electron-beam current Io exceeds the space-charge-limiting current, Izo, for the cavity. Maximum average ion energies are obtained when Io/Izo ~> 3 : the ion (proton) energy spectrum is rather broad with the spectrum peak occurring at ~ Ee, and the maximum energy cutoff at approximately 3 E,, where Ee is the electron-beam kinetic energy. R B Miller and D C Straw, J Appl Phys, 47 (5), 1976, 1897-1908. 34 2214. Infrared detection and mixing in heavily doped Schottky-barrier diodes. (USA) Transit-time and series-resistance effects in heavily doped Schottkybarrier diodes operating in the thermionic mode are evaluated by applying the laws of vacuum tube electronics to the space-charge region and by incorporating the treatment of the series-resistance effect into the discussion. It is assumed that the field distribution in the space-charge region is linear, that collisions in the space-charge region can be neglected, and that the signals are so small that the equations of motion can be linearized. The case of a uniform field distribution is also briefly dealt with; the results are not significantly different from the linear field case. At a frequency equal to three times the plasma frequency of the bulk semiconductor the detection and mixing efficiency is about 0.25 times the low-frequency value. The
Classified abstracts 2215-2225 effect of the high-frequency conductance of the space-charge region turns out to be quite small when the series-resistance effect is taken into account; the performance is then determined by a single transittime factor Ig(j~o) 12 and a series-resistance factor that is independent of transit time. The effect of tunnelling on the device performance is ,discussed briefly. Satisfactory detection and mixing performance seems feasible for the 10M0-/~m wavelength range. A van der Ziel, J Appl Phys, 47 (5), 1976, 2059-2068. 34 2215. Electron gun for generation of snbnanosecond electron packets at very high repetition rate. (USA) In order to create and study trapped particle modes in a plasma column, we have designed an electron gun which delivers packets with a duration shorter than 1 nsec at energies of about 100 eV, and with a repetition frequency of several hundred megahertz. This gun is made with parts of a classical disc-sealed microwave triode. This paper describes the gun itself and the pulse generator used to modulate it. Some results concerning trapped particle modes are shown in conclusion. (France) M Weinfeld and A Bouchoule, Rev Scient Instrum, 47 (4), 1976, 412417. 34 2216. Pulsed microsecond high-energy electron beam accelerator. (USA) Electron beams delivering up to 60 kA at 1.5 MV with pulse durations of 0.5-1.5/~sec were obtained by connecting a low-inductance Marx generator directly across a vacuum diode. Beams with up to 44 kJ and conversion efficiencies up to 51% from Marx energy into electron beam energy were obtained. T H Martin and R S Clark, Rev Scient Instrum, 47 (4), 1976, 460M63. 34 2217. On programmable control of electron-beam welding with space distributed weld. (USSR) Methods for the displacement of the electron beam focused to the surface of welded objects are discussed. Advantages of changing the focusing distance by changes in the focusing current are given. Expenditures necessary for control of focusing distance at welding of space distributed weld are analysed. E N Banya et al, Vestn Kiev Politekh Inst Set Automat Elektropriborostr, No 11, 1974, 137-139 (in Russian). 34 2218. Evaluation of resolution of a three-electrode spherical analyser for investigation of elastically reflected slow electrons. (USSR) Resolution of a three-electrode spherical analyser is calculated on the account of the defocusing action of entrance aperture, defocusing of electron beam at motion in the retarding field between the grid and the specimen and of the scattering action of grid cells. O M Artamonov et al, Uchen Zap LGU, No 370, 1974, 136-140 (in
Russian). 34 2219. Measurement of diffusion length of minority carriers with the aid of an electron beam. (USSR) A method is described for the measurement of small values of diffusion length is the order of tenth microns of minority charge carriers in semiconductors and for the investigation of local changes in diffusion length. The method is based on application of a fine electron beam in vacuum. V I Kononov et al, Coil of Works on Semicon Materials, Dev and Applic, Voronezh, 1973, 182-184 (in Russian). 35 2220. Grain size and film thickness of Nb3 Sn formed by solid-state diffusion in the range 650-800°C. (USA) A simple etching and replication technique for the determination of grain size in thin films of NbaSn has been established. The resulting data compares well with previous reports of grain size as a function of reaction temperature and of the pinning force FPmax as a function of grain size. The possibility of an optimum grain size for Fpmax is indicated. B J Shaw, JApplPhys, 47 (5), 1976, 2143-2145. 35 2221. Inhomogeneities in superconducting niobium surfaces. (USA) Inhomogeneities in superconducting Nb have considerable influence on ac losses in cables; they also give bad rf results. To study inhomogeneities near Nb surfaces, the magnetic induction profile B(x) and hence the critical current density.~(x) were determined as a function of distance x from the surface in samples which were mach-
ined, annealed, and chemically polished or electro-polished and anodized. With identical samples the magnetization and 18-kHz penetration depths were also measured as a function of dc magnetic field, applied parallel to the surface. These measurements on Nb samples, which were all exposed to air, indicate that two superconducting phases exist near the surface. One of them has an Hc2 much higher (50%) and the other somewhat higher (10%) than He2 in the bulk. In addition, strong pinning exists near the surface as evidenced by Jc higher than 106 A/cm z. These effects were found, although to a somewhat lesser extent, even after uhv annealing. The high He2 values and strong pinning near the surface, even above the bulk Hc 2, can be explained by the presence of inhomogeneities there. (Germany) A D Gupta ct al, J Appl Phys, 47 (5), 1976, 2146-2153. 35 2222. Narrow Bloch walls in the RCod-type rare-earth-cobalt compounds. (USA) For compounds of the type YCodxNis _ 5x the saturation moment and Curie temperature were determined. For large Co concentrations the rigid-band approach applies. The magnetic splitting of the 3d band collapses near x = 0.2. Magnetic isotherms were studied at 4.2 K as a function of concentration. The form of the virgin magnetization curve, the presence of large intrinsic coercive forces, and the observation of pronounced thermomagnetic history effects suggest the presence of narrow Bloch walls of only a few interatomic distances in these compounds. It is argued that the presence of narrow Bloch wails is the primary reason for the high coercive forces observed usually in solid pieces of materials of the type RCodxCu5_ 5x. (Netherlands) K H J Buschow and M Brouha, J Appl Phys, 47 (4), 1976, 1653-1656. 35 2223. The role of grain-boundary structure in shock-induced spallation of molybdenum. (USA) Grain boundaries and grain-boundary ledge structures are shown to have a controlling influence on intergranular spallation in molybdenum resulting from shock-wave rarefraction interactions. The structure of grain boundaries and their relationship to spall fracture are examined in detail utilizing the techniques of transmission and scanning electron microscopy, and are shown to be conducive to microcrack nucleation, coalescence, and propagation along the boundary planes. Grain boundaries possessing a high degree of complex structure such as dislocations and ledges are shown to vary significantly in effective dislocation density from the grain matrices, and this phenomenon coupled with the ability of ledges to nucleate cracks appears to account for intergranular spallation in molybdenum shock loaded to pressures of 150 and 250 kbar. L E Murr, J Appl Phys, 47 (4), 1976, 1364--1369. 35 2224. Sharpening of metal tips by heat treatment in vacuum. (GB) It is well known that a metal tip blunts if it is heated in vacuum. According to our hypothesis, a sharpening may occur in the case of an evaporation of the tip material. To test this, molybdenum tips were heated in vacuum. A typical result: at 2400 K, the tip radius decreases from 8 /zm to 0.3 /~m thus confirming the hypothesis. Measured and calculated final radii agree roughly. The pure material evaporation can be replaced by a surface reaction followed by the evaporation of the reaction products. This is shown on tungsten tips heated (1750 K) in the presence of oxygen (0.5 m torr). The evaporation of tungsten oxides results in a sharpening to a radius to 0.05 tzm. Under special conditions, in connection with the formation of solid drops, radii down to 0.01 t~m are obtained. (France) Vu Thien Binh et al, J Phys E: Scient Instrum, 9 (5), 1976, 377-381
(in French). 35 2225. Interaction of metal layers with polycrystalline Si. (USA) Solid-phase reactions of metal films deposited on 0.5-/zm-thick polycrystalline layers of Si grown by chemical vapour deposition at 640°C were investigated by MeV 4He backscattering spectrometry, glancing angle X-ray diffraction, and SEM observations. For the metals A1, Ag, and Au, which form simple eutectics, heat treatment at temperatures below the eutectic results in erosion of the poly-Si layer and growth of Si crystallites in the metal film. Crystallite formation is observed at T ~ 550°C for Ag, T >~ 400°C for AI, and T ~> 200°C for Au films. For the metals Pd, Ni, and Cr, heat treatment results in 497