Very-low temperature specific heat of Torlon

The specific heat of Torlon has been measured in the 0.15-4.2 K temperature range. Data below 1 K can be represented by c(T) = P₁T^1+δ + P₂T³, with P₁ = (5.41 ± 0.08)·10⁻⁶J K^−(2+δ) g⁻¹, P₂ = (2.82 ± 0.03) ·10⁻⁵JK⁻⁴g⁻¹ and δ = 0.28 ± 0.01, as predicted by the tunnelling theory. Above 1 K, the behaviour of c(T) is similar to that of other amorphous materials and can be expressed as: c(T) = P · T^Ω with P = (2.68 ± 0.07)·10⁻⁵JK^Ω+1g⁻¹ and Ω = 3.32 ± 0.02.     

Structure and electrical transport properties of pure and Li2O-doped CuO/MgO solid solution

The different electrical properties, σ, ?′, tan δ and E_σ of pure and Li₂O-doped CuO/MgO solid solution were investigated. The mole fraction of CuO (MF) was varied between 0.048 and 0.2. Pure and doped samples were subjected to heat treatments at 673 and 1073 K. The results revealed that the amount of CuO dissolved in MgO lattice increases progressively by increasing the MF as evidenced from the progressive decrease in the intensity of all diffraction lines of undissolved CuO phase. The dissolution process of copper ions in MgO lattice was accompanied by progressive increase in its lattice parameter. This process being conducted at 1073 K was accompanied by a significant progressive increase in the values of σ, ?′ and tan δ with subsequent decrease in the value of E_σ. The increase in the MF value of CuO from 0.048 to 0.2 led to a significant increase in the value of σ_DC, measured at room temperature, from 6.33 × 10⁻¹² to 9.9 × 10⁻¹⁰ Ω⁻¹ cm⁻¹ and E_σ decreases from 0.76 to 0.58 eV.Li₂O doping of the investigated system followed by calcination at 1073 K resulted in a measurable increase in values of σ, ?′ and tan δ with subsequent decrease in E_σ. These results were discussed in the light of the possible effective increase in the charge carriers concentration (Cu²⁺ions dissolved in MgO lattice) and also to an effective increase in mobility of these charge carriers by Li₂O doping.     

Effect of some VA group modifiers on R2O3 (R = Sb,Bi)–ZnF2–GeO2 glasses doped with CuO by means of spectroscopic and dielectric investigations

Transparent glasses 40Sb₂O₃–20ZnF₂–(40 − x)GeO₂:xCuO, and 40Bi₂O₃–20ZnF₂–(40 − y)GeO₂:yCuO with x = 0, 0.6 and 0 ≤ y ≤ 1 wt% were prepared by melt quenching technique and were characterized by XRD and differential thermal analysis. Spectroscopic studies like optical absorption, FTIR, Raman, EPR and dielectric parameters (such as ?′, loss(tan δ), and σ_ac) were carried out to examine the modifier and dopant effect on zinc germanate glass network. Optical absorption and EPR data have revealed that the environment of Cu²⁺ ions is more ionic in bismuth series rather than antimony glasses. Reduced bismuth ions have been found in pure and at lower concentration of dopant in Bi₂O₃ mixed glasses, which are useful for IR amplifications. FTIR and Raman spectra have indicated the conversion of GeO₄ to GeO₆ structural units by forming cross linking bonds like Bi–O–Ge, Ge–O–Cu, etc., and open the glass network with integration of Bi₂O₃ and CuO doping. It is also confirmed by decreasing T_g and E_g values. The temperature dependence of dielectric parameters at different frequencies was interpreted in terms of structural changes in the glass network.     

Absorption and emission properties of Ho doped lead-zinc-borate glasses

This paper reports on the affect of lead content on the absorption and emission spectra of the Ho³⁺ ion doped lead-zinc-borate glasses in the composition (mol%) of (20 − x)PbO-20ZnO-(59 + x)B₂O₃-1.0Ho₂O₃ where x = 0, 5,10,15 of PbO content with λ_exc = 405 nm. The experimental absorption band energies have satisfactorily been correlated with the theoretical results with an r.m.s deviation of zero with the following correction factors obtained by a least square fit analysis: ΔE¹ = 348.495936 cm^− 1, ΔE² = 1.436043 cm^− 1, ΔE³ =  46.481575 cm^− 1, Δξ_4f = − 28.512979 cm^− 1, Δα = 55.508936 cm^− 1, Δβ = − 1394.339908 cm^− 1 and Δγ = 1208.424336 cm^− 1. By applying the Judd-Ofelt intensity parameter Ω₂ has been found to be linearly decreasing with the PbO content from 5 to 10 mol% and then increasing. And also radiative (A, A_T, β, τ_r) characteristic factors of the luminescent transitions (⁵I₈ ← ⁵F_3,4,5 and ⁵S₂) of the glasses have been evaluated. Stimulated emission cross-sections (σ_p^E) of the measured emission transitions of holmium glasses have also been computed.     

Calcium zinc manganites, Ca4Mn7Zn3O21 − δ (0.5 < δ < 2.5) with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties

Phase-pure calcium zinc manganite, Ca₄Mn₇Zn₃O_21 − δ has been synthesized by the solid-state reaction between CaMnO₃, ZnO and Mn₃O₄ at ∼ 1300 °C. Conversion of Ca₄Mn₇Zn₃O_21 − δ to Ca₄Mn₇Zn₃O_19 ± δ occurs at 1330-1350 °C which is influenced by the oxygen nonstoichiometry associated with increasing Mn³⁺/Mn⁴⁺ ratio. The Ca₄Mn₇Zn₃O_21 − δ phase is rhombohedral (space group = R3¯m) and has the β″-alumina (BA)-type structure consisting dominantly of Mn⁴⁺ (3d³) ions. The Ca₄Mn₇Zn₃O_19 ± δ is hexagonal (space group = P6₃/mmc) having the magnetoplumbite (MP)-type structure containing dominantly Mn³⁺ (3d⁴) ions. The electrical transport properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ are indicative of the activated-type small polaron hopping conductivity below 366 and 392 K respectively. The anisotropic charge transport is influenced only in a limited way by the mixed-valency of Mn prevailing in the manganite spinel blocks. Highly nonlinear current-voltage (I-V) curves with nonlinearity coefficient, α, up to 64 was achieved at the turn-on field strength E_t ∼ 110-160 V/mm. Magnetic properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ have been investigated which showed the antiferromagnetic-insulating (AFI) behavior with T_N = 103 and 80 K and with an AF-ordered effective magnetic moment, μ_eff = 3.54 and 4.28 μ_B/Mn respectively. The Ca₄Mn₇Zn₃O_19 ± δ phase showed the ferrimagnetic interaction below T_N (at ∼ 20 K).     

Surface planarization and masked ion-beam structuring of YBa2Cu3O7 thin films

Surface planarization and masked ion-beam structuring (MIBS) of high-T_c superconducting (HTS) YBa₂Cu₃O_7-δ (YBCO) thin films grown by pulsed-laser deposition (PLD) method is reported. Chemical-mechanical polishing, plasma etching, and oxygen annealing of YBCO films strongly reduce the particulate density (~ 10^-2 ×) and surface roughness (~ 10^-1 ×) of as-grown PLD layers. The resistivity, critical temperature T_c ≈ 90 K and critical current density J_c (77 K) > 1 MA/cm² of films are not deteriorated by the planarization procedure. The YBCO films are modified and patterned by irradiation with He⁺ ions of 75 keV energy. Superconducting tracks patterned by MIBS without removal of HTS material and, for comparison, by wet-chemical etching show same T_c and J_c(T) values. Different micro- and nano-patterns are produced in parallel on planarized films. The size of irradiated pattern depends on the mask employed for beam shaping and features smaller than 70 nm are achieved.     

Approaches for evaluating Young's and shear moduli in terms of a single SAW velocity via the SAM technique

In ultrasonic material investigations Young's modulus, E, and shear modulus, G, conventionally expressed in terms of the velocities of longitudinal (V_L,) and transverse (V_T) modes, are usually difficult to determine from a single measurement, in particular for scanning acoustic microscopy. Therefore, using Viktorov formula and physically acceptable approximations, we derive simple expressions for E and G in terms of the velocity, V_i, of just one propagating mode (including Rayleigh mode V_R). It was found that (E, G) = (α_i, β_i)ρV_i². The validity of these expressions is successfully tested, analytically and graphically, for a great number of materials representing a large spectrum of densities, 1000 kg/m³ < ρ < 22 000 kg/m³, and characterized by a wide interval of velocities, 1400 m/s < V_L < 12 000 m/s.     

Effect of silver concentration on the silver-activated phosphate glass

The effects of silver concentration on the structure and properties of silver-activated phosphate glasses, with the nominal molar compositions xAg₂O·(1 − x)(30Na₂O·10Al₂O₃·60P₂O₅) and 0 ≤ x ≤ 10 mol%, were studied. Increasing the Ag₂O-content decreases the glass transition temperature (T_g), increases the thermal expansion coefficient (TEC) and increases the glass dissolution rate in water. ³¹P nuclear magnetic resonance (NMR) and Raman spectroscopies indicate that the addition of Ag₂O leads to the formation of chain-terminating P-tetrahedra, and ²⁷Al NMR spectra indicate that Al-octahedra are the preferred structural moiety. Optical spectroscopy indicates that Ag₂O-additions shift the UV-absorption edge to longer wavelengths. Irradiating glasses with ≤1.0 mol% Ag₂O with ⁶⁰Co γ-rays creates a photoluminescence (PL) center that emits near 605 nm when excited with UV light (337.1 nm). The intensity of this PL center is proportional to the radiation dosage (up to 200 Gy), and the relative sensitivity is maximized in glasses with 0.05 mol% Ag₂O. When x > 1 mol% Ag₂O, a second PL center, emitting at 470 nm, is activated. The formation of this second PL center is associated with the loss of radiation sensitivity for glasses with greater Ag₂O-contents.     

Mössbauer effect and dielectric behavior of NixCu0.8−xZn0.2Fe2O4 compound

The ferrite system Ni_xCu_0.8−xZn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 was synthesized. XRD measurement confirmed the presence of single-phase spinel structure. The area ratio of Fe³⁺ at the tetrahedral A- and octahedral B-sites was deduced from the spectral analysis of Mössbauer measurements. The results give evidence that Ni²⁺ replaces Cu at B-site in the present unit cell. The dielectric properties ?′, ?″, loss tangent tan(δ) and ac conductivity σ_ac have been studied for the prepared samples in the temperature range (300-600 K) and over the frequency range (10² to 10⁵ Hz). The electrical conductivity results revealed a semiconductor behavior with increasing nickel concentration with a change in the slope at the transition temperature T_c. The variation of the dielectric parameters (?′, ?″ and tan(δ)) with frequency and temperature displayed a strong dependence on nickel concentration. Dielectric anomaly at the transition temperature T_c was pronounced in the relations of ?′ and ?″ with temperature. The determined T_c was found to increase with increasing Ni content. The relation of tan(δ) with frequency at different temperatures showed two relaxation processes where the relaxation time and maximum frequency of the hopping conduction mechanism were determined. The results are explained in the light of cation-anion-cation and cation-cation interactions over the octahedral site in the spinel structure.     

Supramolecular polyurethane networks containing pyridine moieties for shape memory materials

Fabricating smart materials with supramolecular switch is an attractive research topic in recent years. In this communication, shape memory polyurethane supramolecular networks are synthesized from BINA, HDI, MDI and BDO. DSC results show that amorphous soft phase and hard phase are formed through different intermolecular hydrogen bondings at the regions of pyridine units and urethane groups in the resulted BINA containing polyurethanes (BIN-PUs). Moreover, DMA results show that the BIN-PUs have higher modulus ratio (E_g / E_r > 400) and much higher maximum tanδ. These results determine that the BIN-PUs exhibit excellent shape memory effect: higher shape fixity (> 97%) and higher shape recovery (> 91.7%).     

Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Effects of Ti on dielectric and piezoelectric properties of (Pb0.985La0.01)1+y(Nb1−yTiy)2O6 ceramics

Piezoelectric ceramics (Pb_0.985La_0.01)_1+y(Nb_1−yTi_y)₂O₆ (0 ? y ? 0.1) (PLNT) were fabricated by a conventional solid-state reaction method. The crystal structure and microstructures of specimens were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). All PLNT ceramics sintered at temperatures from 1250 °C to 1290 °C are shown to be a single orthorhombic phase. The formation of orthorhombic phase is promoted by excess PbO existed in the Ti-bearing samples during calcination processing. The increasing of the tolerance factor due to substitution of Ti⁴⁺ for Nb⁵⁺ is also responsible for the stability of orthorhombic phase. The high density of ceramics (relative density >95%) with the equiaxed grains and a minor porosity have been obtained. The measurement of dielectric and piezoelectric properties of PLNT ceramics reveals that Ti reduces the dielectric loss (tan δ) and dielectric constant (ε_r) of PLNT ceramics, and enhances the piezoelectric constant (d₃₃) and Curie temperature (T_c). The optimum component with y = 0.075 possesses the excellent electrical properties: ε_r = 182, tan δ = 0.0018, d₃₃ = 84 pC/N and T_c = 564 °C.     

Growth and characterization of tris(8-hydroxyquinoline)-aluminum molecular films

Various tris(8-hydroxyquinoline)-aluminum (Alq3) molecular solid films were grown on top of indium-tin-oxide (ITO) glass substrates using physical vapor deposition. The effect of changing the growth conditions on the properties of the films was studied. From scanning electron microscopy, an Alq3 planar layer over an ITO-substrate was observed at the initial period, and an Alq3 tubular structure (which becomes dominant at substrate temperature T_sub ≧ 90 °C) was found to nucleate from this layer. From X-ray diffraction, the Alq3 planar layer possesses an amorphous character while the Alq3 tubular layer has a triclinic α-phase structure. Based on an Arrhenius plot of the growth rate versus 1/T_sub, the growth behaviors in various T_sub-regions were discussed to be dominated by adhesion (for T_sub < 90 °C), steric effect (90 °C < T_sub < 150 °C), and re-evaporation (T_sub > 150 °C). Then, from optical transmission and photoluminescence spectra performed on the high crystalline Alq3 films, two signals associated with the optical-bandgap E_g absorption and the gap-state absorption were determined and discussed in terms of the optical properties of the constituent Alq3 molecules. Finally, from a fit of E_g(T) by an effective electron-phonon interaction model, the physical significance of these fitting parameters for the Alq3 molecular solid was investigated.     

Temperature shift of the optical gap in some PbO-ZnO-P2O5 glasses

The optical gap (E_g) between 4.54 eV-4.88 eV at room temperature was determined for PbO-ZnO-P₂O₅ glasses with the formal content of P₂O₅ in the region of 30 to 50 mol% and with the formal content of PbO in the region of 50 to 45 mol%, respectively. The temperature (T) dependence of the optical gap (E_g(T)) in the region 80 < T[K] < 600 was determined, and an electron-phonon interaction is suggested to be a major contribution to the temperature shift of the optical gap. In the temperature region of 300-600 K, the E_g(T) dependence can be approximated by a simple linear relation with the temperature coefficient (γ) of the optical gap in the region 6.04 ≤ γ × 10⁴ [eV/K] ≤ 7.39.     

Testing field and annealing temperature dependence of leakage properties in Bi3.25La0.75Ti3O12 thin films

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ thin films annealed at different temperatures were prepared on Pt/Ti/SiO₂/Si substrates by metalorganic decomposition method. The leakage current behavior and the current conduction mechanism were investigated. For all films, the leakage current density − electric field (J−E) characteristic is confined within a “triangle” in the log (J)− log (E) plane bounded by three limiting curves: Ohm's law (J ∝ E), trap-filled-limit (J ∝ E^a, a > 1), and Child's law (J ∝ E²). At high field region, Bi_3.25La_0.75Ti₃O₁₂ thin films with grains of rod-like show higher leakage current, while films with grains of spherical- or planar-like exhibit lower leakage current.     

Optical spectra of Zn1-xBexTe mixed crystals determined by IR-VIS-UV ellipsometry and photoluminescence measurements

Spectroscopic ellipsometry in the photon energy range from 0.04 eV to 6.50 eV is used for investigation of the optical response of Zn_1-xBe_xTe crystals grown by a high-pressure Bridgman method in the composition range x ≤ 0.12. Infrared spectra display absorption bands centred between 411 cm⁻¹ and 420 cm⁻¹ associated with BeTe-type optical phonon modes. The positions of the transverse-optical and longitudinal-optical phonon modes have been found by modelling the line shape of the complex dielectric functions, ε˜ and Im(−ε˜⁻¹), using a classical damped Lorentzian oscillator approach. Ellipsometric measurements in the VIS-UV range allow determination of the fundamental energy-gap (E₀) and the higher threshold energies (E₁, E₁ + Δ₁, E₂) originating from the band edge and spin-orbit splitting critical points. We have found that the Be content x = 0.12 causes an increase of the fundamental energy gap about 0.15 eV at room temperature when compared to the E₀ = 2.23 eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30 K to room temperature. Luminescence at temperature T > 200 K is very weak. The peak positions of the exciton emission lines agree well with the E₀ band-gaps derived from ellipsometric data if corrected for their temperature dependence.     

Impurity induced band-gap narrowing in p-type CuIn1 − xGax(S,Se)2

Green's functions modelling of the impurity induced effects in p-type CuIn_1 − xGa_xS₂ and CuIn_1 − xGa_xSe₂ (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at N_c ≈ 10¹⁷-10¹⁸ cm^− 3 for impurities with ionization energy of E_A ≈ 30-60 meV. (ii) For acceptor concentrations N_A > N_c, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E_g = E_g⁰ − ΔE_g. The energy shift of the valence-band maximum ΔE_v1 is roughly twice as large as the shift of the conduction-band minimum ΔE_c1. (iii) ΔE_v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N_A = 10²⁰ cm^− 3 implies a band-gap narrowing in the order of ΔE_g ≈ 0.2 eV, thus E_g = E_g⁰ − 0.2 eV, and an optical band gap of E_g^opt ≈ E_g⁰ − 0.1 eV.     

Effect of the Mn ion on electrical and magnetic properties of orthorhombic perovskite-type Ca(Mn1−xTix)O3−δ

Orthorhombic perovskite-type Ca(Mn_1−xTi_x)O_3−δ (0 ≤ x ≤ 0.7) was synthesized at 1173 K for 12 h in a flow of oxygen from a precursor gel prepared using citric acid and ethylene glycol. The Mn³⁺ ion was generated by substituting a Ti⁴⁺ ion in CaMnO₃. The average particle size was 100-300 nm and did not depend on x. The lattice constants and the (Mn, Ti)-O distance increased linearly with increasing x. The variation in global instability index (GII) indicated that the instability of the structure increases monotonically with increasing x. Ca(Mn_1−xTi_x)O_3−δ was an n-type semiconductor that had its minimum values of electrical resistivity (ρ) and activation energy (E_a) at x = 0.1. Ca(Mn_1−xTi_x)O_3−δ (x = 0 and 0.1) exhibited a weak ferromagnetic behavior. The variation in μ_eff indicated that the spin state of the Mn³⁺ ion changes from low to high at x = 0.1, then reverts to low in the range of 0.2 ≤ x ≤ 0.7. The variations in ρ and E_a are explained by the number of electrons according to the change in the spin state of the Mn³⁺ ion.     

The chemical stability and conductivity of BaCe0.9−xYxNb0.1O3−σ proton-conductive electrolyte for SOFC

BaCe_0.8Y_0.2O_3−δ and BaCe_0.9−xY_xNb_0.1O_3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.35) were prepared by a solid-state reactions. It was found that the BaCe_0.8Y_0.2O_3−δ samples decomposed into CeO₂ and BaCO₃ after being exposed in the atmosphere (3% CO₂ + 3% H₂O + 94% N₂) at 700 °C for 10 h. However, samples containing Nb remains unchanged in the same conditions, demonstrating a better stability in the presence of CO₂ and H₂O. The conductivity of BaCe_0.9−xY_xNb_0.1O_3−δ increased with the increase of Y content (x ≤ 0.30), and the highest value was observed at x = 0.30 where a significant decrease in conductivity took place at x = 0.35. The conductivity of BaCe_0.6Y_0.3Nb_0.1O_3−δ reaches 0.01 S/cm in humid hydrogen at 700 °C, slight lower than BaCe_0.8Y_0.2O_3−δ, 0.012 S/cm in the same conditions. Fuel cell with BaCe_0.6Y_0.3Nb_0.1O_3−δ as-prepared was successfully prepared and humidified hydrogen was supplied as fuels in evaluating the fuel cell performance. The open circuit voltage, peak power density and interfacial resistance at 700 °C were 1.02 V, 345 mW/cm² and 0.27 Ω cm², respectively.     

Effect of doping with Nd ions on the structural and ferroelectric properties of Ca0.28Ba0.72Nb2O6 single crystal

The crystal structure of Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystal with Nd-doping has been determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4bm and the following parameters: a = b = 12.458 Å, c = 3.954 Å, V = 613.688 Å³, and Z = 5. X-ray diffraction results on a Nd-doped CBN-28 single crystal also have demonstrated that Nd³⁺ and Ca²⁺ occupy the same site in the crystal structure. Dielectric and ferroelectric measurements have been performed. Transition from ferroelectric to paraelectric at around 223 °C has been observed. The Nd-doped crystal has a lower Curie temperature (T_m) than that of undoped CBN-28 crystal. The spontaneous polarization (P_s) and coercive electric field (E_c) also decrease compared with their values in the undoped CBN-28 crystal.