Zinc Doping Profile in AlGaAs/GaAs Heteroepitaxial Structures Grown by Metalorganic Chemical Vapor Deposition

The Zn profile in Al _x Ga_{1 – x} As/GaAs (x = 0.2–0.4) quantum-well heteroepitaxial structures doped during growth by metalorganic chemical vapor deposition is modeled with allowance made for the diffusional broadening of the nominal doping profile. Experimentally determined carrier distributions in the heterostructures are used to refine the diffusion coefficient of Zn at a growth temperature of 770°C. The average value of D _Zn is determined to be 6.0 × 10^–14 cm²/s. The position of the p–n junction in Al _x Ga_{1 – x} As/GaAs heterostructures is assessed as a function of the nominal Zn profile and growth rate. The ways of optimizing the doping profile are outlined.     

Preparation and Properties of Pb1 – xSnxTe1 – ySeyEpilayers

Pb_{1 – x} Sn _x Te_{1 – y} Se _y layers lattice-matched with KCl and BaF₂are grown by liquid-phase epitaxy. Epilayer compositions and growth temperatures are determined at which p- and n-type materials can be obtained without doping. The composition dependences of the Hall coefficient and carrier mobility are analyzed.     

Preparation and electrochemical characterization of Sr- and Mn-doped LaGaO3 as anode materials for LSGM-based SOFCs

Sr- and Mn-doped lanthanum gallate powders (LSGMn, La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} , x = 0.20, 0.35, 0.43) were prepared by glycine-nitrate combustion synthesis. X-ray diffraction patterns indicate the perovskite structure was formed without any second phase after calcining the powders at 1000°C for 4 h. Compacts of powders synthesized under stoichiometric combustion were sintered to densities over 95% of theoretical values. The electrical conductivity of this material in both air and H₂ were characterized using AC impedance spectroscopy. It showed that the conductivity in H₂ atmosphere is lower than that in air due to p-type electrical conduction in this material, and the electrical conductivity increases remarkably with increasing manganese content. Good chemical stability of La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} in H₂ atmosphere as well as the relatively high conductivity makes it an appropriate anode material for Sr- and Mg-doped lanthanum gallate (LSGM)-based IT-SOFCs. Preliminary fuel cell performance measurements were performed, showing promising electrochemical properties of such anode materials.     

Luminescence of Eu3+-Activated Potassium Scandium and Potassium Yttrium Phosphate Vanadates

The photoluminescence spectra of potassium rare-earth phosphate vanadates K₃R_{1 – y} Eu _y (PO₄) _x (VO₄)_{2 – x} (R = Sc, Y) were measured in the range 450–800 nm under excitation at 337 nm. The energies of the Stark components of the ⁷ F _j(j= 0, 1, 2) multiplet were determined, and the crystal-field parameters were evaluated. The effects of the Eu³⁺concentration and PO₄ ^3–/VO₄ ^3–ratio on the luminescence of the materials studied and the concentration quenching of luminescence were analyzed. The materials with high Eu³⁺concentrations are shown to be potentially attractive as photo- and cathodoluminophors.     

Thermoelectric power in Gd3+-substituted Cu-Cd ferrites

Polycrystalline Cd _x Cu_1−x Fe_2−y Gd _y O₄ ferrites fory=0·0 and 0·1 were prepared by ceramic technique. X-ray diffractograms of powder samples show cubic symmetry withx⩾0·2 fory=0·0 and 0·1, while compositions withx=0·0 fory = 0·0 and 0·1 are tetragonal. The thermopower measurements for Gd³⁺-undoped ferrites in the temperature range 300 K to 788 K shown-type conductivity forx⩾0·2. The substitution of Gd³⁺ changedn-type conductivity of the compositions top-type. The mobilities calculated show decreasing trend on Gd³⁺ substitution. The values of activation energy ΔE and drift mobilityE _d suggest polaron formation in substituted samples. The conduction mechanism is explained on the basis of localized model and formation of Gd³⁺+Fe²⁺ stable pairs at B site and Cu¹⁺+Fe³⁺ at A site.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

Synthesis of La-Doped CaTiO3 Perovskite Powders from the Mixture of Metal Salt Solutions by Ultrasonic Mist Combustion/Pyrolysis Processes

The CaTiO₃ and La-doped CaTiO₃ [Ca_1–x (La) _x TiO₃, x = 0.05–0.3] powders were prepared from aqueous solutions by the ultrasonic mist pyrolysis and ultrasonic mist combustion processes. Glycine provides carboxylic acid and amine groups as a fuel in the ultrasonic mist combustion process. In ultrasonic mist pyrolysis, the particles with hollow sphere morphology were obtained; whereas, particles prepared by the ultrasonic mist combustion process had a dense solid morphology with low porosity. The ultrasonic mist combustion process using metal nitrates and glycine as the fuel for a starting material has proved to be a simple and unique approach to preparing dense CaTiO₃ powder and a solid solution of CaTiO₃ with lanthanum.     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

Structure and electrical characteristics of Ce4+-doped Ba2In2O5

The In-site of Ba₂In₂O₅ with Brownmillerite structure was partially substituted for Ce⁴⁺ ions in order to examine the doping effect on the order-disorder transition. Ba₂In_{2 – x} Ce _x O_{5 + x/2} (x = 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5) were prepared by solid state reaction. X-Ray diffraction analyses of these powder samples demonstrated that Ba₂In_{2 – x} Ce _x O_{5 + x/2} (x = 0.1 and 0.2) possesses Brownmillerite structure. With increasing content of Ce⁴⁺ ion the crystal system of Ba₂In_{2 – x} Ce _x O_{5 + x/2} (x = 0.3, 0.5, and 1.0) changed to cubic perovskite structure above the order-disorder transition temperature of Ba₂In₂O₅. Arrhenius plots of the electrical conductivities of Ba₂In_{2 – x} Ce _x O_{5 + x/2} (x = 0.2, 0.3, and 1.0) exhibited no discontinuity. These compounds had high transference numbers of oxide ion above 973 k.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

Characterization of wet processed (Ni, Zn)-ferrites for CO2 decomposition

Ultrafine (Ni, Zn)-ferrites were prepared by two different methods of coprecipitation and hydrothermal synthesis, and their oxygen-deficient ferrites (ODF) produced by hydrogen reduction were investigated on the efficiency of CO₂ decomposition. The crystalline sizes of (Ni, Zn)-ferrites were less than 30 nm with high Brunauer–Emmett–Teller (BET) surface areas, ranging from 77 to 172 m² g^–1. The (Ni, Zn)-ferrites by hydrothermal synthesis resulted in smaller crystalline sizes, higher BET surface areas and better efficiencies of CO₂ decomposition than by coprecipitation. Compared with the binary NiFe₂O_4– ferrite, the ternary (Ni _x , Zn_1–x ) Fe₂O_4– ferrites showed higher efficiency for CO₂ decomposition, indicating a potential catalyst for the reduction of CO₂ emission in the environmental atmosphere.     

A new route to synthesize La1?xSrxMnO3

Self-Propagating High-temperature Synthesis (SHS) was used to producecomplex oxides (La_1–x Sr _x MnO₃ with x = 0, 0.1 and 0.2), which are used as the cathode in solid oxide fuel cells (SOFCs). Thermodynamic predications and experiments show that La_1–x Sr _x MnO₃ can be prepared via SHS under moderate conditions from a mixture of La₂O₃ + SrO₂ + Mn, using either gaseous oxygen or solid NaClO₄ as the oxidant. Partial melting at the high combustion temperature increased product homogeneity. The electrical conductivity of the product was 180 S·cm^–1 at 1000°C in air, matching that of sample made by other synthesis processes. SHS enables a more economical production of La_1–x Sr _x MnO₃ than existing commercial processes.     

Synthesis of new apatite phases by spray pyrolysis and their characterization

This paper describes the synthesis of some apatite compounds, called britholites, by an aerosol pyrolysis technique, which compared to conventional methods of solid-state chemistry, presents the advantages of reducing noticeably the temperature and the duration of the treatment. Britholites are minerals based on Ca_{10 – x} Ln _x (PO₄)_{6 – x} (SiO₄) _x X ₂ formula (with X = F^–, OH^–, X ₂ = O^2–); they are promising materials for nuclear waste storage. In a second part we have synthesized and characterized a new apatite phase where the substitution of calcium by barium in britholite leads to a monophased solid solution Ca_{9 – x} Ba _x Nd(PO₄)_{6 – x} (SiO₄)F₂ for 6 < x < 9. The possible interest of this new material is discussed.     

New ionic conductors Ln2 + xTi2 ? xO7 ? x /2 (Ln = Dy?Lu, x = 0.132)

New oxygen-ion conductors Ln_{2 + x}Ti_{2 – x}O_{7 – x/2} (Ln = Dy–Lu, x = 0.132) with a disordered pyrochlore structure are obtained. Their ionic conductivity attains 10^-3 S/cm at 740°C, owing to the presence of defects in both the cation and anion sublattices. The materials contain ~5% Ln_Ti antisite defects. Ln_2.132Ti_1.868O_6.934 ceramics are shown to be stable in the temperature range 1600–1700°C.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1501–1504.Original Russian Text Copyright © 2004 by Shlyakhtina, Mosunov, Stefanovich, Karyagina, Shcherbakova.     

Thermodynamic Functions of Cu1 �C x Zn x Fe2O4 Ferrite Solid Solutions from 300 to 900 K

Cu_{1 – x} Zn _x Fe₂O₄ ferrite solid solutions were prepared by the ceramic route. As shown by chemical analysis, x-ray diffraction, and thermogravimetry, all of the synthesized materials were suitable for calorimetric studies. The heat capacity of the solid solutions was measured from 300 to 900 K using a DSC-111 differential scanning calorimeter. The results were used to determine the enthalpy, entropy, and Gibbs energy of the ferrites in the range 300–900 K.     

Possibilities for formation of the 110 K phase 2223 in Sb or V doped Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O materials

Superconducting oxide materials with nominal composition Bi_2–x Sb _x Sr₂Ca₂Cu₃O_y (x=0.05–0.3) and Bi_1.6Pb_0.4–x V _x Sr₂Ca₂Cu₃O_y (x=0.1 0.4) were synthesized. It was found that after prolonged synthesis, formation of the 2223 phase in the Bi-Sb-Sr-Ca-Cu-O system is possible. However, the critical temperatures of the samples are around 90 K and are lower than those of the superconducting materials from the Bi-Pb-Sr-Ca-Cu-O and Bi-Pb(Sb)-Sr-Ca-Cu-O systems. It was also found that V inhibits the formation of the phase 2223, raises the resistance of the samples and has a negative effect on theT _c and the phase composition of the Bi-Pb(V)-Sr-Ca-Cu-O materials.     

Structural and Raman analyses of the (A1−x Pb x ) TiO3(A=Ca,Sr, Ba) perovskites

Powder X-ray diffraction and laser Raman spectroscopy were used to study systematically the structural and dynamical changes in three series of plumbous perovskites of composition Ba_1–xPb_xTiO₃Sr_1–xPb_xTiO₃ and Ca_1–xPb_xTiO₃ with 0 <x < 1. For the latter two series prominent phase transitions were found atx ca. 0.5 and 0.55, respectively. As well, Raman spectroscopy provided the means to track the evolution of active vibrational modes characteristic of the end-members across the doping range. The temperature dependence of the Raman spectra showed that the ferroelectric transition temperature of the (Ca, Pb) series can be tailored over a wide temperature range by judicious doping.     

Structure of cobalt and fluorine doped hexaferrites

Hexagonal ferrites in the BaO-Fe₂O₃- CoO system have the M, Y, Z, W and S structures. BaFe_12–x CO _x O_19–x prepared by the glass synthesis method crystallizes with the M structure but forx > O a few CoFe₂O₄ (S) crystallizes at first and then is located at the core of the recovered platelets of hexaferrite M. BaFe₁₆– _x CO_{2 +x} O_27-x F _x prepared in the saure way has not the W structure but is a mixture of cobalt doped M and S.     

Machinable and mechanical properties of sintered Al2O3-Ti3SiC2 composites

Two-phase composites consisting of (1 – x) Al₂O₃ and xTi₃SiC₂ (x = 0–1) were prepared by spark plasma sintering (SPS). Sintered densities larger than 98% of theoretical density were achieved when the specimens were sintered at 1300°C for 5 min (in vacuum, at pressure 30 MPa). When content of Ti₃SiC₂ increased up to 30 wt%, composites were found to be machinable—they could be drilled easily using conventional Fe-Mo-W drills or gravers. The mechanical properties of the (1 – x) Al₂O₃–xTi₃SiC₂ composites were evaluated. The bending strength, Vickers hardness of the specimens had the following ranges: 428 ± 10.2 (x = 0) to 673 ± 15.4 Mpa (x = 1) (bending strength at room temperature); 19.9 (x = 0) to 4.0 GPa (x = 1) (Vickers hardness).     

Molecular Dynamics Simulations of Ba1 – x Gd x F2 + x Solid Solutions over a Wide Temperature Range: II. Structural Characteristics and Fluoride Ion Diffusion

The atomic-scale structure (pair correlation functions and anion-density distribution) and fluoride-ion diffusion in Ba_{1 – x} Gd _x F_{2 + x} solid solutions are investigated over a wide temperature range, including the superionic transition, by molecular dynamics simulations with Born–Mayer–Huggins pair potentials. According to the simulation results for x > 0.1, both the 4b(cubic site symmetry) and 48i positions accommodate fluorine interstitials (F^– _i). Below the superionic transition, the migration of F^– ions at low doping levels is dominated by noncollinear jumps between lattice and interstitial sites. Computer simulations confirm that trigonal Gd³⁺–F^– _i dipolar complexes prevail over tetragonal complexes. It is shown using elements of Monte Carlo simulation that the rare-earth ions in the solid solutions have a tendency to aggregate.