Pulsed laser deposition-induced reduction of SrTiO3 crystals

We report a generic method for fast and efficient reduction of strontium titanate (SrTiO₃, STO) single crystals by pulsed laser deposition (PLD) of thin-films. The reduction was largely independent of the thin-film material deposited on the crystals. It is shown that thermodynamic conditions (450 °C, 10^?7 torr, 10–60 min), which normally reduce STO (0 0 1) substrates to roughly 5 nm into a crystal substrate, can reduce the same crystals throughout their 500 μm thickness when coupled with the PLD. In situ characterization of the STO substrate resistance during thin-film growth is presented. This process opens up the possibility of employing STO substrates as a back-gate in functional oxide devices.     

Exchange bias and magneto-resistance in an all-oxide spin valve with multi-ferroic BiFeO3 as the pinning layer

Modern microscopy techniques indicate that the electrical switching of magnetic domains in multi-ferroic materials is possible. However, the application of such functionality in a real device has yet to be proven. In this work we fabricated an all-oxide spin valve with the ferroelectric anti-ferromagnet BiFeO₃ (BFO) as the pinning layer. The multi-layered heterostructure was grown epitaxially on a (0 0 1) SrTiO₃ substrate and magneto-resistance was achieved at room temperature, which was switchable magnetically in a similar way to conventional metallic spin valves. Some key physical and material issues for building up such a novel device were addressed, in particular the hetero-epitaxy-induced strain effects on the electrical and magnetic properties of each layer and the establishment of exchange bias between BFO and an oxide ferrimagnet, e.g. Zn_0.7Ni_0.3Fe₂O₄ (ZNFO). The strains caused a significant increase in the coercivity but a decrease in the saturation magnetization of the ferrimagnet used. The former is particularly undesirable because it increases the required switching field. The all-oxide architecture allowed the spin valve to be field annealed from a temperature above the high Néel point of BFO (～660 K), after which a very large exchange bias field (H_ex) was achieved at 5 K and kept at a decent value at room temperature. The H_ex–T curve did not follow the widely observed (1 ? T/T_N)^β temperature dependence, but could be explained by the random field model with one-dimensional (1-D) anti-ferromagnetic sublattice magnetization derived from the spin wave theory. Based on the observed 1-D spin wave behavior and the geometric arrangements of the paramagnetic ions at the (0 0 1) surface we propose an atomic model in which only a part of the spin along the diagonal lines in the BFO (0 0 1) surface was strongly exchange coupled with ZNFO.     

BiFeO3 thin films of (1 1 1)-orientation deposited on SrRuO3 buffered Pt/TiO2/SiO2/Si(1 0 0) substrates

BiFeO₃ (BFO) thin films of varying degrees of (1 1 1) orientation were successfully grown on SrRuO₃-buffered Pt/TiO₂/SiO₂/Si(1 0 0) substrates by off-axis radio-frequency magnetron sputtering. They demonstrate much enhanced ferroelectric behavior, including a much enhanced remnant polarization (2P_r ～ 197.1 μC cm^?2 at 1 kHz) measured by positive-up negative-down (PUND), at an optimized deposition temperature of 590 °C. The effects of film deposition temperature on the degree of (1 1 1) orientation, film texture, ferroelectric behavior, leakage current and fatigue endurance of the BFO thin films were systematically investigated. While the degree of (1 1 1) orientation is optimized at 590 °C, the defect concentration in the film increases steadily with increasing deposition temperature, as demonstrated by the dependence of leakage behavior on the deposition temperature. The polarization behavior is shown to strongly depend on the degree of (1 1 1) orientation for the BFO thin film. Oxygen vacancies are shown to involve in the conduction and dielectric relaxation of the BFO thin films deposited at different temperatures, as demonstrated by their dielectric and conduction behavior as a function of both temperature (in the range 294–514 K) and frequency (in the range 10^?1–10⁶ Hz).     

Surface modeling and chemical solution deposition of SrO(SrTiO3)n Ruddlesden–Popper phases

Strontium titanate (STO) is a preferred substrate material for functional oxide growth, whose surface properties can be adjusted through the presence of Ruddlesden–Popper (RP) phases. Here, density functional theory (DFT) is used to model the (1 0 0) and (0 0 1) surfaces of SrO(SrTiO₃)_n RP phases. Relaxed surface structures, electronic properties and stability relations have been determined. In contrast to pure STO, the near-surface SrO–OSr stacking fault can be employed to control surface roughness by adjusting SrO and TiO₂ surface rumpling, to stabilize SrO termination in an SrO-rich surrounding or to increase the band gap in the case of TiO₂ termination. RP thin films have been epitaxially grown on (0 0 1) STO substrates by chemical solution deposition. In agreement with DFT results, the fraction of particular RP phases n = 1–3 changes with varying heating rate and molar ratio Sr:Ti. This is discussed in terms of bulk formation energy.     

Highly (1 0 0)-textured Pb(Zr0.52Ti0.48)O3 film derived from a modified sol–gel technique using inorganic zirconium precursor

Highly (1 0 0)-textured Pb(Zr_0.52Ti_0.48)O₃ films have been prepared on platinized silicon substrate by a modified sol–gel technique using inorganic zirconium precursor. The X-ray diffraction analysis on the crystallinity and texture evolution of sol–gel lead zirconate titanate (PZT) films revealed that the films were well crystallized to perovskite phase when annealed at 550 °C, and that highly (1 0 0) preferred orientation dominated in the PZT films after annealed at 650 °C. The (1 0 0)-oriented PZT film exhibited the remnant polarization of 26.3 μC/cm² and the coercive field of 100 kV/cm.     

Thermal stability of Ti3SiC2 thin films

The thermal stability of Ti₃SiC₂(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to ∼1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti₃SiC₂ by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti₃C₂ slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti₃C₂ slabs into (1 1 1)-oriented TiC_0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.     

Discrepancies in the morphology and physical properties of amorphous and crystalline sprayed lanthanum nickel oxide films

Amorphous LaNiO₃ (a-LNO) and crystalline LaNiO₃ (c-LNO) films were prepared by spraying an aqueous precursor solution of lanthanum and nickel chlorides on hot (450 °C) fused silica substrates followed by annealing at high temperatures (550–850 °C). Thermal analysis of a dried precursor indicated that a stable oxide phase is formed at 560 °C with no distinct crystallization peak. Scanning electron microscopy (SEM) powered with energy-dispersive X-ray spectroscopy (EDX) of as-sprayed films showed rough surfaces with particulate-like deposits and incomplete pyrolysis chloride composition. No chloride contents were detected in annealed films. X-ray diffraction showed that films annealed at 550 °C and 650 °C were a-LNO and those annealed at 750 °C and 850 °C were c-LNO. The c-LNO phase was indexed as a single-phase perovskite structure with (1 1 0) orientation. SEM/EDX showed that a-LNO films have rough surfaces and c-LNO films have uniform crack-free smooth surfaces. Electrical properties measurements showed that c-LNO films have lower resistivity than a-LNO films and both types of LNO films have semiconductor resistant temperature dependence. The activation energy of electric conduction of a-LNO films was found to be much higher than that of c-LNO films. The optical transmittance and reflectance of the films were studied in the UV–visible–near IR range. The optical constants were obtained by modeling the measured transmission and reflection spectra. Because of the discrepancies in the morphology and in the physical properties of a-LNO and c-LNO films, the best fit modeling of transmission and reflection spectra was obtained by using different theoretical models and different geometrical configurations. While the Drude model accounting for larger carrier density was found to be significant for c-LNO, using the Bruggmann model and a configuration of a rough layer on top of a compact film was found to be significant for a-LNO.     

Thermal stability of the Pr3Pt4 compound

The temperature stability of the Pr₃Pt₄ compound was investigated by isothermal annealing at different temperatures, X-ray diffraction (XRD) and differential thermal analysis (DTA). It was found that the decomposition reaction Pr₃Pt₄ → PrPt + PrPt₂ took place at temperatures ranging approximately from 360 °C to 830 °C and it was an exothermal reaction. After annealing at 500–750 °C for 7 days, the Pr₃Pt₄ compound decomposes completely into the two neighboring compounds PrPt and PrPt₂.     

Shifting of the morphotropic phase boundary and superior piezoelectric response in Nb-doped Pb(Zr, Ti)O3 epitaxial thin films

A shift of the morphotropic phase boundary (MPB) and a superior piezoelectric response are observed in Nb-doped Pb(Zr_xTi_1?x)O₃ (PNZT) thin films epitaxially grown on Nb-doped SrTiO₃(1 0 0) (Nb:STO) substrates. X-ray diffraction and Raman spectra characterizations confirm that a phase transition from a tetragonal structure to a rhombohedral structure occurs when the Zr/Ti ratio varies from 20/80 to 80/20. The phenomenological theory and experimental analyses suggest that the MPB of epitaxial PNZT thin films is shifted to the higher Zr/Ti ratio (around 70/30) from the conventional ratio (52/48) due to the misfit compressive stress induced by the substrate. A maximum local effective longitudinal piezoelectric coefficient (d₃₃) up to 307 pm V^?1 is observed at a Zr/Ti ratio of 70/30 in the current compositional range, again confirming the shifting of MPB in epitaxial PNZT thin films. These findings offer a new insight for the fabrication of epitaxial PZT thin films at MPB with a superior piezoelectric response.     

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi₂ and MoSi₂ layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi₂ coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi₂ coatings. Oxidation of the NbSi₂-coated γ-TiAl (NbSi₂/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi₂ coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.     

Preparation and characterization of Ca0.18Na0.32Bi0.50TiO3 ferroelectric thin films by metalorganic solution deposition

Ca_0.18Na_0.32Bi_0.50TiO₃ (CNBT) ferroelectric thin films were prepared by metalorganic solution deposition on silicon substrate and annealed at different temperatures. The morphology and structure of the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystal structure of Ca-doped Na_0.50Bi_0.50TiO₃ films shows no obvious lattice distortion compared with that of un-doped one. The optimal heat treatment process for CNBT films were determined to be high-temperature drying at 400 °C for no less than 15 min followed by annealing at 600 °C for 5 min, which leads to the formation of compact films with uniform grains of 30–50 nm. Ferroelectric property measurement shows that the remanent polarization of CNBT films is 18 times higher than that of un-doped Na_0.50Bi_0.50TiO₃ (NBT) thin films.     

Epitaxial growth of γ-Fe2O3 thin films on MgO substrates by pulsed laser deposition and their properties

Epitaxial γ-Fe₂O₃ films were fabricated by pulsed laser deposition at 350 °C in an oxygen-rich atmosphere onto a (0 0 1) or (1 1 0) MgO substrate utilizing the substrate template effect, while the corundum structure α-Fe₂O₃ was obtained when the same experiment was conducted using sapphire or quartz substrate. X-ray photoelectron spectroscopy analysis and low-temperature SQUID measurements confirmed the formation of γ-Fe₂O₃. After annealing at 500 °C for 1 h under oxygen atmosphere, the γ-Fe₂O₃ phase was still maintained. The saturation magnetization (M_s) of the γ-Fe₂O₃ film was around 400 emu cm^?3 for films 10–50 nm thick, which is in agreement with the bulk value. The ultrathin films showed an enhanced Ms value (489 emu cm^?3). In particular, the M_s of the 5 nm thin film did not diminish even if it was subjected to high-temperature annealing due to the stabilizing effect of the epitaxial growth. The thin films obtained had a flat surface, which is desired for spin filter and other applications.     

Erosion wear behaviour of plasma sprayed NiCrSiB/Al2O3 composite coating

In this work, 60 wt.% NiCrSiB–40 wt.% Al₂O₃ composite coating was produced on AISI 304 substrate material using the atmospheric plasma spraying technique. The coating surface has been characterised using a scanning electron microscope (SEM), optical microscope and X-ray diffractometer (XRD). The microhardness, porosity, density and surface roughness of the coating were measured. The adhesion strength of the coating was measured using pull off adhesion tester. The erosion behaviour of plasma sprayed coating was studied at 450 °C using hot air jet erosion testing machine. The erosion rate of coated and uncoated samples was evaluated at 30° and 90° erodent impact angles. The SEM images of the eroded samples were taken to analyse the erosion mechanism. The test results reveal that the coating protects the substrate at both 30° and 90° impact angles.     

Synthesis of V8C7–Cr3C2 nanocomposite via a novel in-situ precursor method

V₈C₇–Cr₃C₂ nanocomposite has been synthesized by a novel in-situ precursor method, and the raw materials are ammonium vanadate (NH₄VO₃), ammonium dichromate ((NH₄)₂Cr₂O₇) and glucose (C₆H₁₂O₆). The products were characterized by thermogravimetric and differential scanning calorimetry (TG-DSC), X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that V₈C₇–Cr₃C₂ nanocomposite with an average crystallite size of 31.5 nm can be synthesized at 900 °C for 1 h. The powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 100 nm. The weight loss ratio of the precursor throughout the reaction process reaches 70 wt.%, and it changes rapidly before 400 °C (about 35 wt.%). Four endothermic peaks and three exothermic peaks occur during the reaction. The surface of the specimen is mainly composed of V, Cr, C and O four elements. The synthesis temperature of V₈C₇–Cr₃C₂ nanocomposite by the method (900 °C) is 500 °C lower than that of the conventional method (1400 °C).     

Roles of interface roughness and internal stress in magnetic anisotropy of CoPt/AlN multilayer films

Magnetic anisotropy of CoPt/AlN multilayer films has been studied by systematically varying the nominal thickness of CoPt layers, t_CoPt (1–10 nm), and the annealing temperature, T_a (300–500 °C). The as-deposited films show in-plane magnetic anisotropy in the full range of t_CoPt, whereas the annealed films show perpendicular magnetic anisotropy (PMA) within small t_CoPt but change to in-plane magnetic anisotropy when t_CoPt is over a certain thickness. The critical thickness for such anisotropic transformations increases as the T_a increases. The maximum PMA obtained in this work is 1.13 × 10⁷ erg cm^?3. The interface roughness was analyzed by cross-sectional high-resolution electron microscopy and X-ray reflectivity using an abrupt interface model with a Debye exponent shape. The internal stress was analyzed by X-ray diffraction using an equal biaxial stress model. The results show that the CoPt/AlN interface roughness decreases from 0.385 nm to 0.158 nm and the internal stress increases from ?2.36 GPa (compressive) to 1.73 GPa (tensile), as the T_a increases to 500 °C. The roles of the interface roughness and the internal stress in the magnetic anisotropy of CoPt/AlN multilayer films are studied.     

Oxidation behavior of LPS-SiC ceramics sintered with AlN/Y2O3 as additive

In this work, the oxidation behavior of SiC ceramics sintered with additives based on AlN–Y₂O₃ system was investigated. SiC ceramics doped with different AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.% were sintered at 2080 °C for 1 h under nitrogen atmosphere, obtaining ceramics with relative density near to 96% in both compositions. Samples were oxidized at 1200 °C, 1300 °C or 1400 °C in air for up to 120 h. Oxidation was monitored by the weight gain of the samples as function of exposition time and temperature. A parabolic growth of the oxidation layer has been observed and the coefficient of the growth rate has been determined by relating the weight gain and the surface area. In oxidation testing performed at 1200 °C, samples containing lower Y₂O₃ amounts showed greater oxidation resistance; however, by raising the temperature (to 1400 °C), the samples containing higher Y₂O₃ amounts showed greater oxidation resistance. The oxidized layer characterized by X-ray diffraction presented SiO₂ and Y₂Si₂O₇ as crystalline phases. Furthermore, the activation energy for oxidation of 780 kJ/mol and 405 kJ/mol for AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.%, respectively.     

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH^? and H₂O in the KN unit cell, tetragonal KNbO₃ (KN) nanowires were formed when the synthesis was carried out at 120 °C for 48 h. However, when the fabrication was conducted at high temperatures (?150 °C) or at 120 °C for a long period of time (?72 h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120 °C for 60 h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb⁵⁺-doped SrTiO₃ substrate, and their d₃₃ values were measured for the first time. A tetragonal KN nanowire exhibited a d₃₃ value of 23.5 pm V^?1, which is larger than that of the orthorhombic KN nanowire (11.6 pm V^?1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d₃₃ value of 40.0 pm V^?1. The d₃₃ values of KN nanowires increased to 104.5, 137.1 and 146.0 pm V^?1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [1 0 0] direction by application of a DC voltage of 10 V.     

Phase transition behavior and electrical properties of [(K0.50Na0.50)1−xAgx](Nb1−xTax)O3 lead-free ceramics

The effect of AgTaO₃ on the electrical properties of (K_0.5Na_0.5)NbO₃ lead-free ceramics was systematically investigated, and the phase transition behavior of the ceramics was also studied in terms of high temperature X-ray diffraction. The experimental results show that Ag⁺ and Ta⁵⁺ ions diffuse into the (K_0.5Na_0.5)NbO₃ lattices to form a stable solid solution with orthorhombic structure, and also lead to the decrease in the orthorhombic to the tetragonal phase transition temperature and the Curie temperature. The 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramics exhibit optimum electrical properties (d₃₃ = 183 pC/N, k_p = 41%, T_c = 356 °C, T_o–t = 158 °C, ?_r ～ 683, and tan δ ～ 3.3%) and good thermal-depoling behavior. The piezoelectric properties of (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramics are much superior to pure (K_0.5Na_0.5)NbO₃ ceramics. X-ray diffraction patterns for the 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramic at different temperatures indicated a pure perovskite phase with an orthorhombic structure at below 160 °C, a tetragonal structure at 160–350 °C, and a cubic structure at above 360 °C. As a result, the (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramic is one of the promising candidate materials for lead-free piezoelectric ceramics.     

Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi₂ was produced. However with continued milling for 48 h α-MoSi₂ and Mo₅Si₃ phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi₂. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi₂ to α-MoSi₂ and the formation of Mo₅Si₃. The crystallite size of α-MoSi₂ and Mo₅Si₃ were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.     

Effect of Fe2O3 on Sm-doped ceria system solid electrolyte for IT-SOFCs

The effect of Fe₂O₃ addition (0–2.5 mol%) on the densification, crystal structure, ionic conductivity, and aging behavior of Ce_0.8Sm_0.2O_1.9 (SDC) was studied. The addition of Fe₂O₃ promotes densification, reducing sintering temperature by ～100–150 °C. X-ray diffraction showed that these materials exhibit a fluorite structure; a second phase of Fe₂O₃ is identified when Fe₂O₃ content was ≥1.5 mol%. Impedance spectroscopy measurements indicated that SDC with 0.25 mol% Fe₂O₃ has the highest conductivity, about 10% higher than that of SDC at 700 °C. Conversely, a reduction in conductivity is observed in all samples after aging in air at 800 °C for 120 h. Because of the harmful effect of aging, conductivity rapidly decreases as Fe₂O₃ content in the samples exceeded 0.25 mol%. However, SDC with 0.25 mol% Fe₂O₃ shows the same magnitude of decrease in conductivity compared with that of SDC after aging. This indicates that SDC with 0.25 mol% Fe₂O₃ continues to present the best conductivity even after high-temperature aging.