Showing papers on "Transition temperature published in 1998"
TL;DR: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K as mentioned in this paper.
Abstract: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K. The Curie temperature determined from inflection points of the experimental magnetic susceptibility and heat capacity is in excellent agreement with those obtained from the magnetocaloric effect and Arrot plots. Above 2 T the temperature of this transition increases almost linearly with the magnetic field at a rate of $\ensuremath{\sim}6\mathrm{K}/\mathrm{T}$ in fields up to 7.5 T. The spin reorientation transition, which occurs at 227(2) K in the absence of a magnetic field, has been confirmed by susceptibility, magnetization, and heat-capacity measurements. Magnetic fields higher than 2--2.5 T apparently quench the spin reorientation transition and Gd retains its simple ferromagnetic structure from the ${T}_{C}(H)$ down to $\ensuremath{\sim}4\mathrm{K}.$ The nature of anomaly at $T\ensuremath{\cong}132\mathrm{K},$ which is apparent from ac susceptibility measurements along the $c$ axis, is discussed. The presence of large amounts of interstitial impurities lowers the second-order $\mathrm{paramagnetic}\ensuremath{\leftrightarrow}\mathrm{ferromagnetic}$ transition temperature, and can cause some erroneous results in the magnetocaloric effect determined in pulsed magnetic fields. The magnetocaloric effect was studied utilizing the same samples by three experimental techniques: direct measurements of the adiabatic temperature rise, magnetization, and heat capacity. All three techniques, with one exception, yield the same results within the limits of experimental error.
862 citations
TL;DR: In this article, it was shown that a first-order ferromagnetic phase transition with a transition temperature nearly equal to the bulk value can be found in trifluoroethylene polymers with diameters as small as 10'A (two monolayers) even in these almost two-dimensional films.
Abstract: Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films1,2, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature3. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature4. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 A in diameter5, in perovskite films 100 A thick6 and in crystalline ferroelectric polymers as thin as 25 A (7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size11,12,13. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 A (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.
754 citations
TL;DR: In this article, the authors demonstrate that the onset of non-exponential relaxation corresponds to a well defined temperature below which the depth of the potential-energy minima explored by the liquid increases with decreasing temperature, and above which it does not.
Abstract: Most materials attain a glassy state at low temperatures under suitable methods of preparation. This state exhibits the mechanical properties of a solid, but shows microscopic structural disorder1,2. A comprehensive understanding of the glassy state is, however, still lacking3. A widespread assumption is that the non-exponential relaxation processes observed in the dynamics of glasses — and also in protein dynamics, protein folding and population dynamics — are (in common with other manifestations of complex dynamics) strongly influenced by the underlying energy landscape associated with the structural configurations that the system may adopt. But concrete evidence for this in studies of glass formation has been scarce. Here we present such evidence, obtained from computer simulations of a model glass-forming liquid. We demonstrate that the onset of non-exponential relaxation corresponds to a well defined temperature below which the depth of the potential-energy minima explored by the liquid increases with decreasing temperature, and above which it does not. At lower temperatures, we observe a sharp transition when the liquid gets trapped in the deepest accessible energy basin. This transition temperature depends on the cooling rate, in a manner analogous to the experimental glass transition. We also present evidence that the barrier heights separating potential-energy minima sampled by the liquid increase abruptly at a temperature above the glass transition but well below the onset of non-exponential relaxation. This identification of a relationship between static, topographic features of the energy landscape and complex dynamics holds the promise of a clearer, possibly thermodynamic, understanding of the glass transition.
719 citations
TL;DR: The freezing process in lead magnesium niobate (PMN) has been investigated by measurements of the frequency-dependent complex dielectric constant and its third harmonic component as discussed by the authors.
Abstract: The freezing process in lead magnesium niobate (PMN) has been investigated by measurements of the frequency-dependent complex dielectric constant and its third harmonic component. The linear complex dielectric susceptibility was analyzed by a temperature-frequency plot in order to determine the temperature dependence of the dielectric relaxation spectrum and to identify the freezing temperature. It was found that both the shape of the relaxation spectrum and its temperature behavior in the PMN relaxor show remarkable similarities to dipolar glasses, i.e., the longest relaxation time diverges according to the Vogel-Fulcher law, while the bulk of the distribution of relaxation times remains finite even below the freezing temperature. The frequency and the temperature dependence of the third harmonic susceptibility, similar to the behavior observed in linear dielectric response, indicate that the same underlying relaxation spectrum and therefore the same slowing-down mechanism is controlling both linear and nonlinear dynamic response. The observed splitting between the field-cooled and zero-field-cooled dielectric constant---comparable to the one obtained in spin glasses---effectively demonstrates the occurrence of typical glassy nonergodic behavior in the vicinity of the transition temperature where the ferroelectric phase would appear above a threshold electric field.
239 citations
TL;DR: In this paper, it was shown that at pressures of around 100 GPa, solid molecular oxygen becomes superconducting, with a transition temperature of 6 K. The transition is revealed by both resistivity measurements and a Meissner demagnetization signal.
Abstract: Among the simple diatomic molecules, oxygen is of particular interest because it shows magnetism at low temperatures Moreover, at pressures exceeding 95 GPa (∼095 Mbar), solid molecular oxygen becomes metallic, accompanied by a structural transition1 The metallization process is characterized by an increase in optical reflectivity2, and a change in the slope of the resistance–temperature curve3 Here we report that at pressures of around 100 GPa, solid oxygen becomes superconducting, with a transition temperature of 06 K The transition is revealed by both resistivity measurements and a Meissner demagnetization signal
212 citations
TL;DR: In this paper, the critical temperature of (Ln1−xMx)2CuO4 superconductors was shown to decrease linearly with increasing A-site disorder, as quantified by the variance in the distribution of A site cation radii.
Abstract: The critical temperatures of (Ln1−xMx)2CuO4 superconductors1, in which Ln3+ (La and other lanthanides) and M2+ (Ca, Sr, Ba) cations are randomly distributed amongst the ‘type A’ lattice sites, are known to depend on the doping level, x, and the mean A-site cation radius, 〈rA〉 (refs 2, 3). Here we show, by studying series of compositions with the same doping level and 〈rA〉, that the critical temperature decreases linearly with increasing A-site disorder, as quantified by the variance in the distribution of A-site cation radii. From this, we are able to show that, in the absence of disorder, the critical temperature should increase quadratically with 〈rA〉 for superconductors containing a single CuO2 layer. Our results therefore show that the critical temperature is very sensitive to lattice strains, as has also been shown for the metal to insulator transition temperature in the magnetoresistive (Ln1−xMx)MnO3 perovskites4.
199 citations
TL;DR: In this paper, vanadium dioxide thin films have been obtained that exhibit both electrical and optical switching around 70°C and the switching temperature together with the shape of the hysteresis loop can be modified by doping VO2 films with foreign cations.
Abstract: Vanadium dioxide thin films have been deposited from vanadium alkoxides VO(OR)3. An amorphous film is formed that transforms into crystalline VO2 upon heating at 500°C under a reducing atmosphere. Optically transparent VO2 thin films are then obtained that exhibit both electrical and optical switching around 70°C. The switching temperature together with the shape of the hysteresis loop can be modified by doping VO2 films with foreign cations. Doped MxVO2 (M = W6+, Nb5+, Ti4+, Cr3+ or Al3+) thin films have been prepared under the same conditions by mixing the vanadium alkoxide and a metal salt in an alcoholic solution. The switching temperature decreases when the film is doped with high-valent cations (W6+) and increases with low-valent cations (Al3+, Cr3+). The transition temperature first decreases and then increases when TiIV is added to the VO2 film while the width of the hysteresis loop is significantly reduced.
153 citations
TL;DR: Two-dimensional (2D) superconductors were achieved by intercalating a long-chain organic compound into bismuth-based high-Tc cuprates and the superconducting transition temperature of the intercalate was nearly the same as that of the pristine material, suggesting the 2D nature of the high- Tc superconductivity.
Abstract: The free modulation of interlayer distance in a layered high–transition temperature (high-Tc) superconductor is of crucial importance not only for the study of the superconducting mechanism but also for the practical application of high-Tc superconducting materials Two-dimensional (2D) superconductors were achieved by intercalating a long-chain organic compound into bismuth-based high-Tc cuprates Although the intercalation of the organic chain increased the interlayer distance remarkably, to tens of angstroms, the superconducting transition temperature of the intercalate was nearly the same as that of the pristine material, suggesting the 2D nature of the high-Tcsuperconductivity
137 citations
TL;DR: In this paper, the authors performed positron lifetime measurements on poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) in the temperature range of 84-454 K and as a function of time (<200h) at four temperatures (T=84, 149, 224, and 249 K).
Abstract: Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84–414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84–454 K and as a function of time (<200 h) at four temperatures (T=84, 149, 224, and 249 K). The glass transition temperature Tg and secondary transition temperature were determined from the average o-Ps lifetime τ3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation σ(τ3) of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature Tg; (ii) the σ(τ3) in PMMA has no observable variation as a function of positron irradiation time, which suggests that the measured free-volume distribution is not influenced by the positron irradiation. The o-Ps formation probability shows large variations with temperature, thermal history, and measuring time. These effects are discussed in the framework of the spur model.
134 citations
TL;DR: In this article, a model is proposed to explain the occurrence of 3D ferromagnetic order, with a sizeable transition temperature, despite the fact that exchange interactions between layers are negligible.
114 citations
TL;DR: In this paper, the authors investigated the structure and magnetic properties of manganates and found that the charge ordering in the presence of magnetic fields has a significant effect on the transition from a ferromagnetic ground state to a metallic state.
Abstract: The charge ordering in $Nd_{0.5}Sr_{0.5}MnO_3 (\langle r_A\rangle = 1.24 \AA),$ which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For $Y{0.5}Ca_{0.5}MnO_3 (\langle r_A\rangle = 1.13 \AA),$ for which the ground state is charge ordered, on the other hand, magnetic fields have no effect on the charge ordering. In order to understand such a marked difference in charge-ordering behaviour of the manganates, we have investigated the structure as well as the electrical and magnetic properties of $Ln_{0.5}Ca_{0.5}MnO_3$ compositions (Ln D Nd, Sm, Gd and Dy) wherein $\langle r_A\rangle$ varies over the range $1.17–1.13 \AA$. The lattice distortion index, D, and charge-ordering transition temperature, $T_{CO}$, for the manganates increase with the decreasing $\langle r_A\rangle$. The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of $Nd_{0.5}Ca_{0.5}MnO_3$ $(\langle r_A\rangle = 1.17 \AA),$ but this is not the case with the analogous Sm, Gd and Dy manganates with $\langle r_A\rangle$ less than $1.17 \AA$. In order to explain this behaviour, we have examined the $\langle r_A\rangle$ -dependence of the Mn–O–Mn bond angle, the average Mn–O distance and the apparent one-electron bandwidth, obtained from these structural parameters. It is suggested that the extraordinary sensitivity of the charge ordering to $\langle r_A\rangle$ arises from factors other than those based on the Mn–O–Mn bond angle and average Mn–O distances alone. It is possible that the competition between the covalent mixing of the oxygen $O:2p\sigma$ orbital with the A-site and B-site cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in $T_{CO}$.
TL;DR: In this article, a fine interplay of magnetic exchange, structural properties and electronic transport in manganate perovskites of the general formula Lnl − xAxMnO3 (Ln = rare earth; A = divalent ion) is discussed in detail.
TL;DR: In this article, the toughness of PP/EPDM blends was measured over a wide range of temperature (25 −132°C) and composition 0 −26 wt% EPDM.
TL;DR: A review of the literature on substitutions and their effect on the properties of the 90 K superconductor YBa 2 Cu 3 O δ (YBCO) can be found in this paper.
Abstract: The 90 K superconductor, YBa 2 Cu 3 O δ (YBCO), has proved to be highly adaptable chemically as it can accommodate a wide variety of cationic and anionic substitutions. Indeed, the majority of the chemical elements, excluding noble gases and actinides, have been reported to substitute to some extent into the YBCO structure. This review covers the literature on such substitutions and their effect on the properties of YBCO. Reported solubility limits are given, together with crystal symmetry and trends in unit cell parameters with dopant concentration. The dopant site is considered; this is additionally complex in the case of copper substitution because of the two distinct copper sites in the crystal structure. The effect of the dopant on the critical temperature, T c , is reviewed; the literature is often contradictory due to the dual effects of variable oxygen content and the nature of the dopant. Preparation methods appear to have an effect on solubility limits, crystal symmetry and T c , Also, the methods used to determine solubility limits are often imprecise which can lead to contradictions. The magnetic properties of doped materials are reviewed; for some dopants, particularly the magnetic lanthanides, antiferromagnetism and superconductivity co-exist. The related RBa 2 Cu 3 O δ phases (R = lanthanide), their structure, properties and behaviour on doping are reviewed in a similar way. For the larger rare earths, the related systems R 1+ x Ba 2- x CU 3 O δ , are reviewed; as x increases, the transition temperature decreases and compositions R 1.5 Ba 1.5 Cu 3 O δ are semiconducting. The upper and lower solubility limit changes with R, and for R = Dy, the upper limit is x = 0 composition, LaBa 2 Cu 3 O δ , cannot be prepared in air since substitution of La onto the Ba site occurs, forming the Ba-deficient solid solutions. The discovery of superconductivity above liquid nitrogen temperatures in YBa 2 Cu 3 O 7 , has generated a vast quantity of research; to date, > 20 000 papers have appeared on the 123 materials (source: BIDS). This review, therefore, cannot be comprehensive, but attempts to highlight important substitutions, their effect on the properties of YBCO and any contradictions in the literature.
TL;DR: In this paper, the second-neighbor Coulomb interaction of the cations on the octahedral sites was investigated and the Verwey temperature shift as a function of the substituent concentration was determined from the temperature dependence of the resistivity.
Abstract: Magnetite single crystals Fe32xMxO4 doped with M5Ni, Co, Mg, Al, Ga, and Ti were grown and annealed under a controlled atmosphere to produce homogeneous and oxygen stoichiometric samples. The cation vacancy concentration of the samples was proved to be lower than 10 26 by means of magnetic disaccommodation spectroscopy. The Verwey temperature shift as function of the substituent concentration was determined from the temperature dependence of the resistivity. The systematics of the transition temperature shift as function of the concentration and nature of the substituents is indicative that the mechanism of the transition is related to the second-neighbor Coulomb interaction of the cations on the octahedral sites. @S0163-1829~98!00846-7#
TL;DR: In this article, the effect of three levels of zirconia additive on the anatase-rutile (A→R) transition in a sol-gel-derived powder was investigated using differential thermal analysis and X-ray diffraction.
TL;DR: In this paper, the Parisi overlap distribution for the three dimensional Ising spin glass in the Migdal-Kadanoff approximation was studied for temperatures T around 0.7Tc and system sizes upto L=32, and they found a P(q) as expected for the full Parisi replica symmetry breaking.
Abstract: We have studied the Parisi overlap distribution for the three dimensional Ising spin glass in the Migdal-Kadanoff approximation. For temperatures T around 0.7Tc and system sizes upto L=32, we found a P(q) as expected for the full Parisi replica symmetry breaking, just as was also observed in recent Monte Carlo simulations on a cubic lattice. However, for lower temperatures our data agree with predictions from the droplet or scaling picture. The failure to see droplet model behaviour in Monte Carlo simulations is due to the fact that all existing simulations have been done at temperatures too close to the transition temperature so that sytem sizes larger than the correlation length have not been achieved.
TL;DR: In this paper, the authors showed that the BaTiO3 films are ferroelectric tetragonal phase crystals and that the transition temperature for the Ferroelectric-paraelectric phase transition of the films is higher than bulk crystal's.
Abstract: Epitaxial ultrathin films of BaTiO3 were prepared using molecular beam epitaxy. For the substrate, (001)-oriented SrTiO3 single crystals were used. Controlling the growth conditions of these films as well as the semiconductor thin films, led to the successful growth of the BaTiO3 films as single crystals, characterized by x-ray diffraction even in the ultrathin range. The ultrathin BaTiO3 films are highly c-axis-oriented tetragonal phaselike bulk BaTiO3 crystals. The tetragonality of the thin film crystals is much larger than bulk crystal’s. We also measured the saturated polarization (Ps) of the BaTiO3 films at temperatures ranging from room temperature to 600 °C. The results confirmed again that the films are ferroelectric tetragonal phase crystals. Moreover, they showed that the transition temperature for the ferroelectric-paraelectric phase transition of the films is higher than bulk crystal’s.
TL;DR: In this paper, the thermal stability of nanocrystalline maghemite O was investigated by using differential thermal analysis (DTA) measurements and x-ray diffraction analyses and it was confirmed for the first time that this exothermic peak corresponds completely to the Maghemite-to-hematite structural phase transition.
Abstract: Nanocrystalline maghemite O was prepared by a wet chemical method. The thermal stability of this material has been investigated by using differential thermal analysis (DTA) measurements and x-ray diffraction analyses. One exothermic peak without weight loss appears irreversibly on the DTA curve. It was confirmed for the first time that this exothermic peak corresponds completely to the maghemite-to-hematite (-to-) structural phase transition. The nanocrystalline maghemite O particles grow very slowly when the temperature is lower than the range of the exothermic peak; whereas, during and after the transition, the nanocrystalline hematite O particles grow rapidly with increasing temperature. Moreover, the transition temperature of nanocrystalline maghemite O is higher than that of the coarse-grained counterpart. In this paper, the authors have also suggested some preliminary physical interpretations of the experimental results.
TL;DR: In this paper, a large single crystal (about 30 mm in diameter by 20 mm long) of the Pb(Zn1/3Nb2/3)O3-PbTiO3 solid solution system was successfully grown using the vertical Bridgeman method and its properties were investigated.
Abstract: A large single crystal (about 30 mm in diameter by 20 mm long) of the Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) solid solution system was successfully grown using the vertical Bridgeman method and its properties were investigated. No defects such as flux inclusions were observed on as-polished {100} plates, except in a small area near the nucleation point. Further, a domain structure with striped patterning, and an opaque area were observed in the light yellow plate. Distribution of the temperature of transition (TRT) from the ferroelectric rhombohedral phase to the tetragonal phase was investigated within the {100} plates. Transition temperatures were found to be scattered in the range of 48°C to 103°C across the area of these plates. Small rectangular samples poled along the [001] direction and cut along the [100] direction from (001) plates containing 9.5 to 10 mol% of PbTiO3, and with a transition temperature of TRT=70–90°C, exhibited high electromechanical coupling constants (rectangular bar mode, k33'>85%) at room temperature.
TL;DR: In this article, the effect of the crystallization tem- perature (Tcr) on the phase transitions presented by P(VDF/TrFE) copolymers cast from dimethylformamide (DMF) solution with molar ratios 60/40, 70/30, and 80/20 was investigated.
Abstract: A systematic study was carried out on the effect of the crystallization tem- perature (Tcr), on the phase transitions presented by P(VDF/TrFE) copolymers cast from dimethylformamide (DMF) solution with molar ratios 60/40, 70/30, and 80/20. The results obtained by differential scanning calorimetry (DSC) showed that two- phase transitions are observed only when the copolymer crystallizes above a certain temperature To, and that the temperatures at which these transitions occur are reduced slightly with Tcr increase. It also was observed that when Tcr increases, the intensity of the endotherm corresponding to the lowest temperature transition is increased, whereas the one corresponding to the highest temperature transition is reduced. In order to explain these phenomena, the existence of two ferroelectric and two paraelectric phases is suggested. The conformational differences between like phases occur due to the distinct origin of each one: the best organized phase crystallizes directly from solution, whereas the least organized is the result of a solid phase transition. Wide angle x-ray diffusion (WAXD) diffractograms corroborate this hypothesis. Phase diagrams for samples crystallized below and above To have been proposed. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 403-414, 1998
TL;DR: In this article, a new route comprising 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPSA) as both the protonating acid and the solvating group, and dichloroacetic acid (DCA) as a solvent was presented.
Abstract: Conductive polyaniline (PANi) films have been prepared via a new route comprising 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPSA) as both the protonating acid and the solvating group, and dichloroacetic acid (DCA) as a solvent. The AMPSA content was varied so that between 30 and 100% of the nitrogen sites on polyaniline could be protonated. The temperature dependence of the conductivity of the films was measured between 15 and 300 K. Above 75-115 K, depending upon the protonation level, a negative temperature coefficient was observed. At 240-260 K a new transition was observed, essentially independent of the protonation level, above which the negative temperature dependency becomes much stronger. The conductivity data were fitted using a simple model employed previously for films of polyaniline camphorsulphonate, and comparisons made between the two systems. Differential scanning calorimetry measurements on a series of films showed an endothermic transition centred at 240-254 K, whose magnitude is dependent upon the AMPSA content. This thermal transition is ascribed to increased motion in the AMPSA anions when heated to above the transition temperature. The AMPSA anion may be considered to behave as a side chain ionically bonded to the PANi backbone, whose particular structure will affect the intrachain molecular dynamics, and hence electrical transport properties of the bulk polymer. The observed thermal transition correlates well with the new transition seen in the conductivity data. It was possible to draw films of uniaxially both at room temperature and at 363 K. The room temperature conductivity along the stretch direction was increased to a maximum value of for a film drawn at 363 K, compared to for an undrawn sample. The films are compared to previous results obtained with polyaniline films protonated with 10-camphorsulphonic acid (CSA).
TL;DR: In this article, the effects of grain size and oxygen stoichiometry in sol-gel and ceramics samples of La0.67Ca0.33MnO3 ± δ have been studied.
TL;DR: In this paper, an ellipsometry study of the wetting of hexane on water by adding salt to the water, the authors were able to tune the Hamaker constant of this system and demonstrate that two rather than one wetting transitions can exist in a single system.
Abstract: We report an ellipsometry study of the wetting of hexane on water By adding salt to the water, we are able to tune the Hamaker constant of this system This allows us to demonstrate, for the first time, that two rather than one wetting transitions can exist in a single system Upon increasing the temperature, a discontinuous (first-order) transition from a microscopic film to a mesoscopic film occurs, followed by a continuous (critical) wetting transition that leads to a thick adsorbed film The latter is due to the Hamaker constant which changes sign with temperature The first-order transition temperature changes by the same amount as the critical wetting temperature upon changing the Hamaker constant
TL;DR: In this article, a simple low-temperature extension of Landau theory is used, where the extent of quantum mechanical effects is characterized by a saturation temperature, which is related to the extent to which changes in the hard phonon modes influence the transition mechanism.
Abstract: At low temperatures, the behaviour of structural phase transitions is modified by the influence of quantum fluctuations. Such fluctuations enhance the stability of the high-symmetry phase, reducing the observed transition temperature. The effect on phase diagrams of temperature versus some other control parameter (e.g. pressure or chemical composition) is described. A simple low-temperature extension of Landau theory is used, where the extent of quantum mechanical effects is characterized by a saturation temperature . The theory is used to model the phase diagrams of the mineral anorthoclase , the ferroelectric materials ( for the 20 K transition), and SbSI . The saturation temperature is related to the extent to which changes in the hard phonon modes influence the transition mechanism.
TL;DR: In this article, the specific volume of the potato starch extruded into a transparent amorphous glass with a density of 1.5 g/cm3 at room temperature was studied.
Abstract: This paper presents dilatometric and calorimetric experiments on potato starch extruded into a transparent amorphous glass with a density of 1.5 g/cm3 at room temperature. The specific heat increment at the glass transition is used to estimate the transition temperature of samples containing up to 25 wt % water. The specific volume of the samples is studied between 25 and 165 C and between 0.1 and 100 MPa. Glass transitions estimated from the compressibility increment at the transition temperature are found in agreement with those detected by calorimetry. In the entire experimental temperature range, a maximal excess volume of mixing is observed at a composition corresponding to three water molecules per anhydroglucose in the mixture. This suggests that the large size difference between the chemical components allows the water molecules to saturate only one hydroxyl group of the anhydroglucose at a time. Specific volumes of starch glasses and melts are superimposed onto a single master curve by a simple e...
TL;DR: In this article, the long-range order (LRO) parameters for L1 0 FePt have been determined by the X-ray diffraction method from powder specimens as functions of time and temperature from 773 K to near 1573 K. The deformation structure investigated by TEM shows that slip and twinning are the two major deformation mechanisms.
TL;DR: In this article, the authors report direct magnetic characterizations of the premartensitic transition for the stoichiometric Ni2MnGa alloy, which is characterized by a peak in M(T) well above the Martensitic temperature Tm.
Abstract: Magnetization as a function of field and temperature for a ferromagnetic Heusler alloy Ni2MnGa is reported. Magnetization above the Martensitic transition displays a field-dependent peak effect, a direct magnetic evidence of premartensitic phase. At low fields, the peak effect occurs at a temperature consistent with the observations of the micromodulated structure transition seen from neutron scattering, electron microscopy, and ultrasonic studies in this compound. At high fields, the peak effect is suppressed. The strong field dependence of the peak temperature suggests a large magnetoelastic interaction in the intermediate phase. @S0163-1829~98!09537-X# The ferromagnetic Heusler alloy Ni22xMn11xGa with x 50 was first studied in the early 1980’s. 1 For the stoichiometric Ni2MnGa, the alloy was found to be ferromagnetic with a Curie temperature of 376 K. A martensitic phase transition from a cubic structure to a complex tetragonal structure at 202 K on cooling was observed from microscopy and neutron-scattering measurements, with a corresponding jump in magnetization at the same temperature. Later studies on alloys with nonstoichiometric compositions show that both the Curie temperature and the Martensitic transition can be varied with x. 2‐4 Recent interest in the Ni22xMn11xGa alloys as shape memory materials has lead to much more careful studies of the structural transition. It has been reported that the Martensitic transition is proceeded by a premartensitic transition as observed from several experiments such as x-ray, electron- and neutron-scattering, and ultrasound measurements. 5‐11 However, it is generally believed that there is no magnetic anomaly corresponding to the premartensitic transition. 2‐4 In this paper, we report direct magnetic characterizations of the premartensitic transition for the stoichiometric Ni2MnGa alloy. Magnetization M as a function of temperature T at various applied field H shows clear evidence of a premartensitic transition. The premartensitic transition is characterized by a peak in M(T) well above the Martensitic transition temperature Tm . The premartensitic transition temperature T p is found dependent on the applied field. The field dependence of T p demonstrates a large magnetoelastic effect in the premartensitic or intermediate state. Samples are prepared with the conventional arc-melt process with the stoichiometric composition of starting materials. 1 Structural analysis confirms the single phase,
TL;DR: In this paper, the spinel-type compound CuIr2(S12xSex)4 was synthesized and a systematic experimental study of structural, electrical, and magnetic properties of the spinels was carried out.
Abstract: The thiospinel CuIr2S4 exhibits a temperature-induced metal-insulator (M -I) transition around 226 K, showing hysteresis on heating and cooling, that manifests itself as a gap in the electronic density of state with increasing electrical resistivity at low temperatures. Conversely, CuIr2Se4 remains metallic down to 0.5 K. We have successfully synthesized the spinel-type compound CuIr2(S12xSex)4 system. In order to see the effect of substitutions of Se at the S sites, we have carried out a systematic experimental study of structural, electrical, and magnetic properties of CuIr2(S12xSex)4 . Mossbauer spectroscopy measurements of Ir have been performed for CuIr2S4 and CuIr2Se4 . The M -I transition of CuIr2(S12xSex)4 for x 0.80. Magnetic susceptibility measurements show the jump at the M -I transition temperature with hysteresis on heating and cooling. The high-temperature metallic phase of CuIr2S4 shows Pauli paramagnetism, having a density of states at the Fermi level, D(eF)50.67 states/eV atom. The insulating phase at low temperatures exhibits diamagnetism, and there is no localized magnetic moment. The Arrhenius regime is observed for the conductivity with a thermally activated process for 0
TL;DR: In this paper, the yield stress behavior of a range of polyethylene materials which differ with respect to their short chain branch content has been studied, and it has been shown that there is a sudden transition in the behavior of yield stress at a temperature which is dependent on both the grade of material and the applied strain rate.
Abstract: The yield stress behavior of a range of polyethylene materials which differ with respect to their short chain branch content has been studied. Measurements carried out over a wide range of temperatures have shown that there is a sudden transition in the behavior of the yield stress at a temperature which is dependent on both the grade of material and the applied strain rate. These results are in agreement with previous results found from analysis of the yield strain behavior.
Above the transition temperature the materials all behave in a nonlinear viscoelastic manner, and the yield process is considered as being propagation controlled. Below the transition temperature the materials all behave in an elastic-plastic manner, and the yield process is considered as being nucleation controlled. Below the transition temperature the temperature dependence of the yield stress is determined by the thickness of the crystalline lamellae. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2177–2189, 1998