BCT Crystal Research Articles

Analysis of Strength Improvement and Crystallographic Tranformations In an Isothermal Oil Quenched on Martempering AISI 4140

This study aims to maintain the phase change process of 4140 steel at the starting point of martensite (Ms) by adjusting the speed and restraint of the transformation process to prevent the formation of the final martensite (Mf) phase. The use of oil with a constant temperature is able to stabilize the austenite phase into retained austenite (RA). The isothermal cooling (IQ) process uses a cooling chamber controlled by a microcontroller. The partitioning process is isothermal quenching deformation (IQDP) by heating the specimen at 850oC for 20 minutes which is rapidly cooled in oil quenching medium at constant temperature. Oil quenching media is used to hold the cooling rate so that the austenite phase transformation (FCC crystal) to martensite phase (BCT crystal) occurs. The effect of IQDP treatment on phase change, crystal orientation and mechanical properties were analyzed by hardness test, tensile test and SEM. The test results show that IQ treatment on SAE 40 oil is able to increase its tensile strength and modulus of elasticity. The application of isothermal quenching using SAE 40 oil with a constant temperature of 40 oC has been proven to increase the ductility of raw materials from 1226.94 MPa to 1361.77 MPa.

High quality crystal growth and characterization of ferroelectric Ba0.77Ca0.23TiO3 single crystal

[Formula: see text][Formula: see text]TiO3 (BCT) single crystal has been widely studied as a promising lead-free ferroelectric material. In this work, high-quality BCT crystal was successfully grown by the Czochralski (CZ) method. The as-grown crystal is crack-free and shows black coloration. It possesses a high dielectric stability over a wide temperature range, while the dielectric loss is rather small below 90[Formula: see text]C. Furthermore, it possesses excellent ferroelectric properties with residual polarization strength ([Formula: see text]) and coercive field ([Formula: see text][Formula: see text]) of 17.93 [Formula: see text]C/cm2 and 8.47 kV/cm, respectively. Besides, BCT crystal shows large electromechanical coupling factors, with [Formula: see text][Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] of 0.535, 0.254, 0.714 and 0.721, respectively. The piezoelectric coefficients [Formula: see text], [Formula: see text] and [Formula: see text] are measured to be − 36.5, 130 and 246 pC/N, respectively.

Growth and electrical properties of lead-free ferroelectric single crystal Ba0.77Ca0.23TiO3

Ferroelectric materials have been widely used in fields such as capacitors and transducers. A lead-free ferroelectric single crystal (Ba0.77Ca0.23TiO3) was grown by an improved Czochralski method. The composition of the crystal was verified by quantitative analysis. The basic electrical properties such as ferroelectric and dielectric of the Ba0.77Ca0.23TiO3 crystals along different orientations have been investigated. A complete set of dielectric, piezoelectric and elastic parameters were characterized by the dynamic resonance method. BCT crystal along the <001> orientation has the largest piezoelectric constants. Its d33, d31 and d15 were 204 pC/N, −58.3 pC/N and 156.7 pC/N, respectively, and the coupling coefficients are k33, k31, k15, kt were 71%, 30%, 45%, 50%, respectively. These values are better than barium titanate (BT) crystal. At the same time, BCT crystal also has a higher dielectric constant and lower dielectric loss like barium titanate.

Reversible Self-Assembly (fcc-bct) Crystallization of Confined Granular Spheres via a Shear Dimensionality Mechanism.

By combining vibrational annealing and shear dimensionality, we experimentally show (1)a fast reversible crystallization fcc-bct (face-centered cubic-body-centered tetragonal) in a granular system that is composed of dissipative millimeter-sized dry spheres, (2)a two-dimensional (planar) shear promotes self-assembly into an fcc crystal, while one-dimensional shear produces a bct crystal, and (3)in analogy with heterogeneous nucleation, a granular temperature gradient leads to the formation of crystal domains showing competition of polymorphic phases in the cold regions. Our findings suggest that controlling the directionality of the interactions steers to reversible crystallization of hard spheres, adds clues for theoretical studies, and provides a novel mechanism for the technological development of the applications of self-assembling phononic crystals.

The modulation mechanism of growth atmosphere on composition and phase transition behavior in barium calcium titanate crystal

Barium calcium titanate (Ba1‐xCaxTiO3, BCT), which can be used as a three‐dimensional nonlinear photonic crystal, is an anomalous ferroelectric with tremendous electromechanical response. Its phase transition behavior depends closely on the Ca content. In particular, the rhombohedral‐orthorhombic and orthorhombic‐tetragonal transitions can disappear at two contents: x = 0.18 and 0.23, respectively, which is important to study a wide range of physical phenomena including the alluring quantum effects. In this work, based on the natural differences of the affinity of oxygen atoms on Ba and Ca atoms, we developed a growth atmosphere modulating mechanism to control the Ca content in BCT crystals aiming at achievement of the desired compositions (x = 0.18 and 0.23). With the same polycrystalline material (Ba0.773Ca0.227TiO3), BCT single crystals with x = 0.189 and x = 0.225 were obtained under N2 and air atmospheres, respectively. The structural and dielectric properties of the as‐grown crystals were investigated and the differences were discussed. Besides, the influence of Ti3+ imperfections on the diffused phase transition was studied, and the specific heats and transition enthalpies were also measured for characterizing the different phase transition properties.

Polarization properties and crystal structures of ferroelectric (Ba,Ca)TiO3 single crystals

We have investigated the spontaneous polarization (P s ) of Ba 1-x Ca x TiO 3 (BCT) by polarization hysteresis measurements using single crystals and by density functional theory (DFT) calculations. Single crystals of BCT (x = 0.07) were grown by a top-seeded solution growth (TSSG) method. The polarization measurements show that the crystals (x = 0.07) have a P s of 26.0 μC/cm2, which is slightly small compared with BaTiO 3 (27.3 μC/cm2). Our DFT calculations based on a supercell approach show that Ca atoms are markedly displaced cooperatively with the adjacent Ti atoms along the P s direction. It is suggested that the CaTiO 3-like octahedral rotation is constructed in the BCT supercell around the Ca atoms, which is the origin of the smaller P s observed for the BCT crystals.

Phase Diagram of Dipolar Hard and Soft Spheres: Manipulation of Colloidal Crystal Structures by an External Field

Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field.

Light-Induced Polaronic Absorption at Low Temperature in Pure and (Fe, Ce, Cr) Doped SrxBa1-xNb2O6 or Ba1-yCayTiO3 Crystals and Photodissociation of Vis Centers Into Small Polarons

Congruent SBN and BCT crystals doped with Fe, Cr, Ce to enhance photorefractive properties, are investigated at low temperature (T > 1 K) under illumination with Ar☎- and Kr☎-laser light. Light-induced absorption changes in a wide spectral range from UV to IR indicate photoinduced charge transfer processes from impurities to polaronic centers. Broad NIR absorptions (at about 0,7 eV) associated with Ti3☎ polarons in BCT or with Nb4☎ polarons in SBN are observed and their non-linear behaviour with illumination intensity, polarization and temperature is described on the basis of the simple model for the photo charge transport (in SBN: Ce3☎☎ Nb5☎↔Ce4☎☎Nb4☎). A broad visible absorption VIS (at about 2 eV) appearing together with the NIR polarons, is shown by photodissociation (with a Kr☎-laser) to consist at least partly of small polarons (in BCT and SBN).

Photosensitive centers and charge transfer processes in barium calcium titanate

The results of a study of charge transfer processes in barium calcium titanate crystals, Ba0.77Ca0.23TiO3, in the temperature range between 4.2 K and 300 K are reported. The nominally pure and Rh doped BCT crystals were investigated with the help of electron paramagnetic resonance, optical absorption spectroscopy and light induced absorption change measurements.

Calculated electronic structure of metastable phases of Cu.

The electronic energy band and ground-state properties for the existing body-centered-cubic (bcc) and body-centered-tetragonal (bct) crystals, and the predicted hexagonal-close-packed (hcp) structure of elemental copper have been calculated by using first-principles density-functional linear muffin-tin orbital methods in a unified scheme. Results are presented in the form of the energy-band structure in k space and the total energy as a function of the lattice constant. A recent proposed generalized gradient approximation scheme gives more accurate values than the standard local-density approximation. The calculated band structure of bct-Cu is in good agreement with that measured in photoemission experiments, on Cu films grown epitaxialy on Pd{001} and on Pt{001}. The equilibrium lattice constants given by us are in good agreement with those obtained from experiments on bct-Cu and bcc-Cu films. The possibility of the existence of an artificial structure of hcp-Cu has been discussed.