Shift Of Phase Transition Temperature Research Articles

Structure, dielectric and piezoelectric properties of (1-x)K0.48Na0.48Li0.04NbO3-xBa0.85Ca0.15Zr0.1Ti0.9O3 ceramics

Lead free piezoelectric ceramics (1-x)KNLN-xBCZT (0 ≤ x ≤ 0.015) are prepared using ultrasonic irradiation assisted solid state reaction process. The coexistence of orthorhombic and tetragonal phases is identified in the range of 0.008 ≤ x ≤ 0.013. The polymorphic orthorhombic-tetragonal phase transition temperature shifts towards room temperature with the increase of the BCZT concentration. Improved polarization and enhanced piezoelectric properties are obtained in 0.99KNLN-0.01BCZT ceramic at room temperature, which is attributed to the shift of their orthorhombic-tetragonal phase transition towards room temperature as well as the high density and high Pr value. These results indicate that 0.99KNLN-0.01BCZT ceramic is a promising lead-free piezoelectric material.

On the nature of phase separation of polymer solutions at high extension rates

ABSTRACTExperiments with stretching moderately concentrated polymer solutions have been carried out. Model experiments were carried out for poly(acrylonitrile) solutions in dimethyl siloxane. Just the choice of concentrated solutions allowed for a clear demonstration of a demixing effect with the formation of two separate phases—an oriented polymer fiber and solvent drops sitting on its surface. An original experimental device for following all subsequent stages in the demixing process was built. It combined two light beams, one transverse to the fiber and a second directed along (inside) the fiber, the latter played the role of an optical line. This gives a unique opportunity to observe processes occurring inside a fiber. The process of demixing starts from the volume phase separation across the whole cross section of a fiber at some critical deformation and the propagation of the front of demixing along the fiber. Then a solvent cylindrical skin appears which transforms into a system of separate droplets. New experimental data are discussed based on a comparison of the current different points of view on the phenomenon of deformation‐induced phase separation: thermodynamic shift of the equilibrium phase transition temperature, growth of stress‐induced concentration fluctuations in two‐component fluids, and mechanically pressing a solvent out from a polymer network. The general belief is that a rather specific (so‐called “beads‐on‐a‐string”) structure of a filament is realized in stretching dilute solutions: beads of a polymer solution connected by oriented polymer bridges forming a single object. The situation in stretching moderately concentrated solutions appears quite different: real phase separation was observed. So, the alternative phenomenon to the formation of the “beads‐on‐a‐string” structure has been experimentally proven. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 559–565

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration.

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.

Optimizing the Magnetocaloric Effect in Ni–Mn–Sn by Substitution: A First-Principles Study

We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization.

Photoluminescence Spectroscopy Technology to Probe Phase Transition Process of Ba0.95Eu0.05TiO3

The high-quality Ba0.95Eu0.05TiO3 ceramic was prepared by the conventional high-temperature solid-state reaction method. X-ray diffraction (XRD) pattern indicates that the sample is single phase, and temperature dependence of photoluminescence (PL) spectroscopy was measured in the temperature range from 83K to 483K with the 514.5 nm radiation of the argon ion laser. The PL peak position is sensitive to temperature and abrupt change exists in the vicinity of phase transition temperature; hence, PL spectroscopy technology is considered as a new method to probe phase transition process of materials. Simultaneously, phase transition temperatures related to BaTiO3 shift to lower temperature as a part of Barium ions are substituted by the trivalent Europium ions. Besides, the physical origination of the phase transition temperatures shift has been discussed.

Effects of gamma ray irradiation on the piezoelectric and ferroelectric properties of bismuth doped barium titanate ceramics

Gamma ray irradiation effects on the piezoelectric, ferroelectric and dielectric properties of bismuth doped barium titanate ceramics have been investigated. Bismuth doped barium titanate ceramics are irradiated at room temperature by Co60 ion beam of energy 200 Gy. A double hysteresis loop has been observed after irradiation, which may be due to the random local strains caused by irradiation. After irradiation, piezoelectric charge constant and bipolar strain decrease because of fractured grains as confirmed from scanning electron micrographs. Phase transition temperature shifts to a lower value with increase in degree of diffuseness after irradiation. The piezoelectric and ferroelectric properties have been deteriorated by gamma ray irradiation.

Effect of millimetre waves on phosphatidylcholine membrane models: a non-thermal mechanism of interaction.

The nonthermal biological effects of millimeter waves have been mainly attributed to the interaction with biological membranes. Several data on biomimetic membrane systems seem to support this conclusion. In this paper a mechanistic hypothesis is evaluated to explain such an interaction taking into account experimental NMR data on deuterium-labeled phospholipid vesicles. These data showed that millimeter waves induce a time and a hydration-dependent reduction of the water ordering around the phosphocholine headgroups. This effect is here interpreted as a change in membrane water partitioning, due to the coupling of the radiation with the fast rotational dynamics of bound water molecules, that results in a measurable relocation of water molecules from the inner to the outer binding regions of the membrane interface. When millimeter wave exposure is performed in the vicinity of the transition point, this effect can lead to an upward shift of the membrane phase transition temperature from the fluid to the gel phase. At a macroscopic level, this unique sensitivity may be explained by the universal dynamic behaviour of the membranes in the vicinity of the transition point, where a pretransitional increase of membrane area fluctuations, i.e., of the mean area per phospholipid headgroup, is observed. Exposure to millimeter waves increases the above fluctuations and enhances the second order character of the transition.

Phase transitions in KNO3 embedded in MCM-41 films with regular nanopores

This paper reports on a comparative study of phase transitions in nanocomposites made up of KNO3 embedded in 10-μm-thick MCM-41 films with unidirectional pores 4.0 nm in size on an aluminum substrate and of nanocomposites prepared in the form of potassium-nitrate-filled pressed MCM-41 powders with 3.7-nm pores. The temperature dependences of linear permittivity and the amplitude of third harmonic generation have been measured under heating and cooling. The structural transition from phase II to phase I shifts under heating relative to that occurring in bulk KNO3 toward lower temperatures for potassium nitrate in the film and toward higher temperatures for the pressed MCM-41-based nanocomposite. A significant difference has been observed also within the region of existence of ferroelectric phase III. The data obtained suggest that the shifts of phase transition temperatures observed in the conditions of nanoconfinement are influenced markedly not only by pore size and geometry but also by other factors.

Size effects in KDP-porous glass ferroelectric nanocomposites

An influence of the size effect on the phase transition temperature in potassium dihydrogen phosphate (KDP)-porous glass nanocomposites was studied in this work. Results of the dielectric measurements in a wide temperature range for KDP crystals embedded in porous glasses are presented. The phase transition temperature shift toward higher temperatures was observed for KDP embedded in porous glasses with the mean value of the pore size of 312 and 160 nm, whereas for smaller pore sizes (71 and 23 nm) the phase transition temperature decreased. The phase transition temperature dependence of the mean values of the pores dimensions of KDP-porous glass nanocomposites is the experimental evidence showing the non-monotonical character of the size effect on the phase transition temperature in ferroelectrics particles. It was found that the dielectric permittivity of KDP embedded into the porous glasses with the mean values of pore sizes of 160 nm and 312 nm practically did not depend on frequency, whereas for KDP embedded into the pores with dimensions of 71 nm and 23 nm a dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases.

Negative thermal expansion and shift in phase transition temperature in Mo-substituted ZrW2O8 thin films prepared by pulsed laser deposition

Mo-substituted ZrW2O8 (ZrW1.1Mo0.9O8) thin films have been deposited on quartz substrates by the pulsed laser deposition (PLD) method. The effects of oxygen pressure, substrate temperature and annealing temperature on the morphologies and phase compositions of the ZrW1.1Mo0.9O8 thin films were systematically investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The negative thermal expansion and shift in phase transition temperature in cubic ZrW1.1Mo0.9O8 thin films were characterized using high temperature X-ray diffraction. The results indicate that as-deposited ZrW1.1Mo0.9O8 thin films show amorphous phases. Crystallized cubic ZrW1.1Mo0.9O8 thin films were prepared by heating at 1050°C for 7min. The growth of the ZrW1.1Mo0.9O8 thin films was strongly influenced by the substrate temperature and oxygen pressure. The ZrW1.1Mo0.9O8 thin film deposited at 500°C with an oxygen pressure of 10Pa was smooth and compact, and its thickness was about 720nm. The high temperature X-ray diffraction analyses demonstrated that the cubic ZrW1.1Mo0.9O8 thin film exhibited strong negative thermal expansion and its thermal expansion coefficient was calculated to be −8.65×10−6K−1 from 100°C to 600°C. The substitution of Mo in ZrW2O8 thin film leads to a remarkable decrease in phase transition temperature, with the α to β structure phase transition occurring below 100°C. However, with increased testing temperature, the substitution results in part of the cubic ZrW1.1Mo0.9O8 thin film gradually changing into a trigonal phase.

Phase Behavior and Thermal Stabilization Study of Blue Phase Liquid Crystal-Unfunctionalized Nanoparticle Blends

We have studied the phase behavior of blue phase liquid crystal (BPLC)-unfunctionalized nanoparticle (NP) blends. Two different types of NPs which have different anchoring properties have been studied. Interestingly, Silica NP doped BPLC blends have revealed a small but clear positive shift of the Isotropic-BP phase transition temperature for lower concentration of NPs whereas with higher percentage of NPs we have observed a mixed phase consisted with chiral nematic (N*) and BP. On the other hand, low concentration polyhedral oligomeric silsesquioxanes (POSS) NPs which have stronger anchoring force than Silica NPs have enabled BP to stabilize thermally. Our experimental results agrees with the recent theory predicted that NP-defect interaction strongly depends on the interfacial energy between NP and LC which may be responsible for the observed effects in unfunctionalized NP-BPLC composite system.

Phase and dielectric behaviors of a polymorphic liquid crystal doped with graphene nanoplatelets

We report on the phase behavior and dielectric properties of the liquid crystal (LC) 4′-n-octyloxy-4-cyanobiphenyl dispersed with graphene nanoplatelets (GNPs). The temperature-dependent dielectric permittivity at 104 Hz and its derivative with respect to the temperature reveal that the incorporation of GNPs in a LC cell leads to the modification of crystalline polymorphism and shift in phase transition temperature owing to the enhanced positional and orientational order. Additionally, the dielectric data between 1 and 103 Hz show that the dopant reduces the ionic concentration and alters the diffusivity in the LC mesophases.

Influence of the A/B nonstoichiometry, composition modifiers, and preparation methods on properties of Li- and Ta-modified (K,Na)NbO3 ceramics

Properties of Li- and Ta- modified (K,Na)NbO3 piezoceramics with the base composition near the orthorhombic-tetragonal phase boundary have been investigated with respect to variation of temperature, stoichiometry, compositional homogeneity, atmosphere of thermal treatment, and dopants (Ba, Mn). Although the influence of the most of the abovementioned factors has already been reported in the literature, the paper focuses on several aspects of the composition—property relationships that still remain controversial or poorly elucidated. In particular, we show that improvement of compositional homogeniety in these ceramics emphasizes the instability of piezoelectric response with respect to variation of temperature in the vicinity of the orthorhombic-tetragonal phase transition. Ba dopant is shown to suppress conductivity in ceramics sintered in air, though it makes conductivity more sensitive to variation of the oxygen partial pressure. Mn dopant is shown to suppress conductivity and strongly reduce the influence of the oxygen partial pressure on conductivity of the ceramics. Finally, we show that chemical modifications to the ceramic composition, such as Mn dopant or variation of nonstoichiometry affect the piezoelectric response mainly by the shift of the orthorhombic-tetragonal phase transition temperature.

Piezoelectric properties of tetragonal K0.95Li0.05Ta x Nb1− x O3 lead-free single crystals near tetragonal-cubic phase transition boundary

Tetragonal K0.95Li0.05Ta x Nb1 − x O3 lead-free single crystals have been grown by top-seeded solution growth technique. Dielectric and piezoelectric properties of poled samples were investigated as a function of temperature and dc bias. Both orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures shift to lower temperatures as the Ta content increased. High piezoelectric activity was found to the crystal near the tetragonal–cubic phase transition boundary, which is due to phase transition as well as domain instability. For K0.95Li0.05Ta0.59Nb0.41O3 single crystal, piezoelectric coefficient (d 31) and electromechanical coupling factor (k 31) achieved −340 pC N−1 and −40%, respectively, which were much larger than those of lead-based Pb(Zr, Ti)O3.

Magnetoelectricity in the PbFe1/2Nb1/2O3 Ceramics

The magnetodielectric effect (the influence of a magnetic field H on the dielectric constant ɛ) and the magnetoelectric effect (the influence of an electric field E on the magnetoelectric constant α) of the PbFe1/2Nb1/2O3 ceramics have been investigated at temperatures T in the range from 50 to 200°C, including the Curie point TC ≊ 98°C. It has been demonstrated that there is a correlation of these effects with the shift of the ferroelectric-paraelectric phase transition temperature in a magnetic field.

Effects of Li content on the phase structure and electrical properties of lead-free (Ag0.85−xLixNa0.1K0.05)NbO3 piezoelectric ceramics

Lead-free ceramics (Ag0.85−xLixNa0.1K0.05)NbO3 (ALNKN−x, x = 0−0.16) doped with 0.5 mol. % CuO were prepared by solid state reaction method under air atmosphere. The effects of Li content on the phase structure and electrical properties of the ALNKN−x ceramics were investigated. A morphotropic phase boundary (MPB) between orthorhombic and rhombohedral phases is found at x ≈ 0.08−0.14. Both the ferroelectric–ferroelectric phase transition temperature (TFE) and Curie temperature (TC) shift toward higher temperatures with increasing x. Only the phase transition peak near the TC is observed when x ≥ 0.12. The ferroelectric phase of the ALNKN−x ceramics becomes more stable with increasing x. Furthermore, the ceramics with x = 0.12 exhibit relatively good electrical properties along with a high Curie temperature (d33 = 44 pC/N, kp = 17.7%, Pr = 14.5 µC/cm2, Ec = 37.2 kV/cm, and Tc = 335 °C). The results provide helpful guidance to develop new lead-free piezoelectric materials with good temperature stability.

Characteristic features of the molecular dynamics and phase transitions of mixed [(NH4)1−xRbx]3H(SO4)2 (x=0, 0.8, and 1) crystals using 1H and 87Rb NMR

Abstract In this study, we grew [(NH4)1−xRbx]3H(SO4)2 (x = 0, 0.8, and 1) mixed crystals using a slow evaporation method and determined their lattice constants by X-ray diffraction. We investigated the molecular dynamics and phase transition temperatures of the crystals using 1H and 87Rb nuclear magnetic resonance (NMR) relaxation and differential scanning calorimetry. When a fraction of rubidium ions in the pure Rb3H(SO4)2 crystal is partial replaced by ammonium ions, we compared the physical properties of the three types of crystals. A small amount of NH 4 + added to Rb3H(SO4)2 led to a change in the physical properties, bringing about a shift in phase transition temperatures and showed a superprotonic phase transition.

Microwave induced shift of the main phase transition in phosphatidylcholine membranes

Numerous experimental evidence show that exposure of biological systems to extremely high frequency microwaves may induce significant effects even at low powers. These effects are thought to occur via nonthermal mechanisms involving primarily the interaction of microwaves with phospholipid membrane structures. However, no conclusive experimental evidence that biomembranes exhibit remarkable sensitivity to this radiation has been provided up to now. Here, deuterium nuclear magnetic resonance spectroscopy is used to study the effects of microwaves on 1,2-Dimyristoyl-sn-glycero-3-phosphatidylcholine/ 2H 2O multilamellar vesicles that serve as biomimetic membranes. Here we show that, if the membrane is brought into close proximity to the transition point, microwaves induce a reduction of water ordering at the membrane interface, an upward shift of the main phase transition temperature and a broadening of the transition region. A deep dosimetric analysis shows that the above effects are nonthermal, indicating the need for a nonthermal hypothesis to explain them. This study suggests that exposure to high-frequency microwaves can have far reaching consequences on active biological systems.

Structures and Piezoelectric Properties of (Bi0.5Na0.5)TiO3-(Na0.5K0.5)NbO3 Based Lead-Free Ceramics

Lead-free (1-x)(Bi0.5Na0.5)TiO3-xNa0.5K0.5NbO3, (x = 0–8 mol%) (abbr., BNT-xNKN) solid solutions were fabricated by mechanical alloying technique. XRD and SEM showed the crystal structure of the samples become more tetragonal and their grain size become small with the increase of substitution. Dielectric studies revealed that the phase transition temperatures shift to low temperature with the increase of NKN. The piezoelectric properties of BNT-xNKN can not be enhanced with the increase of NKN, but their dielectric properties have been improved. The results show that the BNT-xNKN ceramics are one of the promising lead-free materials for capacitor applications.

Magnetic structure and magnetoelectric coupling in bulk and thin filmFeVO4

We have investigated the magnetoelectric and magnetodielectric response in FeVO$_4$, which exhibits a change in magnetic structure coincident with ferroelectric ordering at $T_{N2}$$\approx$15 K. Using symmetry considerations, we construct a model for the possible magnetoelectric coupling in this system, and present a discussion of the allowed spin structures in FeVO$_4$. Based on this model, in which the spontaneous polarization is caused by a trilinear spin-phonon interaction, we experimentally explore the magnetoelectric coupling in FeVO$_4$ thin films through measurements of the electric field induced shift of the multiferroic phase transition temperature, which exhibits an increase of 0.25 K in an applied field of 4 MV/m. The strong spin-charge coupling in \fvo\, is also reflected in the significant magnetodielectric shift, which is present in the paramagnetic phase due to a quartic spin-phonon interaction and shows a marked enhancement with the onset of magnetic order which we attribute to the trilinear spin-phonon interaction. We observe a clear magnetic field induced dielectric anomaly at lower temperatures, distinct from the sharp peak associated with the multiferroic transition, which we tentatively assign to a spin reorientation cross-over. We also present a magnetoelectric phase diagram for FeVO$_4$.