Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)

Epitaxial Bi₂Sr₂Co₂O_y thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO₃ substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi₂Sr₂Co₂O_y films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step-terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi₂Sr₂Co₂O_y films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

Structural and transport properties of SrRuO3 thin films grown by MOCVD on (0 0 1) SrTiO3 substrates: The role of built-in strain and extra phases

Pure SrRuO₃ (SRO) thin films and SRO thin films containing the extra metallic phases Ru, RuO₂ and Sr₃Ru₂O₇ were deposited by MOCVD on (0 0 1) SrTiO₃ substrates under different conditions (Ru/Sr and Ar/O₂ ratio in the gas phase, substrate temperature, supersaturation). The single-phase compressively-strained SRO film is of high structural quality and shows a ferromagnetic transition at a suppressed Curie temperature (T_c) of about 142 K and low electrical resistivity (230 μΩ cm). Under certain deposition conditions Ru and RuO₂ extra phases form leading to a reduced room temperature resistivity of 100 μΩ cm. On the other hand, the presence of Sr₃Ru₂O₇ increases the resistivity to 385 μΩ cm. We have observed that the existence of the extra phases caused a slight shift of T_c towards the bulk value, while relaxation of the lattice strain resulted in increase of T_c to 160 K. The deviation from the stoichiometric composition in films with extra phases is also confirmed by the residual electrical resistivity ratio. On the other hand, the pure SRO films, the compressively strained and the plastically relaxed exhibit a stoichiometric ratio.     

Microwave direct synthesis of MgB2 superconductor

Intermetallic compound superconductor MgB₂ was synthesized from spherical magnesium powder and lower purity amorphous boron powder by microwave direct heating. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is homogeneous and the particle size is about several hundreds of nanometers. The onset superconducting transition temperature of the MgB₂ sample measured by the temperature dependence of magnetization measurement is about 37.6 K. The critical current density J_c calculated according to the Bean model are about 2.0 × 10⁵ A/cm² at 20 K in self-field and 1.0 × 10⁵ A/cm² at 20 K in 1 T applied field.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Epitaxial growth and exchange coupling of spinel ferrimagnet Ni0.3Zn0.7Fe2O4 on multiferroic BiFeO3

Thin films of the zinc nickel ferrite, Zn_0.7Ni_0.3Fe₂O₄ (ZNFO), were deposited by the RF magnetron sputtering on a number of substrates, including (001) oriented single crystals of LaAlO₃ (LAO) and SrTiO₃ (STO), polycrystalline Pt/Si, and epitaxial films of BiFeO₃ (BFO) and LaNiO₃ (LNO). Except for the films on Pt/Si, the ZNFO films grown on other substrates were epitaxial and their magnetic properties were affected by the heteroepitaxy induced strains. Typically, the coercivity (H_c) was increased with the strain, i.e. H_c varied from 31 Oe for the 150 nm thick polycrystalline films grown on Pt/Si, to 55 Oe and 155 Oe for the 20 nm thick epitaxial films grown on BFO and LAO, respectively. The saturation magnetization of the epitaxial films was reduced accordingly to about 470 emu/cm³ from 986 emu/cm³ in the polycrystalline films. The all-oxide architecture allowed field-annealing to perform at the temperature above the Neel temperature of BFO (~ 370 °C), after which clear exchange bias was observed.     

Characteristics of NixFe100−x films deposited on SiO2/Si(1 0 0) by DC magnetron co-sputtering

Ni_xFe_100−x films with a thickness of about 200 nm were deposited on SiO₂/Si(1 0 0) substrates at room temperature by DC magnetron co-sputtering using both Fe and Ni₈₀Fe₂₀ targets. Compositional, structural, electrical and magnetic properties of the films were investigated. Ni₇₆Fe₂₄, Ni₆₅Fe₃₅, Ni₆₀Fe₄₀, Ni₅₅Fe₄₅, Ni₄₉Fe₅₁ films are obtained by increasing the sputtering power of the Fe target. All the films have a fcc structure. Ni₇₆Fe₂₄, Ni₆₅Fe₃₅, Ni₆₀Fe₄₀ and Ni₅₅Fe₄₅ films grow with crystalline orientations of [1 1 1] and [2 2 0] in the direction of the film growth while the Ni₄₉Fe₅₁ film has the [1 1 1] texture structure in the direction of the film growth. The lattice constant of the film increases linearly with increasing Fe content. All of the films grow with thin columnar grains and have void networks in the grain boundaries. The grain size does not change markedly with the composition of the film. The resistivity of the film increases with increasing Fe content and is one order of magnitude larger than that of the bulk. For all the films the magnetic hysteresis loop shows a hard magnetization. The Ni₇₆Fe₂₄ film has the lowest saturation magnetization of 6.75×10⁻² T and the lowest saturation field of 8.36×10⁴ A/m while the Ni₄₉Fe₅₁ film has a largest saturation magnetization of 9.25×10⁻² T and the largest saturation field of 1.43×10⁵ A/m.     

Synthesis and electrochemical properties of Ti doped Li3 − xFe2 − xTix(PO4)3/C cathode materials

Li_3 − xFe_2 − xTi_x(PO₄)₃/C (x = 0-0.4) cathodes designed with Fe doped by Ti was studied. Both Li₃Fe₂(PO₄)₃/C (x = 0) and Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C (x = 0.2) possess two plateau potentials of Fe³⁺/Fe²⁺ couple (around 2.8 V and 2.7 V vs. Li⁺/Li) upon discharge observed from galvanostatic charge/discharge and cyclic voltammetry. Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C has higher reversibility and better capacity retention than that of the undoped Li₃Fe₂(PO₄)₃/C. A much higher specific capacity of 122.3 mAh/g was obtained at C/20 in the first cycle, approaching the theoretical capacity of 128 mAh/g, and a capacity of 100.1 mAh/g was held at C/2 after the 20th cycle.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoxCo2xBi1 − 3xO3

Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCo^II_xCo^III_2xBi_1 − 3xO₃ (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO₃ thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ₂₃), thermistor constant (B_25/85) and activation energy are in the range of 1.995-2.975 Ω cm, 1140-1234 K and 0.102-0.111 eV, respectively.     

Microwave characterization of laser ablated Y1Ba2Cu3O7−x thin films

In the present study we report the measurements of microwave surface resistance (R_s) of YBCO thin films on LaAlO₃ substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T_c(R = 0) of the films is 90 K and J_c > 10⁶ A/cm² at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R_s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R_s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R_s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R_s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.     

Investigations on TlBa2Ca2Cu3Ox superconducting thin films formation on LaAlO3 substrate

Investigations on pure superconducting phase TlBa₂Ca₂Cu₃O_x (Tl-1223) thin films formation, of about 100-125 nm in thickness, on (001) LaAlO₃ single crystal substrate, were made using radio-frequency sputtering deposition of Ba₂Ca₂Cu₃O_x precursor films and ex-situ thallination in sealed quartz tube. The precursor films were thallinated under different conditions of partial oxygen pressure, temperature, time and y thallium source content using unreacted pellets of composition Tl_yBa₂Ca₂Cu₃O_x. In all cases, strongly c-oriented multiphase films were obtained. A correlation between the Tl-1223 phase purity and the precursor film conditions of thallination is established. Temperature and time of thallination as well as the thallium source content and the partial pressure of oxygen play a key role in the quality of the obtained film. The films' onset temperature of the superconducting transition ranges between 90 and 103 K. It is shown that the best samples can be obtained from a dense precursor film and relatively medium thallination time.     

A comparative study of the properties of thermally evaporated CuIn2n + 1S3n + 2 (n = 0, 1, 2 and 3) thin films

CuInS₂, CuIn₃S₅, CuIn₅S₈ and CuIn₇S₁₁ compounds were synthesized by the horizontal Bridgman method using high-purity copper, indium and sulphur elements. Crushed powders of these ingots were used as raw materials for the vacuum thermal evaporation. So, CuIn_2n + 1S_3n + 2 (n = 0, 1, 2, and 3) thin films were deposited by single source vacuum thermal evaporation onto glass substrates heated at 150 °C. The structural, compositional, morphological, electrical and optical properties of the deposited films were studied using X-ray diffraction (XRD), energy dispersive X-ray, atomic force microscopy and optical measurement techniques. XRD results revealed that all the films are polycrystalline. However, CuInS₂ and CuIn₃S₅ films had a chalcopyrite structure with preferred orientation along 112 while CuIn₅S₈ and CuIn₇S₁₁ films exhibit a spinel structure with preferred orientation along 311. The absorption coefficients of the all CuIn_2n + 1S_3n + 2 films are in the range of 10⁻⁴ and 10⁻⁵ cm⁻¹. The direct optical band gaps of CuIn_2n + 1S_3n + 2 layers are found to be 1.56, 1.78, 1.75 and 1.30 eV for n = 0, 1, 2, and 3, respectively. CuIn₃S₅ and CuIn₅S₈ films are p type with electrical resistivities of 4 and 12 Ω cm whereas CuInS₂ and CuIn₇S₁₁ are highly compensated with resistivities of 1470 and 1176 Ω cm, respectively.     

Electrical conductivity of 5 mol.% Yb2O3 and 5 mol.% Gd2O3 co-doped Sm2Zr2O7

Sm₂Zr₂O₇ co-doped with and without 5 mol.% Yb₂O₃ and 5 mol.% Gd₂O₃ were prepared by a pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectra measurements. Both Sm₂Zr₂O₇ and (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramics exhibit a single phase of pyrochlore-type structure. The grain conductivity, grain-boundary conductivity and total conductivity obey the Arrhenius relation, respectively, and gradually increase with increasing temperature from 723 to 1173 K. (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramic is the oxide-ion conductor in an oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The grain conductivity, grain-boundary conductivity and total conductivity of (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ with dual Yb³⁺ + Gd³⁺ doping are higher than those of undoped Sm₂Zr₂O₇ at identical temperature levels.     

Double electrodeposition and heat treatment process for the fabrication of superconducting oxides on the intensified {1 1 3} <1 2 1> Ag substrate

The Tl-1223 coated conductor (Tl_0.8Pb_0.2Bi_0.2Sr_1.8Ba_0.2Ca_2.2Cu₃O_δ) was synthesized by using electrodeposition on the intensified {1 1 3} <1 2 1> Ag substrate applying the new method of the repeating electrodeposition/heat treatment. The films were fabricated conducting the first electrodeposition and then heat treatment, after that the second electrodeposition on the first electrodeposition/heat treatment coated conductors. The second electrodeposition on the first electrodeposition/heat treatment/the first electrodeposition/heat treatment coated conductor showed the better quality of the Tl-1223 phases than that of the first electrodeposition/heat treatment coated conductor, showing more compact and dense grains on the films. The thin Tl-1223 films caused by the thinning process were discussed by considering the properties of Tl and the epitaxial growth aspects. The purer Tl-1223 grains obtained at the double electrodeposition/heat treatment are due to the growth from Tl-1223 grains already synthesized during the first electrodeposition/heat treatment, facilitating the epitaxial growth easier than that of the Ag substrate. The second electrodeposition process was successfully performed, obtaining 1.9 × 10⁵ A/cm² of J_c at 0 T at 10 K.     

High-temperature thermoelectric properties of non-stoichiometric Ag1 − xInTe2 with chalcopyrite structure

Our group has revealed that AgGaTe₂ with the chalcopyrite structure exhibits relatively high thermoelectric (TE) figure of merit (ZT) when it has a deviation of Ag content from the stoichiometry. The maximum ZT value of 0.77 was obtained for Ag_0.95GaTe₂ at 850 K. On the other hand, the TE properties of AgInTe₂ with the same chalcopyrite structure with AgGaTe₂ have not been investigated. In the present study, we examined the high-temperature TE properties of AgInTe₂ with the composition of Ag_1 − xInTe₂ (x = 0, 0.01, 0.03, and 0.05). The deviation of Ag content from stoichiometry slightly enhanced the ZT value. The maximum ZT value was 0.07 at 600 K obtained in the samples of x = 0.03 and 0.05.     

Josephson transport in Tl-cuprate bicrystal weak links

Superconducting epitaxial Tl₂Ba₂CaCu₂O₈ (Tl-2212) and Tl₂Ba₂CuO₆, (Tl-2201) thin films are synthesised on single crystal and bicrystal LaAlO₃ substrates by an ex-situ process. Tl-2212 films were optically smooth, with T_cs from 105 to 108K and T_c <2K, J_c reaching 2·10⁶A/cm² at 77K. Tl-2201 films exhibited tetragonal or orthorhombic symmetry depending on the thallium content. The T_c of the as-synthesised Tl-2201 films was in the range 40–55K. By oxygen depletion, T_c was increased by 15 – 20 K. Properties of weak links in Tl-2212 and Tl-2201 thin films are reported.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Basic properties of Sr1 − xBaxSi2

Basic properties, such as the phase relationship, crystal structure, and energy gap E_g, have been investigated in Sr-rich Sr_1 − xBa_xSi₂. Sr_1 − xBa_xSi₂ (0 ≤ x ≤ 1.0) has two phases: one with the SrSi₂-type structure and another with the BaSi₂-type structure. The SrSi₂ phase exists at x ranging from 0 to 0.13, and the BaSi₂ phase exists at x ranging from 0.24 to 1.0. The volume increases with x in both the SrSi₂ and BaSi₂ phases. A volume jump of 13.7% appears at the structural phase transition from the SrSi₂ phase to the BaSi₂ phase. E_g increases with x in SrSi₂-phase Sr_1 − xBa_xSi₂ but E_g decreases with x in the BaSi₂-phase Sr_1 − xBa_xSi₂. In Sr-rich BaSi₂-phase Sr_1 − xBa_xSi₂, Ba atoms at a specific crystallographic site, the A1 site, are preferentially substituted by Sr atoms, as well as in Ba-rich BaSi₂-phase Sr_1 − xBa_xSi₂.     

Distribution of pyrochlore phase in Pb(Mg1/3Nb2/3)O3-PbTiO3 films and suppression with a Pb(Zr0.52Ti0.48)O3 interfacial layer

Thin films of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) on Pt/Ti/SiO₂/Si (Pt/Si) substrates both with and without a Pb(Zr_0.52Ti_0.48)O₃ (PZT) interfacial layer were investigated. Perovskite and pyrochlore coexistence was observed for PMN-PT thin films without a PZT interfacial layer. Interestingly, most of the pyrochlore phase was observed in single-coated films and in the first layer of multi-coated films. The pyrochlore phase exhibited grains with an average size of about 25 nm, which is smaller than those of the perovskite phase (about 90 nm). In contrast, for PMN-PT thin films grown on a PZT interfacial layer, the formation of a pyrochlore phase at the interface between PMN-PT layers and the substrate is completely suppressed. Moreover, small grains are not observed in the films with a PZT interfacial layer. The measured polarization-electric field (P-E) hysteresis loops of PMN-PT films with and without PZT layers indicate that enhanced electrical properties can be obtained when a PZT interfacial layer is used. These enhanced properties include an increase in the value of remanent polarization P_r from 2.7 to 5.8 μC/cm² and a decrease in the coercive field E_c from 60.5 to 28.0 kV/cm.