Dielectric properties, ac conductivity and thermal behaviour of flux grown cadmium titanate crystals

The dependence of loss tangent (tan δ) and both real and imaginary parts of the dielectric constant (′ and ″) on temperature in the range 298–923 K and frequency in the range 10³–10⁶ Hz for flux grown CdTiO₃ single crystals is reported. The ln σ_ac versus T plots suggest the conduction mechanism to be ionic hopping conduction. From ln σ_ac versus frequency curves, it can be seen that the slope decreases with the rise in temperature, suggesting that the ionic hopping conduction diminishes with the rise in temperature. The activation energy at various fixed frequencies is calculated from the slope of the graph between ln σ_ac versus 1/T (×10³ K⁻¹). Thermal behaviour of flux grown CdTiO₃ crystals using thermoanalytical techniques including TG, DTA and DTG is discussed. Thermal analysis suggests decomposition of CdTiO₃ in the temperature interval of 1386–1693 K leading to the formation of TiO₂ as the final product. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of kinetics of thermal decomposition suggest contracting cylinder model as the one that is relevant to the decomposition of CdTiO₃. The kinetic parameters viz. the order of reaction, activation energy, frequency factor, and entropy of activation using the above mentioned models are computed.     

Synthesis and crystal structure of [2-NH2-5-CH3C5H4N]4P4O12·6H2O

Chemical preparation, crystal structure, IR absorption and thermal analysis of a new cyclotetraphosphate [2-NH₂-5-CH₃C₅H₄N]₄P₄O₁₂·6H₂O are reported. This compound is triclinic P-1 with unit-cell parameters: a = 10.206(5), b = 11.778(1), c = 9.991(4) Å,  = 110.40(6), β = 117.74(6), γ = 86.41(3)°, V = 989.1(8) Å³, Z = 1, D_x = 1.445 g cm⁻³. The structure has been determined and refined to R = 0.034 and R_w = 0.044, using 3663 independent reflections. The ring anions and water molecules form layers spreading around (a, b + c) planes via OHO hydrogen bonds. Between them are anchored 2-amino-5-methylpyridium cations, which establish H-bonds to interconnect the different adjacent layers and so contribute to the cohesion of the three-dimensional network. Tautomerization of (C₆H₉N₂)⁺ groups was evidenced in the present structure.     

Crystal structure and characterization of [2,5-(CH3O)2C6H3NH3]4P4O12·2H2O

The preparation, crystal structure, TG–DTA analysis and spectroscopy investigation are reported for the 2,5-dimethoxy phenyl ammonium cyclotetraphosphate dihydrate [2,5-(CH₃O)₂C₆H₃NH₃]₄P₄O₁₂·2H₂O. This new compound is triclinic P with unit cell dimensions: a = 7.438(5) Å, b = 11.841(7) Å, c = 12.354(4) Å,  = 96.61(4)°, β = 98.35(4)°, γ = 102.60(6)°, Z = 1 and V = 1038.0(1) Å³. Its crystal structure has been determined and refined to R = 0.049, with 5128 independant reflections. The structure can be described by rows of P₄O₁₂ ring anions along the a axis; between these rows are located the organic groups, connected to them by hydrogen bonds.     

First principles calculations of thermal equations of state and thermodynamical properties of MgH2 at finite temperatures

We present the first principles calculations of the thermodynamical properties of magnesium hydride (MgH₂) over a temperature range of 0–1000 K. The phonon dispersions are determined within the density functional framework and are used to calculate the free energy of MgH₂ within the quasiharmonic approximation (QHA) at each cell volume and temperature T. Using the free energies the thermal equation of state (EOS) is derived at several temperatures. From the thermal EOS structural parameters such as the equilibrium cell volume (V₀) and elastic properties, namely, bulk modulus (K₀) and its pressure derivative are computed. The free energies are also used to calculate various thermodynamical properties within QHA. These include internal energy E, entropy S, specific heat capacity at constant pressure C_P, thermal pressure P_thermal(V, T) and volume thermal expansion ΔV/V (%). The good agreement of calculated values of S and C_P with experimental data exhibits that QHA can be used as a tool for calculating the thermodynamical properties of MgH₂ over a wide temperature range. P_thermal(V,T) increases strongly with T at all the volumes but it is a slowly varying function of volume for T = 298–500 K. According to Karki [B.B. Karki, Am. Miner. 85 (2000) 1447] such volume based variations can be neglected and so it is possible to estimate the thermal EOS only with the knowledge of the measured P_thermal(V, T) versus temperature at ambient pressure and isothermal compression data at ambient temperature. Temperature dependence of ΔV/V(%) shows that V₀ increased with increase in temperature. However, the percentage decrease in K₀ superseded this percentage increase in V₀ even at temperatures moderately higher than 298 K. Therefore, we suggest application of temperature (T > 298 K) as an approach to enhance the hydrogen storage capacity of MgH₂ because of its better compressibility at these temperatures.     

Frequency dependent electrical conductivity and dielectric relaxation behavior in amorphous (90V2O5–10Bi2O3) oxide semiconductors doped with SrTiO3

We report on the experimental results of frequency dependent a.c. conductivity and dielectric constant of SrTiO₃ doped 90V₂O₅–10Bi₂O₃ semiconducting oxide glasses for wide ranges of frequency (500–10⁴ Hz) and temperature (80–400 K). These glasses show very large dielectric constants (10²–10⁴) compared with that of the pure base glass (≈10²) without SrTiO₃ and exhibit Debye-type dielectric relaxation behavior. The increase in dielectric constant is considered to be due to the formation of microcrystals of SrTiO₃ and TiO₂ in the glass matrix. These glasses are n-type semiconductors as observed from the measurements of the thermoelectric power. Unlike many vanadate glasses, Long's overlapping large polaron tunnelling (OLPT) model is found to be most appropriate for fitting the experimental conductivity data, while for the undoped V₂O₅–Bi₂O₃ glasses, correlated barrier hopping conduction mechanism is valid. This is due to the change of glass network structure caused by doping base glass with SrTiO₃. The power law behavior (σ_ac=A(ω^s) with s<1) is, however, followed by both the doped and undoped glassy systems. The model parameters calculated are reasonable and consistent with the change of concentrations (x).     

Crystal structure, thermal analysis and IR spectroscopic investigation of (C6H9N2)H2XO4 (X = As, P)

Crystals of 2-amino-4-methylpyridinium dihydrogenmonoarsenate (C₆H₉N₂)H₂AsO₄ and 2-amino-4-methylpyridinium dihydrogenmonophosphate (C₆H₉N₂)H₂PO₄ have been prepared and grown at room temperature. These materials are isotypic with the following unit cell dimensions (C₆H₉N₂)H₂AsO₄: a = 12.4415(5) Å, b = 6.8224(3) Å, c = 11.3524(5) Å, Z = 4, V = 963.60(6) Å³; (C₆H₉N₂)H₂PO₄: a = 12.4410(9) Å, b = 6.7165(3) Å, c = 11.3417(5) Å, Z = 4, V = 925.09(10) Å³. The common space group is Pnma. The structure of these compounds has been determined by X-ray data collection on single crystals of (C₆H₉N₂)H₂AsO₄ and (C₆H₉N₂)H₂PO₄. Due to the strong hydrogen-bond network connecting the H₂XO₄ groups, the anionic arrangement must be described as a linear organization. The chains composed by the macroanion spread along the b-direction, approximately centered by x = 0 and 1/2. All atoms of the structure, except one oxygen atom, are located in the mirror planes situated at y = 1/4 and 3/4, imparting an internal mirror symmetry to the anionic and the cationic entities. The linear macroanions are crossed by organic cations lying in mirrors perpendicular to the b-direction; this atomic arrangement is then described by a three-dimensional network of hydrogen bonds, built up by two types, O–HO bonds inside the chains and N–HO bonds linking adjacent chains. The thermal properties of both compounds are investigated as well as the IR properties supported by group theoretical analyses.     

The effect of second principal stress on the fatigue propagation of mode I surface crack in Al2O3/Al alloy composites

Using a plate made of A2017-T6 metal matrix composites reinforced with 10 volume % and 20 volume % Al₂O₃ particles and Al alloy possesses the same composition as matrix alloy, the crack propagation rate da/dN of a mode I surface crack by the simultaneous action of plane bending and cyclic torsion are studied. And the effects of crack tip opening stress σ_top, crack opening displacement COD, biaxial stress ratio C (=second principal stress/first principal stress) and the surface roughness of crack section are examined. When stress intensity factor range ΔK is lower than the specific level, da/dN decreases with the increase of volume fraction of Al₂O₃ in C=0 and C=−0.55. But, da/dN of Al alloy becomes minimum in C=−1 and the effect of Al₂O₃ particles disappears. σ_top rises with the increase of volume fraction of Al₂O₃ particles and the decline of C. On the other hand, COD doesn’t always rise with the decline of C. These phenomena can be explained by the residual compressive stress formed at the surface layer of the specimen by the fatigue test and the surface roughness of crack section.     

Synthesis, structure refinement and characterization of tetrahydrated acid gadolinium diphosphate HGdP2O7·4H2O

Synthesis and single crystal structure are reported for a new gadolinium acid diphosphate tetrahydrate HGdP₂O₇·4H₂O. This salt crystallizes in the monoclinic system, space group P2₁/n, with the following unit-cell parameters: a = 6.6137(2) Å, b = 11.4954(4) Å, c = 11.377(4) Å, β = 87.53(2)° and Z = 4. Its crystal structure was refined to R = 0.0333 using 1783 reflections. The corresponding atomic arrangement can be described as an alternation of corrugated layers of monohydrogendiphosphate groups and GdO₈ polyhedra parallel to the () plane. The cohesion between the different diphosphoric groups is provided by strong hydrogen bonding involving the W4 water molecule.

IR and Raman spectra of HGdP₂O₇·4H₂O confirm the existence of the characteristic bands of diphosphate group in 980–700 cm⁻¹ area. The IR spectrum reveals also the characteristic bands of water molecules vibration (3600–3230 cm⁻¹) and acidic hydrogen ones (2340 cm⁻¹). TG and DTA investigations show that the dehydration of this salt occurs between 79 and 900 °C. It decomposes after dehydration into an amorphous phase. Gadolinium diphosphate Gd₄(P₂O₇)₃ was obtained by heating HGdP₂O₇·4H₂O in a static air furnace at 850 °C for 48 h.     

Crystal structure and spectroscopic studies of NaGd(PO3)4

Single crystals of gadolinium–sodium polyphosphate NaGd(PO₃)₄ were grown for the first time using a flux method and characterized by X-ray diffraction. This phosphate crystallizes in a monoclinic system with P2₁/n space group and with the following unit-cell dimensions: a = 9.767(3) Å, b = 13.017(1) Å, c = 7.160(2) Å, β = 90.564(5)°, V = 910.3(4) Å³ and Z = 4. The crystal structure was solved from 3451 X-ray independent reflections with final R(F²) = 0.0219 and R_w(F²) = 0.056 refined with 164 parameters (). The atomic arrangement can be described as a long chain polyphosphate organization. Two infinite (PO₃)∝ chains with a period of eight tetrahedra run along the [0 1 1] direction. The structure of NaGd(PO₃)₄ consists of GdO₈ polyhedra sharing oxygen atoms with phosphoric group PO₄. Each Na⁺ ion is bonded to eight oxygen atoms.     

Structural correlations in light irradiated Ge36Se64 amorphous films—Radial distribution function study

The effect of laser irradiation on the electronic structure of amorphous Ge₃₆Se₆₄ films has been detected by studying the variation of the bond length (r) and the coordination number C_N. The total distribution function T(r) of the as deposited film is characterized by the first coordination sphere corresponding to the superposition of the correlation Ge–Se and Se–Se situated at 2.53 Å. The average estimated C_N is 2.519. The second peak ascribed to the correlation Se–Se lies at 3.85 Å showing good agreement with other published data. After irradiation, the first peak of T(r) shows a considerable shift towards a small r and a reduction of C_N. On the contrary, the second neighbor data shows a slight increase of r and a great increment of C_N value (5.11 before irradiation against 6.59 after irradiation). Study of the variation of both r and C_N values induced by subsequent annealing of the film is also given. The relative concentrations of the GeGe, GeSe and SeSe bonds, as well as, the number of the GeSe₄ tetrahedral per atom are calculated using the continuous random network (CRN) and the chemically order continuous random network (COCRN) models. These calculations argue the presence of Ge₂(Se_1/2)₆ ethane like unit in addition to Ge(Se₄)_1/2 even with the COCRN model. The formation of dynamical bonds during irradiation of the film under study is suggested. Correlation to volume changes during illumination studied by tight binding molecular dynamics computer simulation has been also considered.     

(Fe,Ti) 16N2 films with high saturation magnetization prepared by facing target sputtering

(Fe,Ti)-N films with a Ti concentration of 10 at.% were prepared on Si(100) and NaCl substrates by facing targets sputtering. The effects of the nitrogen pressure (P_N) and the substrate temperature (T_s) on the formation of various (Fe,Ti)-N phases and their microstructures were investigated in detail. X-ray diffractometer and transmission electron microscope provided complete identification of the phases present in the films and the characterization of their microstructures. Films deposited at a lower P_N = 1 3 × 10⁻² Pa or a lower T_s = RT consist of mainly -phase. Films deposited at a higher P_N = 1.3 2 × 10⁻¹ Pa or a higher T_s = 200 °C contain a great many γ' and Fe₂N phases with a higher nitrogen content. When P_N = 4 7 × 10⁻² Pa and T_s = 100 150 °C, it is advantageous to the formation of ′' phase. These films exhibit a high saturation magnetization (M_s) up to the range of 2.3 2.5 T, which is larger than that of pure iron.     

Electric characterization of HfO2 thin films prepared by chemical solution deposition

Hafnium dioxide (HfO₂) thin films were prepared on Si substrates using the chemical solution deposition (CSD) method. The Au/HfO₂/n-Si/Ag structures were characterized by X-ray diffraction (XRD), C–V curves and leakage current measurements. A relative dielectric constant of about 13.5 was obtained for the 65 nm HfO₂ film. Atomic force microscopy (AFM) measurements show uniform surfaces of the films. C–V hysteresis was found for the metal-oxide-semiconductor (MOS) structures with HfO₂ films of 52 and 65 nm thick. It is found that the width of C–V windows is related with the thickness of the HfO₂ films. Furthermore, the C–V hysteresis reveals the possibility of stress-effect, suggesting that it is possible to use HfO₂ to build an MOS structure with controllable C–V windows for memory devices. The leakage current decreases as the film thickness increases and a relatively low leakage current density has been achieved with the HfO₂ film of 65 nm.     

Bi过量以及冷却方式对BiFeO3-BaTiO3陶瓷的相结构及电学性能的影响

采用传统固相烧结法制备了0.7BiFeO₃-0.3BaTiO₃-xBi₂O₃(0≤x≤0.05)无铅压电陶瓷, 研究了Bi补偿量x和冷却方式对其相结构、微观形貌和综合电学性能的影响。结果表明：所有样品均为菱方相(R)和伪立方相(PC)两相共存, 0≤x≤0.01样品为纯的钙钛矿结构, 且x=0.01样品的两相比例C_R/C_PC接近1; x>0.01样品中出现富Bi杂相Bi₂₅FeO₄₀。与冷却方式相比, 优化Bi补偿量更有利于提升BFBT-xBi₂O₃陶瓷的压电性能。随着x增大, d₃₃先增大后减小, 在x=0.01时获得最优值。由于较小的晶粒、较合适的C_R/C_PC以及较大的残余应变, 水冷BFBT-0.01Bi₂O₃陶瓷获得了最优的压电性能(d_33水冷=141 pC/N、k_p=27%)和高T_C=507℃。研究结果表明, BFBT基陶瓷有希望成为兼具高压电性能和高T_C的无铅压电材料体系之一。     

Effect of ferroic domain pattern changes on the Raman spectra of some ferroic crystals

The influence of changes in the pattern of ferroic domain structure on the Raman spectra of β-LiNH₄SO₄ and (NH₄)₃H(SO₄)₂ single crystals were studied. It was shown that the Raman spectra of β-LiNH₄SO₄ passed from the ferroelastic phase differ from those of “as-grown” crystal and those of the crystal, which was in the paraelectric phase. Significant changes could be observed in the Raman bands related to triply degenerated ν₃ and ν₄ vibrations of the SO₄ tetrahedron. Detailed temperature studies of the Raman spectra of β-LiNH₄SO₄ close to the paraelectric–ferroelectric phase transition, exhibit anomaly of some internal vibrations of SO₄ in the temperature range where a regular large-scale structure is observed. Different types of evolution of the ferroelastic domain structure and temperature behaviour of the donor and acceptor vibrations were shown while heating and cooling the (NH₄)₃H(SO₄)₂ crystal. Different values of temperature hysteresis were found in temperature studies of the ferroelastic domain structure (ΔT_S  3–5 K) and in Raman spectra studies (ΔT_S  12 K). No changes were observed in the pattern of ferroelastic domain structure at the temperature T_II–III  265 K, at which C2/c → P2/n structural phase transition takes place. On the other hand, at T_III–IV  135 K additional domains with W′-type of domain wall orientation were found.     

高居里温度BaTiO3-(Bi0.5Na0.5)TiO3无铅正温度系数电阻陶瓷的制备

用传统的固相反应烧结法制备了(1-xmol%)BaTiO₃-xmol%(Bi_0.5Na_0.5)TiO₃(BBNTx)高温无铅正温度系数电阻( positive temperature coefficient of resistivity, PTCR)陶瓷。X射线衍射表明所有的BBNTx陶瓷形成了单一的四方钙钛矿结构。SEM分析结果显示随着BNT含量的增加, 陶瓷晶粒尺寸减小。空气中烧结的0.2mol% Nb掺杂的BBNT1陶瓷, 室温电阻率为~10² Ω·cm, 电阻突跳为~4.5个数量级, 居里温度为~150℃。氮气中烧结的0.3mol% Nb掺杂的BBNTx(10≤x≤60)陶瓷, 同样具有明显的PTCR效应, 居里温度在180~235℃之间。随着BNT含量的增加, 材料的室温电阻率增大, 同时陶瓷的电阻突跳比下降。     

Determining crystallization kinetic parameters of Li2O–Al2O3–SiO2 glass from derivative differential thermal analysis curves

The kinetic parameters such as crystallization activation energy, E, and the frequency factor, ν, of Li₂O–Al₂O₃–SiO₂ glass were determined by a new non-isothermal method. The method is described by the equation

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, where β is the heating rate and T_f is the inflection-point temperature of differential thermal analysis (DTA). The value of T_f is determined as the maximum peak temperature on derivative differential thermal analysis (DDTA) curves. Values of E and ν of Li₂O–Al₂O₃–SiO₂ glass were also determined by two existing non-isothermal methods, namely the Kissinger plot and the Ozawa plot, and compared with those determined by isothermal method. Values of E and ν determined by the proposed equation were 332 kJ/mol and 1.4×10¹³ s⁻¹, respectively. They are excellent agreement with the isothermal analysis results, 336 kJ/mol and 1.8×10¹³ s⁻¹, respectively. In contrast, both the Kissinger equation and the Ozawa equation give much higher values of E and ν.     

The evolution of a-GaAs1−xNx/c-GaAs interface states as a function of Ar-NH3 plasma

This work concerns investigations on electrical properties of amorphous GaAs_1−xN_x thin films grown on GaAs substrates. Film deposition was carried out by RF sputtering of a GaAs target by adding a nitrogen carrier gas (NH₃) to an Ar plasma. Chemical etching of substrates followed by different plasma treatments (like reverse bias and/or NH₃ glow discharge) prior to film deposition have been studied. The effects of substrate and growth temperature and of total pressure in the reactor have been analysed. Electrical characteristics (C–V and C–V(T)) have enabled us to put in evidence the evolution of interface states of the a-GaAs_1−xN_x/c-GaAs junctions. The amorphous GaAs_1−xN_x thin films are potentially interesting to be considered for GaAs-based MIS structures, due to their relatively high resistivity values, or as passivating layers on GaAs devices.     

Structural and optical properties of (100 − x)(Li2B4O7) − x(SrO–Bi2O3–0.7Nb2O5–0.3V2O5) glasses and glass nanocrystal composites

Glasses of various compositions in the system (100 − x)(Li₂B₄O₇) − x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (10  x  60, in molar ratio) were prepared by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). The amorphous nature of the as-quenched glasses and crystallinity of glass nanocrystal composites were confirmed by X-ray powder diffraction studies. Glass composites comprising strontium bismuth niobate doped with vanadium (SrBi₂(Nb_0.7V_0.3)₂O_9−δ (SBVN)) nanocrystallites were obtained by controlled heat-treatment of the as-quenched glasses at 783 K for 6 h. High resolution transmission electron microscopy (HRTEM) of the glass nanocrystal composites (heat-treated at 783 K/6 h) confirm the presence of rod shaped crystallites of SBVN embedded in Li₂B₄O₇ glass matrix. The optical transmission spectra of these glasses and glass nanocrystal composites of various compositions were recorded in the wavelength range 190–900 nm. Various optical parameters such as optical band gap (E_opt), Urbach energy (ΔE), refractive index (n), optical dielectric constant and ratio of carrier concentration to the effective mass (N/m^*) were determined. The effects of composition of the glasses and glass nanocrystal composites on these parameters were studied.     

Bulk moduli of SiO2 polymorphs: Quartz, coesite and stishovite

By combining both Raman and compression data obtained in static high-pressure experiments, the zero-pressure isothermal bulk modulus (B_T,0) and its first pressure derivative (B′_T,0) at room temperature have been calculated for the three SiO₂ polymorphs without employing an empirical equation of state. For -quartz, T_T,0 = 0.372±0.002 Mb and B′_T,0 = 6.0±0.1; for coesite, B_T,0 = 0.94±0.01 Mb and B′_T,0 =B′_T,0 = 6.0±0.1; for coesite, B_T,0 9.8 ± 0.1; and for stishovite, B_T,0 = 2.95 ± 0.02 Mb and B′_T,0 = 1.3±0.1. The values for -quartz compare very well with those determined in ultrasonic experiments using a single-crystal sample, whereas those for coesite are less constrained due to insufficient compression data. The value of B_T,0 for stishovite is compatible with that determined by single-crystal Brillouin scattering, whilst the value of B′_T,0 for stishovite (1.3 ± 0.1) is the first reliable determination of this parameter.     

Microwave dielectric properties and microstructures of MgTa2O6 ceramics with CuO addition

The effect of CuO addition on the microstructures and the microwave dielectric properties of MgTa₂O₆ ceramics has been investigated. It is found that low level-doping of CuO (up to 1 wt.%) can significantly improve the density of the specimens and their microwave dielectric properties. Tremendous sintering temperature reduction can be achieved due to the liquid phase effect of CuO addition observed by scanning electronic microscopy (SEM). The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. With 0.5 wt.% CuO addition, MgTa₂O₆ ceramic can be sintered at 1400 °C and possesses a dielectric constant (_r) of 28, a Q × f value of 58000 GHz and a temperature coefficient of resonant frequency (τ_f) of 18 ppm/°C.