Growth of Ba2Re(Y,Pr)Ru1‐xCuxO single crystals in a high temperature solution drop using an infrared imaging system

Infrared imaging furnace was used to grow single crystals of Ba₂YRu_1‐xCu_xO₆ and Ba₂PrRu_1‐xCu_xO₆ using high temperature solutions of PbO‐PbF₂ in the form of a bubble attached to the feed rods. Several small crystals were found deposited on the cooler portions of the sintered rod as well as in the drop like portion at the end of the rod. These crystals were collected and the morphology, microstructure and magnetic properties were studied. The details of these investigations are presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fast sinter-crystallization of a glass from waste materials

A glass belonging to the CaO-Al₂O₃-SiO₂ system and corresponding to the melting of a mixture of industrial inorganic waste (feldspar mining residues, lime from fume abatement systems of the glass industry and recycled soda-lime glass) has been successfully transformed into dense and strong sintered glass-ceramics, even for very short holding times (30 min at 960 °C) and a very rapid heating, consisting of direct insertion of pressed fine glass powders in furnace (‘fast sinter-crystallization’). The addition of kaolin clay, conceived as binder for pressed glass powders, proved to positively influence the phase balance, the homogeneity and the degree of crystallization of fast sintered glass-ceramics, thus justifying the achievement of remarkable mechanical properties (bending strength exceeding 100 MPa, micro-hardness exceeding 7 GPa).     

Co nanoclusters as origin of ferromagnetism in sol‐gel synthesized Zn1‐xCoxO (x = 0.05, 0.10 and 0.15) samples

We report on the structural and magnetic properties of the polycrystalline samples of Zn_1‐xCo_xO (x = 0.05, 0.10 and 0.15) synthesized via sol‐gel route. The air sintered samples of all compositions exhibit paramagnetic behaviour at room temperature, on the other hand the same samples on annealing in Ar/H₂ atmosphere show room temperature ferromagnetism (RTFM) with enhanced magnetization. The value of magnetization increases with the Co concentration (x) in both the air sintered and Ar/H₂ annealed samples. The observed ferromagnetism in the Ar/H₂ annealed Zn_1‐xCo_xO (x = 0.05, 0.10 and 0.15) samples is attributed to the presence of Co nanoclusters as detected by XRD and FESEM‐EDAX. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of TeO2 nanowires via a facile thermal oxidation method

TeO₂ is of considerable interest for piezoelectric, ther‐moelectric, acousto‐optic and nonlinear optical applica‐tions. In this work, we report a facile synthetic tech‐nique for obtaining high‐quality TeO₂ nanowires (NWs) via a thermal oxidation of Te NWs in a large quantity. The synthetic conditions were systematically optimized to transform from Te NWs to TeO₂ NWs using a furnace oven. The optimal conditions for the formation of high‐quality TeO₂ NWs were found to be 300 °C for 1 hr in an oven furnace. The NWs were characterized using X‐ray diffraction, field‐emission scanning electron mi‐croscopy, high resolution transmission electron micros‐copy and Raman scattering. Diffuse reflectance spec‐trum was also used to determine the band gap (3.78 eV) of synthesized TeO₂ NWs.     

Effect of Sintering Temperature on Thermoelectric Power of Mixed Lithium?Zinc Ferrites

Thermoelectric power of polycrystalline lithium-zinc ferrites having the compositional formula Li_0.5−x/2Zn_xFe_2.0−x/2O₄ (where × = 0.2, 0.4, 0.6, 0.8, and 1.0), sintered at 1200, 1250 and, 1300°C has been investigated over the temperature range 300 to 450 K by the probe method. The Seebeck coefficient (Q) is found to decrease with increasing sintering temperature, whereas the carrier concentration (n) and the charge carrier mobility (μ) are found to increase with increasing sintering temperature. Among all the mixed lithium-zinc ferrites, the one having a zinc content of 0.6 mole has a minimum value of Seebeck coefficient and maximum value of charge carrier concentration.     

Micrometric single crystal germanates obtained using a double‐spherical mirror furnace

Micro‐crystals of two new compounds EuMnGe₂O₇ and SmMnGe₂O₇ were grown performing the flux method in a double‐spherical mirror furnace. One valuable advantage of this system was that the heating profile could be modified easily adjusting lamp positions and orientation as well. The micrometric crystals were observed and analyzed for chemical composition by scanning electron microscopy and energy dispersive spectroscopy. This furnace is perfectly suitable to grow at low price, low temperature and short time new materials as a single crystal for basic research or to obtain raw material. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of foreign phases on the growth of NaBi(WO4)2 crystals

Single crystals of NaBi(WO₄)₂ (NBW) have been grown by Czochralski technique, employing differently synthesized starting charge and different axial temperature gradients. The causes behind inherent problems related to compositional changes and associated lowering in crystallization temperature have been probed by analyzing XRD and DTA patterns of post growth residual charge. Presence of low melting phases viz. Na₂WO₄ and hitherto unreported compound Na₅Bi(WO₄)₄ is thought to be responsible for the observed decrease in crystallization temperature. This problem was tackled by segregating pure phase material through re‐crystallization, under high axial temperature gradient. The use of re‐crystallized charge enabled transformation of almost the entire charge into a single crystal of high transparency. The effect of starting charge synthesis and temperature gradient on the optical transmission characteristics of NBW crystal has also been investigated. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and electrical conduction of Sb2O3 thin films

Antimony trioxide (Sb₂O₃) thin films have been deposited onto glass substrates using thermal evaporation technique at room temperature. The structural feature and surface morphology were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Sandwich‐type structures were deposited with films thickness d = 0.55 μm using evaporated electrodes of silver. Current‐voltage (J‐U) characteristics have been measured at various fixed temperatures in the range 293‐473 K. In all cases, at low electric field (E <10⁴ V/cm), ohmic behavior is observed. However, at high electric field (E >10⁴ V/cm), non‐ohmic behavior is observed. An analysis of the experimental data indicates that in the range of high‐applied electric field, the dominant conduction mechanism is space charge limited currents (SCLC). Using the relevant SCLC theory, the carrier concentration, total trap concentration and the ratio of free charge to trapped charge have been calculated and correlated with changes in the structures of antimony trioxide thin films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Design of Double‐Ellipsoid Mirror Furnace and its Thermal Characteristics for Floating‐Zone Growth of SrxBa1‐xTiO3 Single Crystals

We have designed a double ellipsoid mirror furnace for floating‐zone crystal growth using lamps with rectangular filaments. Its thermal characteristics were studied using an alumina tube for several system configurations. A simple comparison with a commercial furnace that used cylinder lamps for the heating profile was also conducted. By adjusting lamp orientation and positions, one could modify heating profiles easily. In general, the thermal characteristics of the furnace were consistent with the model's prediction [J. Crystal Growth 173 (1997) 561]. The effects of growth chamber and heat pipe were further illustrated. Furthermore, a suitable system configuration leading to better heating uniformity and lower thermal gradients near the growth interface was found for the floating‐zone growth of Sr_xBa_1‐xTiO₃ single crystals.     

Microwave dielectric properties of (1‐x)La(Mg0.5Sn0.5)O3‐x(Sr0.8Ca0.2)3Ti2O7 ceramic system with a near zero temperature coefficient of resonant frequency

In this study, the microwave dielectric properties of (1‐x)La(Mg_0.5Sn_0.5)O₃‐x(Sr_0.8Ca_0.2)₃Ti₂O₇ ceramic system prepared by the conventional solid‐state method have been investigated for application in mobile communication. It was found that the diffraction peaks of (1‐x)La(Mg_0.5Sn_0.5)O₃‐x(Sr_0.8Ca_0.2)₃Ti₂O₇ ceramic system shift to higher angles as x increases from 0.2 to 0.4. It was also found that the X‐ray diffraction patterns of the 0.8La(Mg_0.5Sn_0.5)O₃‐0.2(Sr_0.8Ca_0.2)₃Ti₂O₇ ceramics exhibited no significant phase difference at different sintering temperatures. The average grain size of the (1‐x)La(Mg_0.5Sn_0.5)O₃‐x(Sr_0.8Ca_0.2)₃Ti₂O₇ ceramic system decreased from 6.4 to 4.3 μm as the value of x increased from 0.2 to 0.4 sintered at 1550 °C for 4 h. The dielectric constant increased from 26.6 to 35.9 and the quality factor (Q×f) decreased from 31,600 to 23,300 GHz for (1‐x)La(Mg_0.5Sn_0.5)O₃‐x(Sr_0.8Ca_0.2)₃Ti₂O₇ ceramic system as the x value increases from 0.2 to 0.4 sintered at 1550 °C for 4 h. The average value of temperature coefficient of resonant frequency (τ_f) increased from ‐18 to +8 ppm/ K as the x value increases from 0.2 to 0.4. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bridgman growth of langasite‐type piezoelectric crystals

Three langasite family crystals of Sr₃Ga₂Ge₄O₁₄ (SGG), Ca₃NbGa₃Si₂O₁₄ (CNGS), and Sr₃NbGa₃Si₂O₁₄ (SNGS) were successfully grown by the modified Bridgman method. Among them, SGG crystals up to 2 inches were obtained with the multi‐crucible industrial Bridgman furnace; SNGS crystal grown in any orientation direction other than along a‐axis was realized. Commercially availability SGG boules and the advantage in SNGS crystal indicated that the modified Bridgman technique is a prospective method to realize the mass‐production of the langasite‐type crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Liquid phase epitaxy set‐up designed for in situ X‐ray study of SiGe island growth on (001) Si substrates

In situ X‐ray examination at a synchrotron beamline of the solution growth of self‐assembled SiGe structures on silicon (001) substrates through the backside has been realized by a specific heating equipment and a suitable growth assembly. The furnace allows heating of the growth assembly up to 600 °C. The temperature field and the gas flow in the furnace have been numerically modeled. In this way a meaningful estimate about the power consumption and the thermal gradient across the sample has been reached. Despite its low heat capacity and, thus, fast heating and cooling ability the furnace can be stabilized to ± 0.1 K by a high‐performance temperature controller. The growth assembly has been prepared within three separate stages carried out in conventional slideboat liquid phase epitaxy equipment. Such growth assembly allows carrying out then intended experiments without H₂ as normally used in liquid phase epitaxy in favor of N₂, meeting the demand of minimized risks at beamlines. The equipment ensures an easy handling of the growth assembly. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,structure and electrochemical performance of cobaltite‐based composite cathodes for IT‐SOFC

The development of novel and high‐performance cathodes is a critical issue to be addressed in order to reduce Solid Oxide Fuel Cells (SOFCs) operation temperature to the 600‐800 °C range or less. The performance of CeO₂‐based composite cathodes is very attractive to such operational temperatures. In this work, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and Ce_0.8Sm_0.2O_1.9 (SDC) powders were synthesized by different synthesis methods and mechanically mixed to prepare LSCF‐SDC composite cathodes. Screen‐printed LSCF‐SDC/CGO/LSCF‐SDC symmetrical cells were sintered at 1150 °C for 4 h and characterized by electrochemical impedance spectroscopy in static air. X‐ray diffraction and scanning electron microscopy were employed to characterize the powders. Area specific resistance values of 0.72 and 2.77 Ω cm² at 800 °C were found for composite cathodes containing SDC powder synthesized by modified Pechini and microwave‐assisted combustion methods, respectively. Furthermore, the activation energy of the composite cathode containing SDC derived from modified Pechini method is 1.18 eV, i.e., much lower than 1.73 eV, value determined for LSCF with SDC from microwave‐combustion method.     

Investigation on the blue photorefractive properties varied with MgO codoping in Ru:Fe:LiNbO3 crystals

Mg:Ru:Fe:LiNbO₃ crystals with various doping concentration of MgO have been grown by Czochralski method. The type of charge carriers and photorefractive properties in Mg:Ru:Fe:LiNbO₃ crystals were measured by two‐wave coupling method using Kr⁺ laser (476 nm) and He‐Ne laser (633 nm) as light sources. We found that holes were the dominant charge carriers under blue light irradiation while electrons were the dominant charge carriers under red light irradiation. Mg²⁺ ions behaved no longer as damage resistant, but promoter to the photorefractive properties at 476 nm wavelength. The photorefractive properties under blue light improved with the increase concentration of Mg²⁺ ions. The enhancement mechanisms of the blue photorefractive were suggested. Experimental results definitely showed that Mg‐doped two‐centre Ru:Fe:LiNbO₃ was a promising blue photorefraction material for holographic volume storage.     

Correlation Between the Impurity Content,the Average Charge State of Chromium Cations and Optical Absorption in Sillenite Crystals Doped by Chromium in Wide Range of Concentrations

Single crystals of sillenites (Bi₁₂SiO₂₀, Bi₁₂TiO₂₀) doped with chromium in a wide range of concentrations from 1 x 10^‐5 up to 1.8 x 10^‐2 wt. % were grown by the Czochralski and top‐seed solution growth (TSSG) techniques. To estimate the content and the average charge state of chromium in the grown crystals the chemical analysis by the modified diphenylcarbazide method was applied. The dependencies of the chromium distribution coefficient, the average charge state of Cr cations, and optical absorption on the Cr concentration were found. Both the shift of the absorption edge toward lower frequencies and the appearance of an additional absorption band in the near IR were observed when the chromium concentration in the crystals was increased. The experimental data suggest that chromium has at least two charge states and occupies probably different positions in the sillenite unit cell.     

Microwave dielectric properties of La(1‐2x/3)Bax(Mg0.5Sn0.5)O3 ceramics

This study examined the potential applications of microwave dielectric properties of La_(1‐2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics in rectenna. The La_(1‐2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid‐state method with various sintering temperatures. An apparent density of 6.62 g/cm³, a dielectric constant of 20.3, a quality factor of 51,700 GHz, and a temperature coefficient of resonant frequency of ‐78.2 ppm/K were obtained for La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sol-gel processing of TeO2-TiO2-PbO thin films

TeO₂-TiO₂-PbO thin films were prepared by sol-gel processing from tellurium(IV) isopropoxide precursor and their transmittance spectrum was measured, from which the refractive index was calculated. The hydrolysis of tellurium(IV) isopropoxide and the decomposition process of its hydrolysis product were investigated. The discrepancy between the observed hydrolysis behavior of tellurium(IV) isopropoxide and the partial charge model on the reactivity of metal alkoxides toward hydrolysis, being calculated with the Pauling electronegativity, is explained by a combination of the inductive and steric effects of isopropoxide groups and the electronic configuration of the tellurium atom with lone pair electrons.     

Growth of ZnO single crystals by an induced nucleation from a high temperature solution of the ZnO‐PbF2 system

To grow ZnO single crystals from a high temperature solution of the ZnO‐PbF₂ system, a gas cooling system was assembled at the bottom of the crucible to induce nucleation in the initial growth stage. The growth experiments were carried out in a homemade vertical Bridgman furnace and Pt crucible of 28 mm in diameter was used. The furnace temperature was set to 1100°C and the flow rate of the oxygen gas was optimized as 3.0 l/min. ZnO crystal up to 5∼8mm in the thickness was obtained with the lowering rate of 0.3 mm/h. XRD patterns showed that the as‐grown crystal was pure ZnO Wurtzite phase. The impurity ions were analyzed by the glow discharge mass spectroscopy (GDMS) as 390.0 ppm and 40.0 ppm for Pb²⁺ and F^‐, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impedance spectroscopy of Na and Nd doped strontium bismuth titanate

Sr_0.2Na_0.4Nd_0.4Bi₄Ti₄O₁₅ is prepared by Sol‐Gel route using the modified polymeric precursor method based on the Pechini process and its electrical properties are investigated by Complex Impedance Spectroscopy (CIS). Results are presented in two ways, complex plane plots (real versus imaginary) and spectroscopic plots (real and imaginary against frequency). CIS has provided convincing evidence for the existence of both grain (bulk) and grain‐boundary effects that were separated in the frequency domain of the impedance spectrum. Experimental data are fitted to two parallel RC equivalent circuits in series. The values of resistance and capacitance of bulk and grain boundary contributions are also calculated. Relaxation times are calculated for both grains and grainboundaries. Conduction and charge carrier behaviors are studied from frequency and temperature dependence of ac conductivity. The ac conductivity follows a two‐power law behavior at different temperatures with frequency exponents s₁ and s₂. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)