Technique for testing aspherical surfaces

Conclusions The above technique was used for measuring and appropriately processing the results obtained for spherical (concave and convex), parabolic, elliptical, and hyperbolic surfaces (in the case of a spherical surface the measured distance d is the radius r).The obtained results were found to be so precise that they can be recommended for application in the technique for precise practical measurements of surface shapes. This method is based on comparison with surfaces of predetermined shape whose meridional cross section can be represented by an equation for a circle, parabola, and hyperbola.After the surface measurements the meridional cross-section curves were identified according to the method under consideration. Discrepancies in the determined parameters were discovered by comparing the parameters of surfaces with a given shape to the identified surfaces. Thus, the discrepancies for the different surfaces are: spherical — 1·10^–3, elliptical and hyperbolic — 2.10^–2, parabolic — 6·10^–3.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 13–15, January, 1981.     

Trapping Fermionic 40K and Bosonic 87Rb on a Chip

We demonstrate the loading of a Bose–Fermi mixture into a microfabricated magnetic trap. In a single-chamber vacuum system, laser-cooled atoms are transported to the surface of a substrate on which gold wires have been microfabricated. The magnetic field minimum formed near these current-carrying wires is used to confine up to 6 × 10⁴ neutral ⁴⁰K atoms. In addition, we can simultaneously load 2 × 10^{5 87}Rb atoms, demonstrating the confinement of two distinct elements with such a trap. In a sequence optimized for ⁸⁷Rb alone, we observe up to 1 × 10⁷ trapped atoms. We describe in detail the experimental apparatus, and discuss prospects for evaporative cooling towards quantum degeneracy in both species.     

Molecular dynamics modeling of microstructure evolution during growth of amorphous carbon films

Summary Classical molecular dynamics simulations, using Brenner's bond-order interatomic potential model, is used to study the bonding microstructure formation during quench from liquid and during growth on a diamond surface. For a 64-atom quench simulation we find 56 sp³- and 8 sp²-bonded carbon atoms, in qualitative agreement with tight-binding simulations. The growth of amorphous carbon films was simulated by depositing carbon and hydrogen atoms onto a diamond surface at energies up to 100 eV The simulated films are amorphous with a maximal density near the deposition energies (20–40 eV) used to grow films on magnetic disks. Lower deposition energies yield open graphitic structures, while much higher deposition energies cause the surface to ablate, leading to a poorly defined interface. The hardness calculated from the densest simulated films is about twice that found experimentally.     

Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

Measurement of rapidly changing temperatures of conducting solid bodies by means of thermocouples

Conclusions A thermocouple with welded electrodes possesses a considerable thermal inertia. Thermocouples without junctions are virtually inertialess and are capable of measuring rapidly changing temperatures on the surfaces of solid conducting bodies.The contact resistance between the thermal electrodes and the body with which they make contact has the maximum effect on the precision of measurements in the case when current-measuring instruments are used. This error can be reduced by increasing the contact pressure up to 2–2.5 kg-wt/mm² (196.13·10⁵–245.17·10⁵ N/m²), cleaning the contact surfaces and preheating the contact.     

Oxidation kinetics of silicon carbide whiskers studied by X-ray photoelectron spectroscopy

Silicon carbide whisker surfaces were characterized by X-ray photoelectron spectroscopy (XPS) to determine changes in the surface oxide film which occurred as a result of heating in air at temperatures from 600 to 800 °C. Equations were derived for the calculation of surface oxide film thickness from the SiC to SiO₂ 2p intensity ratios. Oxidation was found to follow a linear rate law in this temperature range for the first 10 nm of oxide growth. An activation energy of 17.2 ± 2.8 kcal mol^–1 (72 ± 12 kJ mol^–1) was measured.     

Behaviour of bismuth during simulated processing of model aluminium capacitor foils

The production of high specification, aluminium-based, electrolytic capacitors requires optimization of material composition and heat and surface treatments in order to maximize the area available for formation of the dielectric, anodic film. Commercial foils contain low additions of copper and lead in order to achieve this goal. The present study examines the effects of heat and surface treatments on aluminium foil containing either 50 or 1000 ppm bismuth, as a replacement for lead, by a combination of Rutherford backscattering spectroscopy and scanning electron microscopy. Heat treatment at 823 K results in segregation of bismuth to the surface regions of the foils, with enrichments in the range 4–8 × 10¹⁴ Bi atoms cm^–2, localized mainly just beneath the thermal oxide, for the selected treatment conditions. The enrichment reduces following alkaline etching, to the range 1–4 × 10¹⁴ Bi atoms cm^–2. This level of enrichment is maintained during subsequent anodizing, with the enrichment partitioned between the metal and the outermost layers of the anodic films. The enrichment of the metal is about 4 × 10¹³ Bi atoms cm^–2. Electropolishing in perchloric acid solution eliminates the enrichment developed during the heat treatment, probably due to activation during the polishing process. The enrichment remains very low or negligible during subsequent anodizing. The general behaviours of bismuth and lead are similar in aluminium foils subject to the selected heat and surface treatments. However, additional studies are needed of tunnel etching to determine the feasibility of substitution of lead by bismuth in commercial foils.     

Bonding to aged surfaces: A thermally-aged epoxy

It is common experience that aged surfaces are often difficult to bond to. We report an examination of bonding to thermally-aged epoxy surfaces, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 ° C, with the ageing time varying from 2 to 8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 ° C were 0.077, 0.059, and 0.050 kJ M^–2, respectively. These compare to 0.13 kJ M^–2 for a bond to an unaged surface and 0.21 kJ m^–2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with good adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was felt for some distance.     

Friction and wear properties of monolithic silicon-based ceramics

Friction and wear properties of two forms of silicon nitride (reaction bonded and hot pressed) and two of silicon carbide (reaction sintered and sintered) are reported. The materials were slid against themselves under unlubricated conditions. Tests were run using a simulated inertial sample dynamometer. Sliding speeds ranged from 0.5 to 5.5 m sec^–1 with applied loads of 225 or 450 N. Friction coefficients were found to be in the range of 0.15 to 0.8 for both types of material. Friction response was qualitatively correlated with changes in surface chemistry at the sliding interface. Wear rates were of the order of 10^–14 to 10^–13m³ (Nm)^–1, in order-of-magnitude agreement with previous pin-on-disc results reported in the literature. Wear surfaces exhibit plastic deformation, ploughing, and oxide film formation and removal.     

Phonon-limited mean free path in the Sondheimer oscillation of aluminum

The phonon-limited part of the electron mean free path l _Hp ,in aluminum has been investigated by the Sondheimer size effect for a magnetic field along the three principal crystallographic orientations in the temperature range 1.8–12 K. It is found that l _Hp decreases with temperature approximately as T ^–3 and varies from 1.4 × 10^–3 to 1.3 × 10^–2 cm, with the orbits responsible for the oscillations at 20 K. The results are in agreement with a calculation based on the 4-OPW Fermi surface model and the anisotropic electron-phonon scattering time given by Leung et al.Work supported in part by a Grant-in Aid for Scientific Research from the Ministry of Education.     

Properties of GaN grown on sapphire substrates

Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1 ¯102) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H₂. Faceting was also reduced when the GaN was doped with Zn and the best surfaces for the (1 ¯102) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1¯102) substrate had a mobility greater than 400 cm² V^–1 sec^–1 and a carrier concentration of about 2 × 10¹⁷ cm^–3. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300cm²V^–1 sec^–1 and a carrier concentration of 1 × 10¹⁸ cm^–3.     

Diffusion bonding of AZ91 using a silver interlayer

Diffusion bonding of AZ91 alloy with a silver interlayer was carried out at 480 °C for different times under 1 MPa in a vacuum of 2 × 10⁻³ Pa. Shear test was applied to measure the shear strengths of the joints in the room temperature. The shear strength values of all bonded samples were found around 65–70 MPa. SEM–EDS studies indicated that the melting occurred along the interface of bonded samples as a result of transfer of atoms between the interlayer and the matrix during bonding. XRD results confirmed that the interlayer dissolved in the interface of joints. Investigations of the fracture surfaces showed that a good bonding was obtained by plastic deformation.     

Development of opto-electronic devices using electrochemically grown thin ZnSe layers

ZnSe thin films of both p and n conducting types were successfully deposited on transparent conducting glass substrates using an electrochemical deposition technique. Thin films were deposited from an aqueous acidic electrolyte containing and . Deposition of thin films in a voltage range of 0.0 to –1.0 V was investigated by means of a voltamogram. The most suitable deposition voltage range was found to be –0.48 to –0.62 V for ZnSe in this aqueous medium. The nature of the surface of the conducting glass and the deposited ZnSe layers were studied using atomic force microscopy (AFM). Although the substrate surface was rough and spiky as observed from the AFM studies, electrodeposited ZnSe layers exhibit good crystallinity when compared to MBE grown ZnSe layers on GaAs (1 0 0) surfaces. The C–V depth profiling studies suggest the level of charge carrier concentration to be in the semiconducting region for these thin film materials. Photoluminescence studies were carried out in order to study the energy states in the band gap region and the results indicate the presence of less number of defect levels in comparison with MBE grown ZnSe layers. The best p–n junction devices fabricated with electrodeposited layers to date display a rectification factor of over five orders of magnitude at 1.0 V.     

Vibrational modes in electrodeposited amorphous silicon: FT-IR analysis

Infrared spectra of 13 samples of amorphous silicon bonded with hydrogen, fluorine and carbon, prepared by electrodeposition using a mixture of ethylene glycol and fluosilicic acid were analysed in the wave number region 4000-400 cm^–1 with a Fourier transform infrared spectrometer. Strong absorption peaks were observed at 1000 cm^–1 due to the SiF_x stretching mode. Small peaks were seen around 2300 and 640 cm^–1 due to SiH stretching and wagging modes of absorption. The number of bonded hydrogen atoms in the film deposited at 0.05 M, 50 mA cm^–2 was calculated to be 6.2579×10²¹ and 1.2302×10²⁰ atm cm^–3 using integrated absorption of the CH and SiH stretching modes, respectively. The absorption coefficient around the SiF_x stretch region was found to vary from 1300–2500 cm^–1 as the molarity of the electrolyte was increased. Binding energy shifts in X-ray photoelectron spectrum were used as a cross check to confirm the silicon bonding with carbon, hydrogen, oxygen and fluorine atoms. The absence of columnar growth in SEM photographs indicates no polysilane formation in the films.     

Magnetic properties of porous iron compacts

Magnetic hysteresis measurements, magnetoacoustic emission, and the Barkhausen effect have been used to investigate the magnetic properties of six porous iron compacts with porosities in the range 0.3–6.2% and pore sizes in the range 1.6–13.2 µm. Coercivity and maximum permeability varied with pore size, showing respectively a maximum and a minimum for pore areas in the range 50–100 (µm)², corresponding to pore diameters 8–11 µm. Significant correlations were found between several of the magnetic parameters including a fundamental relationship between coercivity, initial permeability, and maximum differential permeability. There were also indications of a correlation between magnetic properties and the initial molding pressure, although these were not conclusive. However, none of the magnetic properties was found to vary in a progressive manner with percentage porosity, although it is conceivable that if all other variables, such as pore size and grain size, remained constant, that changes in magnetic properties with porosity could be detected from magnetic measurements.     

Mass-Spectrometric Study of the Temperature Variation in the Dimer-to-Monomer Ratio in the Free-Surface Vaporization Fluxes from Alkali Halide Single Crystals

A mass-spectrometric method is used to investigate the kinetics of the vaporization of LiF (970–1070 K), NaCl (722–889 K), KCl (780–900 K), KBr (724–918 K), and CsI (656–838 K) single crystals. A dimer-to-monomer ratio, J_D/J_M, in the fluxes vaporizing from free surfaces of these crystals are calculated, using the currents of M⁺, MX⁺, and M₂X⁺ ions formed via electron impact ionization of the molecules MX and M₂X₂ (M is the alkali atom; X is the halogen). It is found that the dimer-to-monomer ratio increases with temperature at a continually increasing rate in the LiF and NaCl cases. In the KCl, KBr, and CsI cases, this rate passes through a maximum. Such a specific temperature variation in J_D/J_M is discussed through the analysis of proposed mechanisms of dimer formation in light of the terrace–ledge–kink and surface charge models.     

The low temperature magnetic properties of austenitic Fe---Cr---Ni alloys. 3. The effect of chromium concentration in Fe---Cr-17wt% Ni alloys

The magnetic properties of ternary Fe---Cr---Ni alloys containing 17 wt% Ni and 18–24% Cr have been studied over a range of temperature from 4.2–35 K by both ac and dc techniques in fields from 5 Am⁻¹ − 6 MA m⁻¹. At room temperature, the matrix of all the alloys was the paramagnetic fcc austenite phase (with an unavoidable 0.1 – 0.5 wt% δ ferrite) and on cooling all the alloys showed distinct peaks in ac susceptibility, though equivalent peaks in dc susceptibility were only observed at low fields in the alloy containing 18 wt% Cr. An analysis of the magnetization/field/temperature results showed that all the alloys were superparamagnetic at low temperature, due to the formation of ferromagnetic clusters in the disordered antiferromagnetic matrix, and that the chromium atoms make no measureable contribution to the magnetization of these clusters. Appropriate susceptibility values are tabulated for use as design data.     

Brittle-to-ductile transition temperature and its strain rate sensitivity in a two-phase titanium aluminide with near lamellar microstructure

The temperature dependence of tensile properties of a two-phase titanium aluminide with nearly lamellar microstructure has been investigated and brittle-to-ductile transition (BDT) temperatures (T_BDS) have been determined under different strain rates from 10^–5 to 10^–1 s^–1. It is found that T_BD rises with the increase of strain rate. From the positive strain rate sensitivity of T_BD, the apparent activation energy of BDT is determined to be 324 kJ/mol by means of Zener-Hollomon factor. The determined activation energy approximates to the activation energies of self-diffusion of Ti atoms, and inter-diffusion of Ti and Al atoms in TiAl phase. The approximation, fractography analysis and theoretical calculation using the Nabarro Model add up to the speculation that the BDT of the alloy is controlled by dislocation climbing.     

Crack velocity dependence of longitudinal fracture in bone

An evaluation of the fracture characteristics of bovine tibia compact tension specimens associated with controlled crack propagation in the longitudinal direction has been made. The fracture mechanics parameters of critical strain energy release rate (G _c) and critical stress intensity factor (K _c) were determined for a range of crack velocities. A comparative fracture energy (W) was also evaluated from the area under the load-deflection curve. It was found that an increase in the average crack velocity from 1.75 to 23.6×10^–5 m sec^–1 produced increases in G _c (from 1736 to 2796 J m^–2), K _c (from 4.46 to 5.38 MN m^–3/2) and W. At crack velocities >23.6×10^–5 m sec^–1, W decreased appreciably. Microstructural observations indicated that, for crack velocities <23.6 m sec^–1, relatively rough fracture surfaces were produced by the passage of the crack around intersecting osteons (or lamellae), together with some osteon pull-out. In contrast, at a higher crack velocity, fracture was characterized by relatively smooth surfaces, as the crack moved indiscriminately through the microstructural constituents.