Dependence Of Transition Temperature Research Articles

Investigation of Spin Reorientation in Ga Substituted Y-type Hexaferrite based on Mössbauer Spectroscopy

The polycrystalline sample of Ba2Co1.5Mg0.5Fe11.88Ga0.12O22 Y-type hexaferrite, doped with Ga-cation, was prepared by using the solid-state reaction method. The crystalline structure of sample was investigated by x-ray diffractometer (XRD), and the magnetic properties of sample were measured by vibrating sample magnetometer (VSM), and Mossbauer spectrometer. The crystal structure of prepared sample was determined to be rhombohedral with space group R-3m. From the temperature dependence of the magnetization curves under 100 Oe between 4.2 and 740 K, two temperature-dependent magnetic transitions occurred in the Ba2Co1.5Mg0.5Fe11.88Ga0.12O22 sample. Mossbauer spectra of the sample were analyzed at various temperatures ranging from 4.2 to 620 K, and the Ba2Co1.5Mg0.5Fe11.88Ga0.12O22 sample showed abrupt changes in Hhf and EQ at 200 K, indicating the spin transition effect. We have also determined the magnetic transition temperature TC, in addition to the temperature dependent magnetization and ZVC measurements.

High-temperature sliding wear behavior of an intermetallic γ–based TiAl alloy

The high-temperature sliding wear behavior of the γ–based TiAl alloy TNM-B1 was investigated using a pin-on-disk apparatus up to 800 °C in air under variation of the counterpart material. Sliding TNM-B1 against a Ni-based pin led to approximately twice as high wear losses compared to like-on-like wear tests up to 400 °C. These severe wear losses were verifiably attributed to a strong material transfer from the TiAl surface to the pin already existing at low temperatures. Moreover, it was proven that there is a distinctive temperature dependent transition from severe to mild wear for both sliding couples due to the formation of protective oxide scales above 600 °C. In this regard, Al- and Ti-rich oxide scales formed on the worn surfaces minimizing direct metal-to-metal contact.

Spectroscopic signature of trimer Mott insulator and charge disproportionation in BaIrO3

We have measured the reflectivity spectra of the barium iridate $9R$ BaIrO$_3$, the crystal structure of which consists of characteristic Ir$_3$O$_{12}$ trimers. In the high-temperature phase above the transition temperature $T_c\simeq180$ K, we find that the optical conductivity involves two temperature-dependent optical transitions with an ill-defined Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in BaIrO$_3$, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below $T_c$, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in BaIrO$_3$.

Glass transition and primary crystallization of Al86Ni6Y4.5Co2La1.5 metallic glass at heating rates spanning over six orders of magnitude

The glass transition and primary crystallization of melt-spun Al86Ni6Y4.5Co2La1.5 metallic glass have been investigated at continuous heating covering more than six orders of magnitude of heating rates. The heating rate dependence of the glass transition temperature is described by a generalization of the Bartenev-Ritland equation utilizing the Vogel-Fulcher-Tammann equation for the description of the viscosity and the Maxwellian relaxation time. This result is confirmed by measurements of primary crystallization performed also over a wide range of scanning rates. In the analysis a theoretical model is employed which takes into account the effect of decoupling of viscosity and diffusion.

Thermal Properties of Open-Cage Silsesquioxanes: The Effect of Substituents at the Corners and Opening Moieties

Abstract Incompletely condensed polyhedral oligomeric silsesquioxane (IC-POSS), which has an open-cage structure, exhibits high optical transparency even incorporating 30 wt% of the IC-POSS fillers in a poly(methyl methacrylate) (PMMA) matrix dependent on their substituents. However, real understanding of the substituent-dependent dispersibility of the IC-POSS fillers is unclear. In this work, we studied structure-dependent bulk thermal properties of the IC-POSS derivatives substituting isobutyl, phenyl and cyclohexyl groups at the Si corners and dimethylsilyl, trimethylsilyl, dimethylethylsilyl, dimethylvinylsilyl, dimethylphenylsilyl, and dimethylethylcyclohexyl groups at the opening moieties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement revealed that both the substituents at the corners and opening moieties significantly affected the thermal properties of the IC-POSS derivatives. We found volcano-type dependence of the phase transition temperature on the molecular weights of the substituents at the opening moieties in all the tested IC-POSS derivatives. The trimethylsilyl groups and the dimethylphenylsilyl groups at the open moieties showed highest and lowest phase transition temperatures, respectively. Especially, the phenyl groups at the opening moieties lead to low melting point and high thermal stability.

An optimized sample preparation approach for atomic resolution in situ studies of thin films.

This work provides the details of a simple and reliable method with less damage to prepare electron transparent samples for in situ studies in scanning/transmission electron microscopy. In this study, we use epitaxial VO2 thin films grown on c-Al2 O3 by pulsed laser deposition, which have a monoclinic-rutile transition at ~68°C. We employ an approach combining conventional mechanical wedge-polishing and Focused Ion beam to prepare the electron transparent samples of epitaxial VO2 thin films. The samples are first mechanically wedge-polished and ion-milled to be electron transparent. Subsequently, the thin region of VO2 films are separated from the rest of the polished sample using a focused ion beam and transferred to the in situ electron microscopy test stage. As a critical step, carbon nanotubes are used as connectors to the manipulator needle for a soft transfer process. This is done to avoid shattering of the brittle substrate film on the in situ sample support stage during the transfer process. We finally present the atomically resolved structural transition in VO2 films using this technique. This approach significantly increases the success rate of high-quality sample preparation with less damage for in situ studies of thin films and reduces the cost and instrumental/user errors associated with other techniques. The present work highlights a novel, simple, reliable approach with reduced damage to make electron transparent samples for atomic-scale insights of temperature-dependent transitions in epitaxial thin film heterostructures using in situ TEM studies.

The Temperature Dependences of the Electron-Piezoelectric Potential Phonon Interacting System of Quasi Two Dimensional System in GaN and ZnS.

We investigated theoretically the temperature dependence of the quantum optical transition of quasi 2-Dimensional Landau splitting system, in GaN and ZnS. We apply the Quantum Transport theory (QTR) to the system in the confinement of electrons by square well confinement potential. We use the projected Liouville equation method with Equilibrium Average Projection Scheme (EAPS). Through the analysis of this work, we found the increasing properties of Quantum Transition Line Shapes (QTLSs) and the Quantum Transition Line Widths (QTLWs) of CdS and GaN with the temperature we also found that QTLW, γ(T) of ZnS < γ(T) of GaN in γ = 394 μm.

A re-assessment of the isosteric heat for CCl4 adsorption on graphite

We have carried out molecular simulations of carbon tetrachloride adsorption on graphite, in order to investigate the role of the octopole in potential models for the CCl4/graphite system, and the temperature dependence of the first-order gas-liquid transition in the first adsorbate layer. Two classes of potential model for carbon tetrachloride were considered: the first has 5 LJ sites and the second includes five partial charges to model the leading octopole. Both models are adequate to represent the vapour-liquid equilibrium, suggesting that the octopole makes an insignificant contribution to the properties of the bulk phase. Both models show that adsorbed CCl4 molecules are delocalized on a graphite surface because of the strong intermolecular interactions. It is found that the LJ sites on the chlorine atoms, not the octopole, play the most important role in matching the experimental isotherm and isosteric heat data with simulation. The heat is constant, across the first-order transition of the first adsorbate layer. The simulation results show that both the magnitude of the density jump, and the isosteric heat across the first-order transition, decrease as the temperature increases. This is in qualitative agreement with the 1972 experimental data of Avgul and Kiselev, but these experimental data exhibit an unusually strong decrease in the isosteric heat, and the coexistence region between the two phases displays an unusual asymmetrical shape. Detailed analysis of our simulation results, together with the calculated isosteric heat from the experimental isotherms of Machin and Ross, show that there may be errors associated with the heat data of Avgul and Kiselev at high temperatures.

Conservation of Ovary-Specific Genes, Foxl2, Aromatase, and Rspo1, in the Common Indian Garden Lizard, Calotes versicolor, That Lacks Chromosomal or Temperature-Dependent Sex Determination

Foxl2,Rspo1, and Aromatase are genes important in the ovary developmental pathway in mammals and birds. Here, we show their presence in the lizard, Calotes versicolor, which is known to lack a chromosomal as well as a temperature-dependent mode of sex determination and has an indeterminate, bipotential gonad throughout embryonic development. The expression of the 3 genes, as well as that of CvSox9 and Wnt4 - the known testis and ovary pathway genes - was studied by RT-PCR and whole tissue RNA in situ hybridization (WRISH) on the developing mesonephros gonadal complex (MGC). The expression of all 3 genes was initiated in the gonad shortly after its evagination from the mesonephros (day 5 onwards). CvFoxl2 generally was expressed in those MGCs in which CvSox9 was either not expressed or lowly expressed and vice versa. On the other hand, CvArom was expressed rather sporadically and randomly, showing no association with CvFoxl2, CvRspo1, or CvSox9, though in later stages WRISH preparations showed its coincidence with CvWnt4. CvRspo1 was expressed in almost all embryos right from day 5. Immunofluorescence localization of Rspo1 and Foxl2 proteins showed their presence in the gonads from day 10 onwards, and by day 25 it was primarily confined to the cortex but away from the coelomic epithelium of the gonadal cortex. Apparently both proteins were localized in the pregranulosa cells, Rspo1 in the cytoplasm and Foxl2 in the nucleus. Thus, it is clear that both CvFoxl2 and CvRspo1 are active in ovary formation, but whether they are expressed in the same or different cells is unknown. Though the transcription pattern of CvArom remains circumspect for its role in differentiation of the ovary, earlier evidence on aromatase inhibitor-induced reversal to the male sex indicates its importance in ovary function.

The effect of Ag addition on the crystallization kinetics and glass forming ability of Zr-(CuAg)-Al bulk metallic glass

In this study, the effect of Ag addition on the crystallization kinetics and glass forming ability (GFA) of Zr-(CuAg)-Al based bulk metallic glasses were investigated. X-ray diffraction (XRD) has been used to identify the amorphous nature and crystalline phases of the as cast samples. The crystallization and glass transition kinetics of the as-cast samples have been examined under non-isothermal conditions using differential scanning calorimetry (DSC). The DSC experiments have been carried out at different heating rates for all the samples. The heating rate dependence of crystallization and glass transition temperatures has been used to evaluate activation energy associate to these processes. In addition to this Avrami exponent has been measured at all heating rate using Various Heating Rate (VHR) method. The GFA and thermal stability of the samples have been investigated using the Trg, γ and fragility parameters. These parameters were found to be composition dependent, which indicated that among the studied bulk metallic glass systems the sample having 9 at. % Ag possessed maximum thermal stability.

Topological constraint model of alkali tellurite glasses

A topological constraint model is developed to elucidate the coordination environment of alkali tellurite glasses. The model is derived from temperature-dependent constraint theory and provides a quantitative connection between the connectivity of the glass network and the compositional dependence of the glass transition temperature (Tg). Our model gives support to the model proposed by Barney et al. [J. Phys. Chem. Lett.4, 2312–2316 (2013)], confirming their model of the coordination environment in alkali tellurites. The model simultaneously serves as a framework that can be used in binary alkali glass-formers to predict an expected coordination the network forming cation in an oxide glass.

Analytical description of domain morphology and phase diagrams of ferroelectric nanoparticles

Analytical description of domain structure morphology and phase diagrams of ferroelectric nanoparticles is developed in the framework of Landau-Ginzburg-Devonshire approach. To model realistic conditions of incomplete screening of spontaneous polarization at the particle surface, it was considered covered by an ultra-thin layer of screening charge with effective screening length. The phase diagrams, calculated for spherical Sn2P2S6 nanoparticles in coordinates "temperature" and "effective screening length" by finite element modelling, demonstrate the emergence of poly-domain region at the tricritical point and its broadening with increasing the screening length for the particle radius over a critical value. Metastable and stable labyrinthine domain structures exist in Sn2P2S6 nanoparticles with radius (8–10) nm and more. We derived simple analytical expressions for the boundaries between paraelectric, single-domain and poly-domain ferroelectric phases, tricritical point and the necessary condition for the appearance of labyrinthine domains, and demonstrated their high accuracy in comparison with finite element modelling results.Analytical expressions for the dependence of the ferroelectric-paraelectric transition temperature on the particle radius in the single-domain and poly-domain states of the particle were compared with analogous dependences experimentally measured for SrBi2Ta2O9 nanoparticles and simulated for Sn2P2S6 nanocrystals by Monte Carlo method within the framework of axial next-nearest-neighbours Ising model. The analytical expression for the ferroelectric-paraelectric transition temperature in the poly-domain state quantitatively agrees with experimental and simulated results, and it accurately reproduces empirical Ishikawa equation at all temperatures, justifying it theoretically. Analytical description shows that phase diagrams and domain morphologies, which are qualitatively similar to the ones calculated in this work, can be expected in other ferroelectric nanoparticles covered by the screening charges, being rather different for the ferroelectrics with the first and second order ferroelectric-paraelectric transitions respectively.

Thermal analysis of structural phase transition behavior of Ln2Ni1−xCuxO4+δ (Ln = Nd, Pr) under various oxygen partial pressures

Structural phase transition behavior between orthorhombic and tetragonal in Ln2Ni1−xCuxO4+δ (Ln = Nd, Pr), which attracts interest as new cathode material for solid oxide fuel cells, has been investigated with thermal analysis under controlled oxygen partial pressures (P(O2)). For Nd2Ni1−xCuxO4+δ with 0.0 ≤ x ≤ 0.1, temperature, enthalpy change (ΔH) and change in excess oxygen content (Δδ) at the phase transition decreased with increasing Cu content. The phase transition temperature decreased with decreasing P(O2), whereas little variation was observed in ΔH and Δδ. For Nd2Ni1−xCuxO4+δ with x ≥ 0.15, crystal structure was tetragonal and no phase transition was detected below 750 °C. For Pr2Ni1−xCuxO4+δ with x = 0.0 and 0.1, phase transition was observed by DSC, showing similar dependence of phase transition temperature and ΔH on Cu content and P(O2) with that of Nd2Ni1−xCuxO4+δ. Δδ was not detected in TG curve of Pr2Ni0.9Cu0.1O4+δ, which could be attributed to too small Δδ. From Ellingham diagram prepared using temperature and P(O2) at the phase transition, variation of standard enthalpy (ΔH°) and standard entropy (ΔS°) was evaluated. It was revealed that variation of phase transition temperature by Cu content and difference of the phase transition temperature between Nd2Ni1−xCuxO4+δ and Pr2Ni1−xCuxO4+δ showed correspondence with variation of ΔH°. Excess oxygen content, δ, in Nd2Ni1−xCuxO4+δ, evaluated with reduction in TG apparatus, decreased with increasing Cu content. It was suggested that some amount of δ was required for stabilization of orthorhombic phase and that low δ by Cu substitution stabilized the tetragonal structure of the specimens with x ≥ 0.15.

MADS-Box Transcription Factor MadsA Regulates Dimorphic Transition, Conidiation, and Germination of Talaromyces marneffei.

The opportunistic human pathogen Talaromyces marneffei exhibits a temperature-dependent dimorphic transition, which is closely related with its pathogenicity. This species grows as multinucleate mycelia that produce infectious conidia at 25°C, while undergoes a dimorphic transition to generate uninucleate yeast form cells at 37°C. The mechanisms of phenotype switching are not fully understood. The transcription factor madsA gene is a member of the MADS-box gene family. Previously, it was found that overexpression of madsA gene resulted in mycelial growth instead of yeast form at 37°C. In the current study, the madsA deletion mutant (ΔmadsA) and complemented strain (CMA) were constructed by genetic manipulation. We compared the phenotypes, growth, conidiation, conidial germination and susceptibility to stresses (including osmotic and oxidative) of the ΔmadsA with the wild-type (WT) and CMA strains. The results showed that the ΔmadsA displayed a faster process of the yeast-to-mycelium transition than the WT and CMA. In addition, the deletion of madsA led to a delay in conidia production and conidial germination. The tolerance of ΔmadsA conidia to hydrogen peroxide was better than that of the WT and CMA strains. Then, RNA-seq was performed to identify differences in gene expression between the ΔmadsA mutant and WT strain during the yeast phase, mycelium phase, yeast-to-mycelium transition and mycelium-to-yeast transition, respectively. Gene ontology functional enrichment analyses indicated that some important processes such as transmembrane transport, oxidation-reduction process, protein catabolic process and response to oxidative stress were affected by the madsA deletion. Together, our results suggest that madsA functions as a global regulator involved in the conidiation and germination, especially in the dimorphic transition of T. marneffei. Its roles in the survival, pathogenicity and transmission of T. marneffei require further investigation.

The spin-reorientation magnetic transitions in Ga-doped SmCrO3

Orthochromite SmCrO3 is known to show spin reorientation around 34 K and which was assigned as transition from Γ4 to Γ2. Our work reports the spin reorientation transitions in a series of Ga-doped SmCrO3: the spin reorientation transition from Γ4 to Γ2 at TSR2 = 88 K and another spin reorientation transition of Γ2 to Γ1 at TSR1 = 15 K were observed simultaneously when the content of Ga is 30%. Importantly, the evolution of spin reorientation transitions from the single spin reorientation of Γ4 → Γ2 to double ones of Γ4 → Γ2 → Γ1, and then to the single one of Γ2 → Γ1 is caused by the increase of Ga doping concentration. The M-T and M-H measurements have been made to study the temperature and magnetic field dependence of these spin reorientation transitions. These temperatures and doping dependence of spin reorientation transitions in RCrO3 (R = rare earth or yttrium) can appeal to the magnetic switching devices design based on these findings.

CuAgSe nanocrystals: colloidal synthesis, characterization and their thermoelectric performance

CuAgSe is a promising thermoelectric (TE) material for its superior carrier mobility and ultralow lattice thermal conductivity. Herein, we present a scalable colloidal method to prepare monodisperse CuAgSe nanocrystals with high yield. The collected powder sample was washed by a sulfur-free reagent of NaNH2 to remove the surface organic ligands (CuAgSe-W) and then annealed (CuAgSe-W-A). Both kinds of ligand-free samples were then hot pressed into dense pellets to measure the TE property. The results revealed that the crystal structure of both samples changed from low-temperature β-phase to high-temperature α-phase at around 465 K. Sample CuAgSe-W shows interesting temperature-dependent transition from N-type to P-type, which could be potentially used as thermal control transistor. Sample CuAgSe-W-A does not display this transition state but it exhibits potential for intermediate temperature TE applications with a figure-of-merit zT reaching 0.68 at 566 K.

Mössbauer Studies of Z-Type Sr3Co2Fe24O41 Strontium Ferrite

Sr3Co2Fe24O41 plocrystalline sample was synthesized by using a polymerizable complex method. We investigated the crystallographic and the magnetic properties of sample by using X-ray diffractometer, vibrating sample magnetometer, and Mossbauer spectrometer. Based on the Rietveld refinement, the crystal structure of the sample was found to be single-phased and to be hexagonal with space group of P63/mmc. The hysteresis curves of sample were measured under 15 kOe at various temperatures ranging from 4.2 and 295 K. From the temperature dependence of the magnetization curves under 100 Oe at temperatures between 4.2 and 740 K, three temperature-dependent magnetic transitions were found to have occurred in the Sr3Co2Fe24O41 sample. The Mossbauer spectra of the samples were obtained at various temperatures ranging from 4.2 and 295 K.

Composition dependence of transition temperatures on quaternary Co-Fe-Ni-Ga melt-spun alloys

The composition dependence of transition temperatures on quaternary Co-Fe-Ni-Ga melt-spun alloys with constant Ga content are examined. A few at.% of Fe replacement for Co in Co50Ni22Ga28 drastically reduce martensitic transformation temperature (TM) whereas ferromagnetic transition temperature (Tc) remains above 400 K. Increasing Ni instead of Co in Fe doped specimen can raise TM around room temperature again. Further increase of Fe content with this manner, the alloys finally connected to ternary Ni-Fe-Ga of ferromagnetic shape memory region. The martensitic transition start temperature (MS) of Co-Fe-Ni-Ga28 alloys show good correlation with the valence electron ratio (e/a). The MS vs e/a plot clearly shows that trend lines are separated for each Fe content, and intervals of trend lines seem to be proportional to Fe content at lower Fe region. The Tc and saturation magnetization of Co-Fe-Ni-Ga28 alloys show almost linear relation with magnetic valence Zm.

Bipolar Conduction as the Possible Origin of the Electronic Transition in Pentatellurides: Metallic vs Semiconducting Behavior

The pentatellurides, ZrTe5 and HfTe5 are layered compounds with one dimensional transition-metal chains that show a never understood temperature dependent transition in transport properties as well as recently discovered properties suggesting topological semimetallic behavior. Here we show that these materials are semiconductors and that the electronic transition is due to a combination of bipolar effects and different anisotropies for electrons and holes. We report magneto-transport properties for two kinds of ZrTe5 single crystals grown with the chemical vapor transport (S1) and the flux method (S2), respectively. These have distinct transport properties at zero field: the S1 displays a metallic behavior with a pronounced resistance peak and a sudden sign reversal in thermopower at approximately 130 K, consistent with previous observations of the electronic transition; in strikingly contrast, the S2 exhibits a semiconducting-like behavior at low temperatures and a positive thermopower over the whole temperature range. Refinements on the single-crystal X-ray diffraction and the energy dispersive spectroscopy analysis revealed the presence of noticeable Te-vacancies in the sample S1, confirming that the widely observed anomalous transport behaviors in pentatellurides actually take place in the Te-deficient samples. Electronic structure calculations show narrow gap semiconducting behavior, with different transport anisotropies for holes and electrons. For the degenerately doped n-type samples, our transport calculations can result in a resistivity peak and crossover in thermopower from negative to positive at temperatures close to that observed experimentally. Our present work resolves the longstanding puzzle regarding the anomalous transport behaviors of pentatellurides, and also resolves the electronic structure in favor of a semiconducting state.

Temperature dependence of excitonic transitions in Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum wells from 4 to 750 K

The excitonic optical properties of an Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum well structure were studied by photoluminescence (PL) spectroscopy at various temperatures. An analysis of the temperature dependence of PL peak energy revealed the stronger localization of excitons than biexcitons, consistent with the trend predicted from the difference between the spatial extents of excitons and biexcitons. The PL linewidth of the excitons increased linearly with increasing temperature up to 300 K and then more rapidly above 300 K. This result indicated that the exciton-longitudinal optical phonon interaction became prominent as the broadening mechanism of the exciton linewidth above 300 K. The luminescence of both excitons and biexcitons exhibited low thermal quenching, with their intensities at 300 K remaining at 37% and 47%, respectively, of their intensities at 4 K. These observations reflected the high thermal stability of the excitons and biexcitons in this multiple quantum well structure. In addition, the ratio of the PL intensity of the exciton-biexciton scattering to the product of the exciton and biexciton luminescence intensities was independent of temperature up to 200 K and increased rapidly with increasing temperature above 250 K. The temperature-independent behavior of this ratio up to 200 K originated from the localization of excitons and biexcitons, and the rapid increase in this ratio with temperature above 250 K reflected an increase in the frequency of interactions between the excitons and biexcitons due to the gradual delocalization of the excitons and biexcitons with increasing temperature.