D. C. electrical properties of hot-pressed nitrogen ceramics

The d.c. electrical properties of some hot-pressed polycrystalline nitrogen ceramics have been measured between 18 and 500° C in applied electric fields up to 1.1×10⁴ Vcm^–1. The materials examined were Si₃N₄, 5wt%, MgO/Si₃N₄ and two sialons havingz=3.2 andz=4.0. The conduction in all the materials showed similar general features. The time dependent charging (I _c) and discharging currents (I _D) were observed which followed a I(t)t_–n law at room temperature withn=0.7 to 0.8. The exponentn forI _c decreased with increasing temperature. The current density-field (J _s-E) characteristics were ohmic in applied fields of less than 3×10³ Vcm^–1; conductivity increased with electric field above that range. Above about 280° C, a was independent ofE, its temperature dependence following log T ^–1. Below about 230° C conductivity fitted a exp (–B/T _1/4) law in both low and high fields. There is a good correlation between the temperature and field variations of time dependent current and the steady current. The conductivities were in the range of 10^–15 to 10^–16–1 cm^–1 at 18° C and rose to 4×10^–10 to 2×10^{–12 –1} cm^–1 at 500° C. The activation energies were in the range of 1.45 to 1.80 eV and 0.05 to 0.15 eV at above 300° C and near room temperature respectively. Various models to explain the data are considered.     

Space-charge-limited current analysis in amorphous InSe thin films

The study of space-charge-limited currents (SCLC) in amorphous InSe thin films is presented. The temperature-dependent current–voltage (J–V) measurements were carried out for TO/a-InSe/Au sandwich structures in the range of 200–320 K. For all samples, ohmic behavior was observed up to an electric field strength of about 2×10⁵ V cm^–1. From the temperature dependence of conductivity data, the position of the thermal equilibrium Fermi level E _fo is determined as 250 meV above the valence band E _v. At higher electric field strength values in the SCLC regions, the proportionality constant of voltage changes is between 2 and 2.9 with temperature. The analysis of J–V characteristics using the SCLC method and analytical approach for the determination of density of states (DOS) in the energy range of 190–250 meV shows that DOS changes between 3.8×10¹⁷–1.7×10¹⁸ eV^–1cm^–3 with energy. The energy distribution of DOS is temperature independent indicating that the SCLC in these amorphous films is related to the bulk, not to the surface layer between the contact and the film.     

The surface diffusion coefficients of MgO and Al2O3

From the measurement of neck size and neck curvature during the sintering of two spheres the surface diffusion coefficients of MgO and Al₂O₃ were determined. The spheres of both materials were machined from single crystals. The following values of surface diffusion coefficients were found: for MgO,D _{s s} = 3.7 × 10^–4 exp (407.8 kJ mol^–1/RT m³ sec^–1; for Al₂O₃,D _{s s} = 1.5 × 10^–2 exp (518.7 kJ mol^–1/RT) m³ sec^–1.     

Measurement of the surface resistance of YBa2Cu3O7?x thin films using stripline resonators

We review methods of measuring surface resistance (R _s ) of thin films using stripline resonators, and present our measurements of theR _s of YBa₂Cu₃O_7–x films as a function of frequency, temperature, and r.f. magnetic field. The films were deposited on LaAlO₃ substrates by two methods: (1) electron-beam coevaporation of Y, BaF, and Cu followed by annealing in O₂, and (2) single-targetin situ sputtering. The measurements were obtained at frequencies from 0.4 to 10 GHz, temperatures from 4 to 90 K, and an r.f. magnetic field range from 0 to 30 Oe. At low temperature and low r.f. field at 0.4 GHz, theR _s values obtained for the two deposition methods are approximately 7×10^–6 and 4×10^–6 , respectively.     

Surface Diffusion of Solid Hydrogen

A number of experiments have shown that hydrogen molecules can migrate on films of solid hydrogen. This was thought to be a diffusive process but recent experimental findings have led others to a different conclusion, so a question remains about the process by which the hydrogen molecules migrate over their own solid surface. We report new measurements using a hole-burning technique which confirm these recent experiments but we interpret them differently. We have developed a model for the recovery of the hydrogen which accounts for these results and shows that the H₂ does move diffusively. The diffusion is thermally activated and we have determined the diffusion constant, D ₀=4.03×10^–4±2.5×10^–5 m²s^–1, and the activation energy, E=43±7 K, for hydrogen.     

Effect of Growth-Induced Linear Defects on High Frequency Properties of Pulse-Laser Deposited YBa2Cu3O7?δ Films

Growth-induced linear defects are shown to strongly affect the microwave surface resistance, R_s, of highly biaxially oriented high temperature superconductor (HTS) YBa₂Cu₃O_7– (YBCO) films. Measured R_s(77 K) turned out to be 4–5 times higher than in single crystals. The films were deposited by modified pulse-laser technique, J_c(77 K) = (3-6) × 10⁶ A/cm², onto LaAlO₃ substrates. R_s(T) was measured at 134 GHz and 20–100 K. TEM/HREM study of YBCO films deposited at T_s = 750°C–780°C revealed a reduction of edge dislocation density with T_s increase (from 2 · 10¹¹ to 10¹⁰ lines/cm²). YBCO films deposited at T_s = 780°C exhibited the smallest R_s(77 K, 134 GHz) < 120 m and the lowest density of dislocations detected by HREM and X-ray analysis. A nature of the dislocation effect is discussed within a model of local anisotropic elastic deformation in a vicinity of dislocation cores, where T_c variation and an enhancement of normal quasiparticle density are significant.     

Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

Superelastic property of shape memory alloys (SMAs) is becoming increasingly important for impact applications due to their large recoverable strains and high capacity to dissipate energy. In this work, tensile behavior of superelastic NiTi SMA wires at impact strain rates was studied by instrumented tensile-impact technique, which allows to obtain material properties on the order of 1–10² s⁻¹. The results show that even at impact strain rates, martensite can be induced by tension in NiTi. At impact, a plateau stress appears during transformations similar to that at quasi-static strain rates, but 100–150 MPa higher in stress. This is due to the higher temperatures achieved during the deformation due to the close to adiabatic nature of the impact event. The influence of the strain rate over the mechanical behavior of NiTi was spread to the quasi-static strain rates so that the evolution of several parameters was also studied on the range 10⁻⁵–10² s⁻¹. Therefore, forward stress-induced martensitic (SIM) transformation stresses (σ^Ms and σ^Mf) and deformation energy (E_d) increase with strain rate, but they are strain rate independent from 10⁻¹ s⁻¹ at least until 10² s⁻¹. Reverse SIM transformation stresses (σ^As and σ^Af), recoverable strain energy (E_r), and dissipated energy (W_d) depend mainly on maximum strain achieved during the deformation, but for strains corresponding to a load–unload cycle with complete SIM transformation, σ^As, σ^Af and E_r are higher at impact than at quasi-static strain rates, and W_d shows similar values at very low strain rates and at impact.     

Space charge limited current, variable range hopping and mobility gap in thermally evaporated amorphous InSe thin films

We have analyzed the properties of as-deposited InSe thin films, deposited onto well cleaned glass substrates under a vacuum of 10^–5 Torr, using X-ray diffraction, Rutherford back scattering, energy dispersive analysis of X-rays, optical transmittance and current–voltage (120–390 K) measurements. Allowed and indirect transition was identified and the mobility gap was determined as 1.44 eV. Under low field (<1×10⁵ V cm^–1) and in the temperature range of 130–200 K, the conductivity in the films was behaving like that of Mott's variable-range hopping (VRH) type. Mott's parameters such as characteristics temperature (T ₀), hopping range (R _hop), hopping energy (W _hop), values of localized states density N (E _F), and activation energy (E _a) were estimated. In the temperature range 210–290 K, thermionic conduction mechanism plays a dominant role and its activation energy was calculated. At high field (>2×10⁵ V cm^–1) and in the temperature range of 300–390 K, space charge limited conduction currents (SCLC) mechanism was observed and the related parameters, such as electron density (n ₀), trap density (n _t), the ratio between free electron density to the total electron density (), mobility () and the effective mobility (_eff) of the InSe film of typical thickness 265 nm were calculated and the results are discussed.     

Pitting susceptibility of UNS NO. 8904 stainless steel in bromide-sulphide solution

The effect of sulphide ion on the pitting corrosion behaviour of UNS* no. 8904 (904L) stainless steel (SS) in 0.6 M NaBr at 25 °C has been investigated by using potentiodynamic anodic polarization and electrochemical impedance spectroscopy techniques. The pitting potential, E _pit, in 0.6 M NaBr + 10^–2 M Na₂S was more negative than that obtained in 0.6 M NaBr, and decreased with increasing temperature in the range 25–60 °C. Scanning electron microscopy observation showed that elemental sulphur formed on the steel surface before E _pit was reached in 0.6 M NaBr + 10^–2 M Na₂S at 60°C. E _pit of 904L SS pH independent in the pH range from 3–10 of 0.6 M NaBr + 10^–2 M Na₂S at 25 °C, while at high pH values the pitting was suppressed. The impedance measurements showed that the charge transfer resistance, R _t, decreased with time, when the controlled potential became higher than E _pit.     

Cell structure and compressive behavior of an aluminum foam

The plastic collapse strength, energy absorption and elastic modulus of a closed cell aluminum foam are studied in relation to cell structures. The density, node size and the cell wall thickness of the aluminum foams decrease with increasing cell size. The failure of the foam cells under compressive load progresses successively from the top or/and bottom to the mid-layer of the compression specimens, and no initial rupture of the foam cells is observed in the mid-height of the foam samples. When foam density increases from 0.11 to 0.22 g/cm ³, the plastic collapse strength rises from 0.20 to 1.29 MPa, while the elastic modulus of the closed cell aluminum foam increases from 0.70 to 1.17 GPa. In contrast, the energy absorption of the foams decreases rapidly with increasing cell size. When cell size increases from 4.7 to 10.1 mm, the energy absorption drops from over unity to 0.3 J/cm ³. The normalized Yong’s modulus of the closed cell aluminum foam is E^*/E_s = 0.208 (ρ^*/ρ_s), while the normalized strength of the foams, σ */σ_s is expressed by σ */σ_s = c ⋅ ρ */ρ_s where c is a density-dependent parameter. Furthermore, the plastic collapse strength and energy absorption ability of the closed cell aluminum foams are significantly improved by reducing cell size of the aluminum foams having the same density.     

Mössbauer spectral invetigations of ultrafine rare-earth iron garnets

Ultrafine rare-earth iron garnets (RIG) in the crystalline size range 1.0–35 nm, prepared by the citrate precursor method have been investigated by Mössbauer spectroscopy at room temperature. The crystallites of 10–35 nm size show two six-line spectra corresponding to tetrahedral and octahedral Fe³⁺ with isomer shifts, , and magnetic hyperfine fields, H _hf, comparable to the bulk polycrystalline garnets. However, 1.0–1.5 nm crystallites (X-ray amorphous) show a quadrupole split doublet due to superparamagnetic nature with isomer shifts (0.3 mms^–1) internediate between tetrahedral and octahedral values of bulk garnets. The relative increase of tetrahedral and the decrease in the octahedral indicate, respectively, an increase of tetrahedral Fe-O bond distance and a decrease of octahedral Fe-O bond distance in the ultrafine state, in complete agreement with the observations made earlier in the spectrochemical investigations. The lines are broad (linewidth=0.65 mms^–1) and the quadrupole splittings are relatively larger (E _Q = 1.0 mms^–1) indicating large deviation from cubic symmetry at the Fe³⁺ sites.     

Ionic transport studies of the glassy silver vanadomolybdate system

Ionic conductivity of the Ag₂O-MoO₃-V₂0₅ system has been studied over a wide range of frequency, temperature and composition. A narrower glass forming region has been found in comparison to the corresponding Ag₂O-MoO₃-P₂O₅ and Ag₂O-B₂O₃-P₂O₅ systems. The highest conductivity at room temperature, _rt, = 3.21 × 10^–6–1 cm^–1 (d.c.) with an activation energy,E _act, of 0.466 eV, was observed for the glass former's ratio of unity. Further, it reached a maximum value of 2.2 × 10^–2¨-1 cm^–1 withE _act = 0.153 eV when the oxide-base glass was dissolved with Agl. D.c. conductivity, hopping rate and relaxation time in the present system have been found to be characterized by the same activation energy.     

Effect of structural defects on optical properties of amorphous selenium films

Thickness of film, energy of incident photons and glass transition temperature all affect the structural bonding between neighbours and are considered to be the main factors in studying the optical properties of amorphous selenium films. The results indicate that in the lowtemperature range (T <T _g), a shift in the absorption edge to lower photon energies with increasing film thickness occurs. Increasing the thickness is accompanied by a decrease in the optical energy gap,E _g ^opt , with a gradient of 5×10^–4eV nm^–1. In the high-temperature range (T >T _g), the value ofE _g ^opt for a given thickness decreases by more than 50% due to pronounced modulation of the structural defects under incidence of isoenergetic photons of 1.8 eV. The isothermal curves ofT,R=f(t), atT >T _g, take place via three time-dependent stages. These results are interpreted and are correlated with the temperature dependence of the morphological changes declaring the formation of spherulites having a lamellar structure. The kinetic parameters controlling the structural transition are computed and the results are discussed.     

High thermal expansion KAlSiO4 ceramic

Orthorhombic kalsilite (KAlSiO₄) was prepared by solid-state reaction from K₂CO₃, Al₂O₃, and SiO₂. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10^–5°C^–1 for the a-axis, 1.6×10^–5°C^–1 for the b-axis, 2.8×10^–5°C^–1 for the c-axis, and 2.0×10^–5°C^–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li₂CO₃. The KAlSiO₄ ceramic sintered at 1200°C for 2 h with 5 wt% Li₂CO₃ had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10^–5 °C^–1 from room temperature to 600°C.     

Alginate polyelectrolyte ionotropic gels

The kinetics and mechanism of exchange of Ni²⁺ by H⁺ ions in ionotropic nickel alginate polymembrane gels of capillary structure have been studied by pH-metric and conductimetric techniques. The rate of exchange conforms to; rate=k ₁ [nickel alginate] [H⁺]- k _–1 [alginic acid] [Ni²⁺] with k ₁=6.34×10^–2dm³mol^–1s^–1 and k _–1=8.83×10^–4 s^–1 at 20°C, respectively. The activation parameters have been evaluated and a mechanism consistent with the kinetic data is discussed.     

Prediction of welded joint thinning by the method of polarization resistance

The equivalent electric circuits for determiningR _p in electrolytic cells with two dissimilar electrodes are investigated. For welded joints, we suggest a relation for estimating the corrosion rate in terms ofR _p and describe the design of a solion for determining this rate in a local zone of a welded joint.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 94–96, September–October, 1993.     

Electrical properties of In doped CdS thin films

Indium doped polycrystalline cadmium sulphide CdS:In thin films have been prepared by the spray pyrolysis technique on glass substrates in an enclosed dome. The different scattering mechanisms such as lattice, impurity and grain boundary scattering for CdS:In films are observed at low temperature, in the range of 303 to 120 K. The experimentally determined mobilities due to these scatterings are well interpreted with those of theoretically calculated mobilities. The d.c. conductivity for CdS:In films has also been studied in the same temperature region. The Mott variable range hopping conduction process followed below the temperature of 150 K. The Mott parameters such as N(E _F ), R, W and are found to be 1.26 × 10¹⁹ eV^–1cm^–3, 9.8 × 10^–-7cm, 0.02 eV^–1 and 2.38 × 10⁶cm^–1, respectively from the conductivity data.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.     

Analysis of structural defect annealing in copper-base alloys exhibiting the shape memory effect

The process of martensite stabilization is investigated by resistometric method and positron annihilation spectroscopy in Cu-Al-Zn and Cu-Al-Ni alloys exhibiting the shape memory effect. A strong stabilization of martensite observed in Cu-Al-Zn alloys is explained by diffusion of divacancies and monovacancies formed during quenching with the following values of the Arrhenius relation E _2V ^M = (0.69 ± 0.02) eV, K ₀₄ = 6.4 × 10^8.0±0.1s^–1 and E _1V ^M = (0.84 ± 0.02) eV, K ₀₃ = 6 × 10^9.00±0.15s^–1, respectively. It was found that the energy of vacancy formation in Cu-8.5Al-14.5Zn(%wt.) alloy was 0.56 eV. The proposed model has been confirmed by applying the positron annihilation technique. In Cu-Al-Ni alloys the disappearance of quenched-in vacancies is described by one recovery process only with the diffusion energy of monovacancy 0.74 eV.