Effects of heating rate on the magneto-optical properties of bismuth-substituted yttrium iron garnet films prepared via modified metal-organic decomposition

This work investigated the effects of heating rate and annealing on the magneto-optical properties of bismuth-substituted yttrium iron garnet (Bi-YIG) thin films on glass and (111)-oriented single-crystalline gadolinium gallium garnet (GGG) substrates fabricated by metal-organic decomposition (MOD). We modified the MOD method by eliminating the pre-annealing process. We performed annealing at various temperatures to determine the optimal temperature for obtaining the Bi-YIG phase. We then annealed at the optimized temperature using various heating rates. The optimal conditions were annealing for 1 h at 750 °C at a heating rate of 30 °C/min on GGG to obtain highly crystallized fine grains. The Faraday rotation for this film was about −10.5°/μm. The optimized heating rate enhanced the magneto-optical properties due to improved crystallinity and saturated magnetization. The Bi-YIG thin films prepared by this prescribed MOD method exhibited excellent magneto-optical performance and are potential candidates for applications in optical devices.     

Structural comparison of gadolinium and lanthanum silicate films on Si(1 0 0) by HRTEM, EELS and SAED

High-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED) were used to study gadolinium and lanthanum silicate films deposited on Si(1 0 0) substrates using electron-beam evaporation from pressed-powder targets. As-deposited films consist of an amorphous silicate layer without an interfacial layer. After annealing at 900 °C in oxygen for 2 min, an interfacial SiO₂ layer is formed in the gadolinium silicate film, while this interfacial layer is a SiO₂-rich lanthanum silicate layer in the lanthanum silicate film. The formation of interfacial silicate layers is thermodynamically more favorable for the lanthanum films than for the gadolinium films. The gadolinium silicate films crystallize at a temperature between 1000 and 1050 °C, while the crystallization temperature for the lanthanum silicate films is between 900 and 950 °C.     

Austenite grain growth kinetics and mechanism of grain growth in 12Cr ultra-super-critical rotor steel

ABSTRACT

In this paper, the austenite grain growth behaviour of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of heat treatments. The heat treatments at heating temperatures of 900°C–1250°C and holding time of 1?h–20?h were conducted in an electric box-type heating furnace. Experimental results showed that the sizes of austenite grain were affected by heating temperatures and holding time, and heating temperature was the dominant factor. In addition, the grain growth rate changed significantly before and after the turning points of 1050°C and 1250°C. Meanwhile, an austenite grain growth mathematical model was established at different heating temperature stages, and possession of the capability to accurately predict austenite grain size was confirmed. Furthermore, the microstructure of austenite grain in the heating process was observed by optical microscope (OM) and transmission electron microscopy (TEM), which revealed the mechanism of austenite grain growth. Analysis indicated that the change of quantity of precipitate particles with increasing heating temperature was the main reason for the difference in austenite grain growth.     

Densification and microstructure of lead zirconate titanate ceramics fabricated from a triol sol–gel powder

Lead zirconate titanate (PZT) nano-powder was prepared by a triol sol–gel process. X-ray diffraction and transmission electron microscopy results showed that as-synthesized amorphous powder started to crystallize at the calcination temperature above 500 °C. The crystalline powder was formed into pellets and sintered at temperatures between 900 and 1300 °C. Co-existence of tetragonal and rhombohedral phase was observed in all ceramics. Microstructural investigation of PZT ceramics showed that uniform grain size distribution with average grain size of ∼0.8–2.5 μm were received with sintering temperature up to 1200 °C. Further increasing the temperature caused abnormal grain growth with the grain as large as 13.5 μm. An attempt to optimize densification with uniform grain size distribution was also performed by varying heating rate and holding time during sintering. It was found that dense (∼97%) sol–gel derived PZT ceramic with uniform microstructure was achieved at 1100 °C with a heating rate of 5 °C min⁻¹ and 6 h dwell time.     

Growth and characterisation of single grain Al-Cu-Ru icosahedral quasicrystals from self-fluxes

ABSTRACT

In order to obtain high-quality single grains of the Al-Cu-Ru icosahedral quasicrystal (iQC), suitable for a structure analysis, the crystal growth conditions with the self-flux method have been studied. The melts of the master alloys with the compositions of Al_57.0+xCu_39.5-xRu_3.5 (x?=?0, 2.5, 5, 7.5, 10) and Al_62.0Cu_34.0+y Ru_4.0-y (y?=?0, 0.5, 1.5) were held at 1150°C for 2?h, then cooled down to 800, 900, or 1000°C at a rate of ?2?K/h, and subsequently retained for various durations, up to 750?h. Single grain iQCs having several millimetre-sizes, which were evaluated their quality by powder X-ray diffraction (XRD), were grown throughout this study. The peak of (664004) reflection in powder XRD of the iQCs grown at 1000°C has approximately 50% narrower width than that grown at 800°C. The inhomogeneity of the compositions intra- as well as inter-grains grown at 800°C was observed. High-quality single grains with homogeneous composition could be achieved with a long-time annealing at 900°C or regardless of the annealing time at 1000°C. By changing the Al/Cu ratio of the master alloys, the composition could also be controlled for the iQCs grown at 1000°C. Single-crystal XRD experiment with synchrotron radiation on Al_66.6Cu_16.4Ru_17.0 iQC, grown at 1000°C, resulted in the collection of 2680 independent Bragg reflections that confirms the high-quality of the sample. The phase retrieval of the diffraction data resulted successfully in obtaining the structure solution, which reveals some characteristic features of this face-centred iQC structure.     

Production and Characterization of Zirconia (ZrO2) Ceramic Nanofibers by Using Electrospun Poly(Vinyl Alcohol)/Zirconium Acetate Nanofibers as a Precursor

Zirconia (ZrO₂) inorganic ceramic nanofibers were produced using electrospinning of the poly(vinyl alcohol)/zirconium acetate as a precursor followed by calcinating and sintering to decompose the polymer and turn the metal salt (zirconium acetate) into the metal oxide. Characterization of the nanofibers, including polymer thermal decomposition, chemical and crystal structure, phase transformations, and fiber morphology were investigated by simultaneous thermal analysis (STA), thermomechanical analysis (TMA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). The results showed that the polymer decomposition started at 250°C and zirconia nanofibers with different phases (tetragonal and monoclinic) were obtained by the calcination of the precursor nanofibers at various temperatures between 500°C and 1100°C. The initially crystallized zirconia phase, which formed at 500°C, was tetragonal and with increasing calcination temperature, zirconia nanofibers with increasing amount of monoclinic phase were formed. Consequently, at 1100°C, the tetragonal phase disappeared and was transformed to the monoclinic phase of the zirconia completely. Increasing the calcination temperature caused the fiber average diameter decrease and grain growth took place due to the removal of the polymer and organic groups; neighboring grains sintered to each other and formed fibers with a high aspect ratio. At 1100°C the grains size was about the same as the fiber diameter.     

Effect of thermal annealing on the structure and microstructure of TiO2 thin films

Nanostructured TiO₂ thin films have been prepared through chemical route using sol-gel and spin coating techniques. The deposited films were annealed in the temperature range 400–1000°C for 1 h. The structure and microstructure of the annealed films were characterized by GAXRD, micro-Raman spectroscopy and AFM. The as-deposited TiO₂ thin films are found to be amorphous. Micro-Raman and GAXRD results confirm the presence of the anatase phase and absence of the rutile phase for films annealed up to 700°C. The diffraction pattern of the film annealed at 800 to 1000°C contains peaks of both anatase and rutile reflections. The intensity of all peaks in micro-Raman and GAXRD patterns increased and their width (FWHM) decreased with increasing annealing temperature, demonstrating the improvement in the crystallinity of the annealed films. Phase transformation at higher annealing temperature involves a competition among three events such as: grain growth of anatase phase, conversion of anatase to rutile and grain growth of rutile phase. AFM image of the asdeposited films and annealed films indicated exponential grain growth at higher temperature.      

Low-temperature synthesis of Zn2SiO4:Mn green photoluminescence phosphor

Zn₂SiO₄:Mn green phosphor having comparable photoluminescence (PL) efficiency with commercial phosphor has been synthesized at 1000 °C using solid state reactions involving ZnO, silicic acid and manganese acetate. The water of crystallization attached to SiO₂ in silicic acid whose dissociation at 1000 °C seem to promote the sintering efficiency of Zn₂SiO₄:Mn. Incremental ZnO addition and re-firing at 1000 °C promote the diffusion rate of ZnO and SiO₂. The formation of a single crystalline phase of willemite structure in the samples was confirmed by powder XRD measurements. The phosphor exhibit an intense excitation band centered around 275 nm and a relatively weak excitation centered around 380 nm while the broad band green emission peaks at 524 nm. Other parameters studied include PL spectra, grain morphology, ZnO/SiO₂ molar ratio, Mn concentration, co-dopant/flux and the effect of chemical forms of Mn dopant as well as silica on the PL efficiency.     

Li‐, Sb‐ and Ta‐modified (K,Na)NbO3 nanopowder prepared via a water‐based sol–gel method

Stable Li‐, Sb‐ and Ta‐modified (K, Na)NbO₃ (LTS‐KNN) sol and gel were successfully prepared via an economical water‐based sol–gel method. Simultaneous thermogravimetry and differential scanning calorimetry (TG‐DSC) and X‐ray diffraction showed that organic compounds were eliminated and a pure perovskite phase formed around 600 °C. Transmission electron microscopy showed that the LTS‐KNN particle size was in the range of 11–34 nm after decomposition at 600 °C. Moreover, high performance LTS‐KNN ceramic was successfully prepared at a low sintering temperature of 1000 °C by use of the nanopowder, and its room‐temperature d₃₃, K_p, K and loss are 311 pC/N, 46.8%, 1545 and 0.024, respectively. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependences of Young’s modulus of biomorphic silicon carbide ceramics

Temperature dependences of the Young’s modulus E of wood-derived biomorphic SiC ceramics fabricated through pyrolysis of eucalyptus and oak with subsequent silicon infiltration were studied using electrostatic resonance excitation of longitudinal vibrations. The decrease in E with increasing temperature observed to occur in eucalyptus SiC in the temperature interval 20–1000°C was found to be accompanied by several jumps (splittings) in the resonance frequency, which persist after the sample is heated to 1000°C. The oak-SiC ceramic exhibits only one jump, which vanishes after heating to 1000°C. The observed anomalies are assigned to the presence of defects (including pores) in the materials studied.     

Raman scattering studies on nanocrystalline BaTiO3 Part II—consolidated polycrystalline ceramics

BaTiO₃ dense ceramics with different grain sizes from 5.6 µm down to 35 nm were thoroughly studied by Raman spectroscopy. The temperature characteristics of optical phonons were compared with those obtained for powders. The micrograined ceramic revealed the well‐known spectrum profiles and transitions, typical for bulk BaTiO₃. On the other hand, the Raman spectra obtained for a nanograined ceramic with an average grain size of 35 nm revealed a tetragonally distorted pure BaTiO₃ phase showing a diffused phase transition behaviour with respect to temperature. Abnormality of phonon damping characteristics for the nanograined ceramic was demonstrated through comparison with powders with various crystallite sizes and the micrograined ceramic. The Curie temperature of the nanograined ceramic was estimated to be 105 °C from the temperature characteristic of a sharp peak at 307 cm⁻¹, which is one of the most specific tetragonal features for bulk BaTiO₃. In the present study, local stabilization of the tetragonal phase in ultra‐fine grains was experimentally demonstrated from comparison between the Raman spectroscopic results for powders and ceramics prepared through microemulsion‐mediated synthesis. Rather long phonon mean free paths can exist even in such ultra‐fine grains, but the phonon characteristics originating from various grains are diffused mainly because of the effect of internal stress. Copyright © 2007 John Wiley & Sons, Ltd.     

Impedance spectroscopy of vanadium modified BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics

In recent years a wide range of Aurivillius layered materials have been introduced. These novel materials are produced in many various forms such as fibers, thin films as well as bulk by using a number of processing routes. As advanced materials they are they have many interesting properties which include a number of useful electrical properties related to separated grain and grain boundary conductivity, impedance, activation energies, etc. In this paper these properties are described and discussed in detail. The electrical properties of the vanadium doped BaBi₂Nb₂O₉ ceramic was measured over a wide range of temperatures by impedance spectroscopy (IS). The separated grain activation energy, calculated from Arrhenius characteristics at temperatures between room temperature and 600 °C, was 1 eV for 0 at.% of vanadium dopant and 1.2 eV for 10 at.%, whereas the activation energies in the grain boundary region were 0.97 and 1.15 eV, respectively. The obtained results suggest the significant role of vanadium dopant, causing ordering the crystalline structure.     

The room-temperature ferromagnetism of defect-rich ZnMgO semiconductor thin films

The positive temperature coefficient of resistance (PTCR) characteristics of Na₂Ti₆O₁₃ (NT)-doped 0.94BaTiO₃–0.06(Bi_0.5Na_0.5)TiO₃ (BBNT) ceramics were investigated in order to evaluate the effect of NT as a new additive for lead-free PTCR thermistor application. The BBNT ceramic sintered at 1325°C exhibited a relatively high Curie temperature (T _C ) of 158°C while its PTCR characteristic was not satisfactory for thermistor application. However, doping with NT significantly influenced the PTCR behavior of BBNT ceramic. It is considered that NT was responsible for grain growth of the BBNT by forming a liquid phase during sintering due to its low melting temperature of 1300°C. The grain growth resulted in the enhanced PTCR characteristics of BBNT ceramic. In particular, 0.1 mol% NT doped BBNT ceramic exhibited excellent PTCR performance of low resistivity at room temperature (1.6×10² Ω cm), resistivity increase near T _C (1.28×10⁴) and high T _C of 158°C, suitable for lead-free PTCR thermistor application.     

Effect of calcium substitution on structural,dielectric, ferroelectric,piezoelectric, and energy storage properties of BaTiO3

The effect of calcium substitution on the structural, dielectric, ferroelectric, piezoelectric, and energy storage properties of BaTiO₃ (BT) ceramics has been investigated. XRD confirmed the phase formation of Barium Calcium Titanate (BCT), and structural Rietveld refinement was used to estimate the lattice parameters. It is evident from the SEM data that the average grain size decreases as calcium is added. At Curie temperature (110 °C), BCT 0.10 ceramic has a good dielectric constant of 15834 and a very low dielectric loss of 0.009. According to the ferroelectric and piezoelectric investigations, BCT 0.10 exhibits maximum spontaneous polarization with the highest piezoelectric charge coefficient of 100 pC/N. BCT 0.10 has a maximum energy storage density of 96.8 mJ/cm³ and a good energy storage efficiency of 53.9%, which is around three times that of pure BaTiO₃. These results suggest that the BCT ceramic has good potential for energy storage applications.     

Mechanosynthesis of YIG and GdIG: A Structural and Mössbauer Study

We have investigated the mechanosynthesis of gadolinium and yttrium iron garnets by high-energy ball-milling of α-Fe₂O₃ and Gd₂O₃ or α-Fe₂O₃ and Y₂O₃, respectively, followed by short thermal annealings conducted at moderate temperatures. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy, in order to determine the influence of the milling time and annealing conditions on the final products. For as-milled samples of each rare-earth system, the results revealed the formation of perovskite phases, in relative amounts that depend on the milling time. The formation of garnet phases was observed in as-annealed samples treated at 1000°C for 2 h or 1100°C for 3 h, i.e., at very modest annealing requirements when compared with ordinary solid-state-reaction processes performed without previous high-energy milling. Also, the occurrence was verified of a milling time for which the relative amount of garnet phases formed by annealing was maximized. This time depends on the rare-earth composing the garnet phase and on the annealing temperature.     

Fractal dimension of the grain boundaries in ceramics with nanodispersed additions

The fractal dimension of the grain boundaries in Al₂O3-MgO-SiO₂ corundum ceramic is measured. It is shown for the first time that a similarity exists between the aggregation of solid disperse particles and the grain formation in the ceramic and that this similarity can be used to reveal grain formation mechanisms. The fractal dimension of the grain boundaries is found to be 1.68 and 1.42 at sintering temperatures of 1200 and 1600°C, respectively. These values correspond to primary recrystallization and normal grain growth in the ceramic. A relationship between the fractal dimension of the grain boundaries and the sintering temperature of the corundum ceramic is obtained.     

Raman and X‐ray diffraction analysis on unburned carbon powder refined from fly ash

The bonding and crystalline structures of oil‐fried fly ash collected from a power plant were analyzed by using Raman spectroscopy and X‐ray diffraction (XRD), respectively. These carbon powders underwent a series of annealing treatment for graphitization and crystallization. In Raman spectra, the refined, unburned carbon contains clearly the D, G and D′ peaks under 2000 or 2700 °C treatment. The Raman spectral line shape of refined, unburned carbon heated at 2700 °C is similar to that of commercial graphite made from graphitization process. In the XRD spectra, the refined, unburned carbon contains (002), (100) and (004) diffraction peaks under heating at 2000 or 2700 °C. The XRD spectral line shape of refined, unburned carbon heated at 2700 °C is similar to that of a commercial graphite bar. The quantitative graphitization level from Raman spectra and crystallization degree from XRD spectra on refined, unburned carbon powders serve as preliminary guide for the qualitative evaluation of these unburned carbon powders. Copyright © 2009 John Wiley & Sons, Ltd.     

Mössbauer and X-Ray studies of MgO-Fe nanocomposite

XRD phase analysis and Mössbauer spectroscopy are used to study the structure of magnesiowustite (Mg_0.9Fe_0.1) obtained via the decomposition of mixed iron-magnesium oxalate in different atmospheres, the structure of MgO-α-Fe composite after reduction by hydrogen in Ar + 5% H₂ gas mixture at 800°C and 1000°C, and the structure of iron at every stage. It was shown that fine particles of α-Fe are obtained upon the decomposition of iron-magnesium oxalate in vacuum at 1000°C. If a precursor is decomposed in high purity Ar, α-Fe particles form during reduction at lower temperature (800°C) due to the partial decomposition of one of the phase components—magnetite.     

Template growth of vertically aligned carbon nanotubes using self-assembled monolayers of SiO2 particles by Langmuir–Blodgett technique

The thermal stability of nanocrystalline 3R-CuCrO₂ obtaining by hydrothermal method was investigated by annealing treatment, XRD, FT-IR, XPS and TG. The three temperature domains corresponding to thermal stability of 3R-CuCrO₂ nanocrystals (25–400 °C), destabilization of nanocrystalline 3R-CuCrO₂ phase (400–800 °C) and recrystallization of 3R-CuCrO₂ in microcrystalline state over 800 °C, were determined by the specific hydrothermal synthesis conditions. This study has indicated that nanocrystals with delafossite structure synthesized by hydrothermal method exhibit nanocrystalline state up to a reasonably high temperature, about 390 °C, which could be interesting for technical applications and the classical theory of the grain growth.     

Structure of the amorphous phase of pyrolisates of lanthanum diphthalocyanine according to X-ray scattering data

Data on X-ray diffraction in lanthanum diphthalocyanine pyrolysates synthesized at temperatures of 800–1800°С demonstrate the formation of an amorphous carbon phase with embedded lanthanum atoms. Low-temperature pyrolysis (800–900°С) creates layered carbon structures. Due to annealing at 1000°С, carbon integrates into globules whose number of atoms is m ~ 100. Such structures with gyration radii of R _g ~ 0.4–0.5 nm on the order of the precursor molecule size are synthesized in the temperature range of 1000–1800°С, and are stable in terms of size and mass. In this case, their density approaches that of graphite.