Formation Of Low-temperature Phase Research Articles

Comparative studies of La and Co modified BiFeO3 (0.05) prepared by sol–gel versus conventional solid state reaction method

In this article, we looked at how the multifunctional characteristics of a multiferroic BiFeO3 were affected by the synthesis process. La and Co modified BiFeO3 powders with x= 0.05 were prepared by conventional solid-state reaction and sol–gel route. The X-ray Diffraction patterns of both the samples, prepared by sol–gel and solid-state route show single-phase formation. It has been observed that sol–gel route attains the low temperature phase formation as compared to the solid state route. The density was optimized by sintering these samples at different loads. Room temperature dielectric plots with the frequency of these samples revealed that the dielectric constant decreases with frequency, and dielectric loss in the solid state reaction (SSR) prepared sample is smaller than the sol–gel route (SGR) prepared sample. The effects of processing on dielectric properties of La and Co modified BiFeO3 have been studied in the different frequency and temperature range from 300 K (RT) to 550 K. Dielectric anomalies in temperature dependent dielectric plots have been observed in both of the samples.

НИЗКОТЕМПЕРАТУРНЫЕ ФАЗОВЫЕ РАВНОВЕСИЯ В БИНАРНЫХ СИСТЕМАХ И ПОЛУЧЕНИЕ ФУНКЦИОНАЛЬНЫХ МАТЕРИАЛОВ

The problems of studying low-temperature phase formation in systems, which are the basis for obtaining functional materials, are selected. The use of salt fluxes (alkali metal nitrates and sulfates) is an effective method of accelerating the achievement of equilibrium in fluoride and oxide systems. Extrapolation of phase equilibria to absolute zero temperature, taking into account the requirements of the third law of thermodynamics, is a powerful method for obtaining relevant information. Summary phase diagrams of the ZrO2-Sc2O3 and albite — anorthite systems are proposed.

Theoretical study on order–disorder phase transition of CH3NH3PbCl3 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 51572219, 51872227, 11204239, and 11447030), the Project of Natural Science Foundation of Shaanxi Province of China (Grant Nos. 2015JM1018, 2013JQ1018, 15JK1759, and 15JK1714), and the Science Foundation of Northwest University of China (Grant No. 12NW06).

Order–disorder phase transitions for CH3NH3PbCl3 are studied with density functional theory. Our calculations show that the disorder is manifested in two aspects in the cubic phase, namely, the disorder of orientation and rotation of organic groups. Organic groups of [CH3] and [NH3] in cubic crystals can easily rotate around its C 3 axis. At the same time, [CH3NH3]+ organic groups can also orient to different spatial directions due to the weak interactions between organic group and inorganic frame. Our results show that its possible phase transition path starts from the deviation of organic groups from the crystal c-axis. Its structural transition changes from disordered cubic phase to hydrogen-only disordered tetragonal structure in the process of decreasing symmetry. The disordered high temperature cubic phase can be expressed as a statistical average of substructures we rebuilt. The electrostatic repulsive force between adjacent organic groups triggers out the formation of low temperature phase on cooling.

Enhanced Magnetic Properties in Ar+-Ion Beam-Bombarded MnBi Thin Films

MnBi is a promising ferromagnetic material for applications at elevated temperatures due to its high Curie temperature, sizable magnetocrystalline anisotropy, and increasing coercivity with temperature. However, fabrication of MnBi thin films with ferromagnetic low-temperature phase (LTP) is conventionally difficult due to the peritectic Mn-Bi reaction and Mn oxidization. In this work, in situ Ar+-ion beam bombardment was introduced to modify the microstructure and magnetic properties of (Mn/Bi)n superlattice. Structural characterization revealed formation of MnBi LTP in the as-deposited ion-beam-bombarded samples. Vacuum annealing further promoted growth of MnBi LTP grains with c-axis orientation, leading to remarkably improved perpendicular magnetic anisotropy. Ion-beam bombardment resulted in a 200% increase in the saturation magnetization of the annealed MnBi thin film. This advancement is attributed to the enhanced interdiffusion and reaction of Mn and Bi in the ion-beam-bombarded thin films. A stability test after 150 days in air revealed a remarkable reduction in ferromagnetism due to MnBi decomposition. This work provides an effective approach for fabrication of high-quality MnBi thin films.

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni2MnGa Compound

In order to explain the formation of low-temperature phases in stoichiometric Ni2MnGa magnetic shape memory alloy, we investigate the phase transformation paths from cubic austenite with Heusler structure to low-symmetry martensitic structures. We used ab initio calculations combined with the generalized solid state nudged elastic band method to determine the minimum energy path and corresponding changes in crystal lattice. The four-, five-, and seven-layered modulated phases of martensite (4O, 10M, and 14M) are built as the relaxed nanotwinned non-modulated (NM) phase. Despite having a total energy larger than the other martensitic phases, the 10M phase will spontaneously form at 0 K, because there is no energy barrier on the path and the energy decreases with a large negative slope. Moreover, a similar negative slope in the beginning of path is found also for the transformation to the 6M premartensite, which appears as a local minimum on the path leading further to 10M martensite. Transformation paths to other structures exhibit more or less significant barriers in the beginning hindering such a transformation from austenite. These findings correspond to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure.

Deformation-induced decomposition of enstatite in mantle peridotite and its role in partial melting and chromite ore formation

Deformed orthopyroxene grains are studied in detail in mantle peridotite. It is shown that deformation of enstatite is accompanied by its decomposition with the formation of low-temperature phases (pargasite, Fe-rich olivine) and restite represented by depleted enstatite, forsterite, and small newly formed chrome spinellide grains. The role of plastic deformation in initiation of partial melting of peridotite and in the formation new chrome spinellide grains is discussed.

Low-temperature phase formation in CaF2–HoF3 system

Phase formation in CaF2–HoF3 system has been studied by coprecipitation followed by X-ray powder diffraction. Aqueous nitrate solutions have been used as initial substances, while hydrofluoric acid has been employed as fluorinating agent. Formation of hydrated nanophases: solid solution Ca1–x Ho x F2 + x (х ≤ 0.1) and HoF3 has been revealed. Dehydration proceeds on heating to 600°C.

Low-temperature phase formation in a Ti – B – C – O system

Low-temperature phase formation in a Ti – B – C – O system

Comparative studies of pure BiFeO3 prepared by sol–gel versus conventional solid-state-reaction method

In this article, we studied the effect of synthesis route on the multifunctional properties of multiferroic BiFeO3. BiFeO3 powders were prepared by conventional solid-state-reaction and sol–gel route. X-ray diffraction (XRD) patterns for these samples were collected at different stages of synthesis to analyze the phase purity of the formation. The XRD patterns reveal that sample prepared by sol–gel route attains the low temperature phase formation as compared to the solid state route. Rietveld refinement has been performed for these samples and lattice parameters, cell volume bond length etc. have been calculated from XRD patterns. Phonon modes were studied by Fourier transform infrared spectroscopy measurements and bond length calculated from XRD shows the good agreement with the bond length calculated from IR spectra. UV–visible spectra showed that BFO nanoparticles exhibit absorption peak at wavelength ~521 nm and band gap is more for the sample prepared by sol–gel route than solid state. The room temperature (RT) magnetic hysteresis (M–H) curve shows the large value magnetization in the sample prepared by sol–gel route in comparison to the sample prepared by solid state route. Similar behaviour is seen in the P–E hysteresis curve. Room temperature dielectric properties of these samples revealed that there is dispersion in the low frequency range that shows normal dielectric characteristics.

Critical phenomena in superconductors and uniaxial antiferromagnets (Review Article)

This overview is inspired by the pioneering works by Eremenko et al. on the first observed similarities in the low-temperature phase formation of conventional antiferromagnets and superconductors near the critical temperature of the transition. In the development of the detected similarities, a comparison was made of the temperature dependence of the magnetic and structural properties of new anisotropic superconductors and antiferromagnets with competing order parameters (multiferroics). Common features in their critical behavior near the ordering temperature Tc are discussed.

Size effects in oxide mechanochemistry

A new size effect has been experimentally found: the mechanochemical synthesis of previously mechanically activated powder mixtures of MgO-Al2O3 oxides with a block size of about 80 nm leads to the formation of MgAl2O4 spinel at room temperature. A model is proposed according to which the mechanism of mechanochemical solid-phase synthesis changes from dislocation transgranular embrittlement to grainboundary sliding, which is accompanied by the generation of dynamic vacancies and low-temperature phase formation via the diffusion-induced migration of grain boundaries.

Structural and magnetic properties of bulk MnBi permanent magnets

Structural and magnetic properties of bulk nanostructural Mn100−xBix (x = 40, 45, 52) permanent magnets prepared by spark plasma sintering technique were studied. The effect of the Mn/Bi ratio on the MnBi low temperature phase (LTP) formation and its magnetic properties were investigated. An increase of the bismuth amount in the magnets leads to better formation of LTP, resulting in the improvement of both magnetization (Ms) and remanence (Mr), but decreasing the coercivity (Hc) of the magnets. At room temperature, Ms increases from 27.87 emu/g for Mn60Bi40 to 45.31 emu/g for Mn48Bi52, whereas Hc decreases from 12 to 7.9 kOe. The microstructure of Mn48Bi52 magnet is composed of fine and uniform grains with an average size of 140 nm as shown in the TEM image. The Mn48Bi52 magnet shows a high Hc of 19 kOe at 423 K, indicating a strong positive temperature coefficient of coercivity for the MnBi magnet.

Catalytic effect of silver addition on the low temperature phase formation ofMgB2

A small amount of silver addition was found to dramaticallydecrease reaction temperature of magnesium and boron forming theMgB2 bulk without degradations in both critical temperature and critical current properties. A sample with∼3% silver addition formagnesium heated at 550 °C for 72 h recorded a high Jc above 2 × 105 A cm−2 at 20 K in low fields reflecting its strongly grain-coupled microstructure. In addition, a sample with∼5% silver additionshowed Jc of∼1 × 105 A cm−2 at 20 K in low fieldseven for heating at 500 °C for 72 h. Although the added silver formed Ag–Mg alloy after the heat treatment,these impurity particles mainly existed at the edge of voids without losing theeffective current path. Our present results will contribute to developing low costMgB2 conductors.

Phase stability and its intrinsic conditions in nanocrystalline materials

The critical grain (particle) sizes of high-temperature phases being stable at room temperature for Co, Fe 80Ni 20 and ZrO 2, which have martensitic transformations in their conventional bulk, coarse-grained samples, are calculated respectively as 35, 35 and 13 nm by our universal theory. However, our experiments on the as-prepared state of nanosized materials seem to be in conflict with our prediction. Based on the concept of phase stability, intrinsic conditions of determining the stability of high-temperature phases were proposed, namely, the high-temperature phase must first form during preparation, then, during cooling, the grain (particle) size of the high-temperature phase must be smaller than its critical grain size. The intrinsic conditions of high-temperature phase stability are verified by experiments on Co nanogranular film sputtered at 773 K, Fe 80Ni 20 nanocrystalline powders annealed at 953 K, and nanosized ZrO 2 amorphous powders annealed at 1573 K. The critical grain size (14 nm) of the high-temperature phase determined by experiments of ZrO 2 is in good agreement with the theoretical value (13 nm) since ZrO 2 does not undergo oxidation during annealing. The origin of low-temperature phase formation in the as-prepared state of nanosized materials is also discussed.

Low Temperature Phase Formation in Mechanically Alloyed Cu-Ga Powders

Low Temperature Phase Formation in Mechanically Alloyed Cu-Ga Powders

Effect of the Surface Temperature on Formation of Low-Temperature Phase in Magnetron-Sputtered Chromium Films

The existence of a low-temperature (LT) chromium phase with the primitive cubic lattice in chromium films is questionable. We revealed that the main condition necessary for LT phase formation in magnetron sputtered Cr films is the deposition of the film from low-energy atomic flux, which is accomplished during deposition at a high working gas pressure. The films deposited from high-energy atomic flux at a low gas pressure crystallized in the ordinary bcc phase independently of the substrate temperature. This effect is explained by assuming the existence of a “hot” layer on the growth surface whose temperature can be noticeably higher than the substrate temperature.

Mechanochemical modification of the phase diagrams of hexagonal oxide ferrimagnets

Mechanical activation and fine comminution of hexagonal oxide ferrimagnets by milling to a nanocrystalline state (ultradisperse powder), followed by sintering, leads to significant changes in the phase diagram, the temperature of synthesis, and the magnetic properties as compared to those of the materials obtained by traditional ceramic technology. The final material structure is formed through “chemical assembly”—sequential consolidation of nanodimensional particles (structure-forming blocks)—rather than through the formation of low-temperature phases and their decomposition with increasing temperature.

Investigation into the Properties of a New Precursor for Preparation of Tetragonal Zirconia

A finely crystalline tetragonal zirconia powder with a crystal size of less than 40 nm is synthesized from zirconium peroxide. The metastable tetragonal phase of zirconia contains ∼1–3% H2O, is stable up to approximately 500–600°C, and possesses a sufficient stability at room temperature. It is demonstrated that the temperatures of the formation of low-temperature phases and their structure and stability depend on the prehistory of the material and the reactants used. The irreversible phase transformation from metastable forms into stable forms is accompanied by changes in the structure and composition of the phases due to the liberation of impurities (OH–, H2O) stabilizing the metastable phases.

Preparation and characterization of K(Sr0.5Ba0.5)2Nb5O15 thin films by sol-gel method

Crack-free K(Sr0.5Ba0.5)2Nb5O15 (KSBN50) thin films could successfully be prepared by the control of the reactions through the sol-gel method. KSBN50 precursor powder was directly crystallized to the tetragonal tungsten bronze phase without formation of low temperature phase at 600°C. KSBNSO thin films on MgO(100) and Pt(100)/MgO(100) also crystallized to the tetragonal tungsten bronze with c-axis highly orientation at 600°C. Two crystal lattice planes of KSBN50 were intergrown at an orientation of 18.5 degrees on MgO(100) substrate. The Curie point of KSBN50 thin films was at around 60°C at 10 kHz. KSBN50 thin films on Pt(100)/MgO(100) substrates exhibited the typical P-E hysteresis.

Interfacial Diffusion Effects and Non-Stability of Disperse Layered Structures

AbstractThe kinetics and atomic mechanism of the low temperature phase formation and accompanying effects in disperse multilayered structures of the binary systems Ni – Cr and Ni – Al have been studied under the conditions when volume diffusion is practically “frozen”.