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Showing papers on "Transition temperature published in 2004"


Journal ArticleDOI
12 Aug 2004-Nature
TL;DR: It is shown that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferro electricity in strontium titanate, a material that is not normally ferroelectric at any temperature.
Abstract: Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

1,861 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of grain size and dc field on the dielectric properties of BZT ceramics was investigated, and the fine-grained sample showed a relaxor-like ferroelectric behavior.

250 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used finite element models to predict material response in internally heated nozzle tests, and the results of the modeling suggest that HfB2 should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity.
Abstract: The thermal conductivity, thermal expansion, Young's Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB2, HfC0.98, HfC0.67, and HfN0.92 ceramics. The mechanical behavior of αHf(N) solid solutions was also studied. The thermal conductivity of modified HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0.98 to 1100°C for HfC0.67 ceramics. The transition temperature of HfB2 was 1100°C. Pure HfB2 was found to have a strength of 340 MPa in 4 point bending, that was constant from room temperature to 1600°C, while a HfB2 + 10% HfCx had a higher room temperature bend strength of 440 MPa, but that dropped to 200 MPa at 1600°C. The data generated by this effort was inputted into finite element models to predict material response in internally heated nozzle tests. The theoretical model required accurate material properties, realistic thermal boundary conditions, transient heat transfer analysis, and a good understanding of the displacement constraints. The results of the modeling suggest that HfB2 should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity. The comparison the theoretical failure calculations to the observed response in actual test conditions show quite good agreement implying that the behavior of the design is well understood.

233 citations


Journal ArticleDOI
19 Feb 2004-Nature
TL;DR: Detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures are reported, substantiating the conclusion that the glass transition of water cannot be probed directly.
Abstract: The formation of glasses is normal for substances that remain liquid over a wide temperature range (the ‘good glassformers’) and can be induced for most liquids if cooling is fast enough to bypass crystallization. During reheating but still below the melting point, good glassformers exhibit glass transitions as they abruptly transform into supercooled liquids, whereas other substances transform directly from the glassy to the crystalline state. Whether water exhibits a glass transition before crystallization has been much debated over five decades1,2,3,4,5,6. For the last 20 years, the existence of a glass transition at 136 K (ref. 3) has been widely accepted2,3,4, but the transition exhibits qualities difficult to reconcile with our current knowledge of glass transitions2,5,6. Here we report detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures. We compare our results to the behaviour of glassy water and find that small endothermic effects, such as the one attributed to the glass transition of water, are only a ‘shadow’ of the real glass transition occurring at higher temperatures, thus substantiating the conclusion6 that the glass transition of water cannot be probed directly.

221 citations


Journal ArticleDOI
TL;DR: In this paper, the authors observed that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting sizedependent optical resonances and size-dependent transition temperatures.
Abstract: The size effects on the optical properties of vanadium dioxide nanoparticles in ordered arrays have been studied. Contrary to previous VO2 studies, we observe that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting sizedependent optical resonances and size-dependent transition temperatures. The collective optical response as a function of temperature presents an enhanced scattering state during the evolving phase transition. The effects appear to arise because of the underlying VO2 mesoscale optical properties, the heterogeneous nucleation behind the phase transition, and the incoherent coupling between the nanoparticles undergoing an order-disorder-order transition. Calculations that support these interpretations are presented. The study of photon-matter interactions at nanometer length scales has as its ultimate goal the optimization of the coupling between selected radiation modes and specific material excitations. Among the most exciting prospects are those that incorporate metal or semiconductor nanostructures into periodic arrays, with potential applications as photonic crystals [1], biochemical sensors [2], and near-field electromagnetic waveguides [3]. In periodic arrays of nanostructures, new properties arise from the combination of nanoscale material features and the periodicity of the arrays. These include the size-dependent and shape-dependent shift of optical resonances, and near-field or far-field coupling already observed in two-dimensional metal nanoparticle arrays [4 –7]. In this Letter, we describe first observations of the collective optical response of vanadium dioxide nanoparticle (NP) arrays during a reversible semiconductorto-metal phase transition (SMT). A previously unreported resonance in the visible region is found in the spectral signature of the SMT, and the resonance peak, apparently due to multipole effects, is blueshifted with decreasing NP size. Unlike the hysteretic response of VO2 NPs with inhomogeneous size and spatial distributions, the hysteresis associated with the NP arrays shows a distinctive three-state behavior resulting from the differential scattering efficiency between the metal and semiconducting phases of VO2. This unique optical response may be viewed as an order-disorder transition in the spectral response of the array, superimposed on the hysteretic response of individual NPs, and contains significant new insights about the stochastic nature of the mechanism that triggers the metal-insulator transition. VO2 exhibits a SMT at a critical temperature Tc 67 C that is a result of an atomic rearrangement. Above T c , VO 2 has a tetragonal rutile structure and exhibits metallic properties. Below Tc, VO2 is a narrow-gap semiconductor with a monoclinic unit cell [8]. The reversible VO2 SMT displays a 10 4 jump in conductivity and

219 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that hadron multiplicities in central high energy nucleus-nucleus collisions are established very close to the phase boundary between hadronic and quark matter.

203 citations


Journal ArticleDOI
TL;DR: In this paper, the Pt57.5Cu14.7Ni5.3P22.5 composition has a liquidus temperature of 795 K, a glass transition temperature of 508 K, and a supercooled liquid region of 98 K.
Abstract: Highly processable bulk metallic glass alloys in the Pt–Co–Ni–Cu–P system were discovered. The alloys show low liquidus temperature below 900 K, excellent processability with low critical cooling rate reflecting in maximum casting thicknesses in quartz tubes of up to 20 mm, and a large supercooled liquid region. The Pt57.5Cu14.7Ni5.3P22.5 composition has a liquidus temperature of 795 K, a glass transition temperature of 508 K with a supercooled liquid region of 98 K. For medical and jewelry applications a Ni-free alloy, Pt60Cu16Co2P22 was discovered with a liquidus temperature of 881 K, a glass transition temperature of 506 K, and a supercooled liquid region of 63 K. Glass formation was observed in a wider composition range. Vickers hardness of these alloys is in the 400 Hv range. The alloys can be processed in the supercooled liquid region in air without any measurable oxidation. In this region, a large processing window is available in which the material does not embrittle. Embrittlement in these alloys is correlated with crystallization. It can be avoided as long as substantial crystallization does not take place during isothermal processing in the supercooled liquid region. Also, liquid processing can be performed in air when flux with B2O3.

167 citations


Journal ArticleDOI
21 Apr 2004-Langmuir
TL;DR: The investigation of pH dependence of two-phase transitions further indicates the PNIPAM brush reveals two separate transitions even with a change in interchain/interparticle association, which indicates that in the first transition PnIPAM segments show much higher cooperativity than in the second one.
Abstract: The thermally induced phase transition of the poly(N-isopropylacrylamide) (PNIPAM) brush covalently bound to the surface of the gold nanoparticles was studied using high-sensitivity microcalorimetry. Two types of PNIPAM monolayer protected clusters (MPCs) of gold nanoparticles were employed, denoted as the cumyl- and the cpa-PNIPAM MPCs, bearing either a phenylpropyl end group or a carboxyl end group on each PNIPAM chain, respectively. The PNIPAM chains of both MPCs exhibit two separate transition endotherms; i.e., the first transition with a sharp and narrow endothermic peak occurs at lower temperature, while the second one with a broader peak occurs at higher temperature. With increase of the MPC concentration, the transition temperature corresponding to the first peak only slightly changes but the second transition temperature strongly shifts to lower temperature. The calorimetric enthalpy change in the first transition is much smaller than that in the second transition. The ratio of the calorimetric enthalpy change to the van't Hoff enthalpy change indicates that in the first transition PNIPAM segments show much higher cooperativity than in the second one. The investigation of pH dependence of two-phase transitions further indicates the PNIPAM brush reveals two separate transitions even with a change in interchain/interparticle association. The observations are tentatively rationalized by assuming that the PNIPAM brush can be subdivided into two zones, the inner zone and the outer zone. In the inner zone, the PNIPAM segments are close to the gold surface, densely packed, less hydrated, and undergo the first transition. In the outer zone, on the other hand, the PNIPAM segments are looser and more hydrated, adopt a restricted random coil conformation, and show a phase transition, which is dependent on both concentration of MPC and the chemical nature of the end groups of the PNIPAM chains. Aggregation of the particles, which may also affect the phase transition, is briefly discussed.

160 citations


Journal ArticleDOI
TL;DR: Temperature-dependent lifetime measurements show that the quenching rate is high, resulting in an on/off behavior, and smaller particles, with higher local ionic strength due to a higher density of charged NH(3)(+) surface groups, experience a lower transition temperature due to stronger local freezing-point depression.
Abstract: Luminescence temperature antiquenching (LTAQ) is observed for water-soluble CdTe quantum dots (QDs) capped with aminoethanethiol (AET). The efficient exciton emission (quantum efficiency of approximately 40% at 300 K) is quenched almost completely as the QD solutions are cooled to below 230 K and is fully recovered around 270 K upon warming up to room temperature (LTAQ). Temperature-dependent lifetime measurements show that the quenching rate is high, resulting in an on/off behavior. No LTAQ is observed for CdTe QDs capped with aminoundecanethiol (AUT). The LTAQ is explained by the influence of solvent freezing on the surface of the QD core. Freezing of the solvation water molecules surrounding the QD will induce strain in the capping shell, due to the interaction between water and the charged heads of the capping molecules. Short carbon chains (AET) will propagate the strain to the QD surface, creating surface quenching states, whereas long and flexible chains (AUT) will dissipate the strain, thus avoiding surface distortion. Freezing-point depression by the addition of methanol results in a lowering of the transition temperature. Additional support is provided by the size dependence of the LTAQ: smaller particles, with higher local ionic strength due to a higher density of charged NH(3)(+) surface groups, experience a lower transition temperature due to stronger local freezing-point depression.

146 citations


Journal ArticleDOI
TL;DR: The superfluid critical temperature and the pair-breaking temperature are determined as functions of the attractive interaction between Fermi atoms, from the weak- to the strong-coupling limit (where bosonic molecules form as bound-fermion pairs).
Abstract: We consider the BCS-BEC (Bose-Einstein-condensate) crossover for a system of trapped Fermi atoms at finite temperature, both below and above the superfluid critical temperature, by including fluctuations beyond mean field. We determine the superfluid critical temperature and the pair-breaking temperature as functions of the attractive interaction between Fermi atoms, from the weak- to the strong-coupling limit (where bosonic molecules form as bound-fermion pairs). Density profiles in the trap are also obtained for all temperatures and couplings.

145 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the transition temperature of nanoparticles of ZnFe 2 O 4 and found that the Neel temperature of the nanoparticles is much higher than that of the bulk material.

Journal ArticleDOI
TL;DR: In this article, the cubic-to-tetragonal transition of zirconia is shown to be displacive, and the transition is first-order in both volume and latent heat.
Abstract: We present a new interatomic potential capable of describing the cubic and tetragonal phases of zirconia. From an analysis of molecular-dynamics simulations of the structural fluctuations in the cubic phase close to the transition temperature, we show that the cubic-to-tetragonal transition is displacive. In addition, the discontinuous change in volume and the latent heat at the transition demonstrate clearly that the transition is first order. The model correctly predicts that doping with yttria tends to stabilize the cubic phase at lower temperatures. A description of the first-order nature of dopant-driven transitions is given.

Journal ArticleDOI
TL;DR: In this article, it was shown that the Zn4Sb3 phase does not exist below 767 K (the β−γ transition temperature) and that the structure of Zn6-δSb5 is similar to that of Sb3 but no Zn/Sb mixture is observed on any Sb site.
Abstract: Composition, crystal structure, and stability of the thermoelectric material, known in the literature as “Zn4Sb3”, has been characterized using low- and room-temperature single-crystal X-ray diffraction techniques, as well as in situ room- and high-temperature powder X-ray diffraction methods. We have found that the Zn4Sb3 phase does not exist below 767 K (the β−γ transition temperature); it is the Zn6-δSb5 phase that is erroneously assigned the Zn4Sb3 composition and is considered to be a promising thermoelectric material. The structure of Zn6-δSb5 is similar to that of “Zn4Sb3” but no Zn/Sb mixture is observed on any Sb site. Instead, a significant deficiency on the Zn site is discovered. There are two, not one, as previously reported, Zn6-δSb5 polymorphs below room temperature. In dynamic vacuum and at elevated temperatures the Zn6-δSb5 phase becomes zinc poorer due to zinc sublimation and eventually decomposes into ZnSb and Zn before reaching its melting temperature of 841 K. The binary Zn1-δSb compou...

Journal ArticleDOI
TL;DR: In this article, the formation of single and two-phase glasses from rare-earth oxide-alumina materials was reported, which were attributed to nucleation and growth of the second liquid at a temperature below the equilibrium liquid-liquid transition temperature.
Abstract: We report formation of single- and two-phase glasses from rare-earth oxide–alumina materials. Liquids with the Y3Al5O12 and Er3Al5O12 compositions underwent a liquid–liquid phase transition which resulted in glasses with a cloudy appearance due to spheroids of one glass in a matrix of a second glass. The two glasses were isocompositional within the limits of experimental error. Clear, brilliant, single-phase glasses were obtained from La3Al5O12, ErLaYAl5O12, and compositions containing ≥5 mol% La2O3 substituted for the other rare-earth oxides. Formation of two glasses is attributed to nucleation and growth of the second liquid at a temperature below the equilibrium liquid–liquid transition temperature. Addition of lanthanum depresses the phase transition temperature below the glass transition temperature and the liquid–liquid phase transition is not observed. The results are discussed in the context of first-order liquid–liquid phase transitions (polyamorphism) and formation of single-phase glass from liquids that contain a high proportion of 4-coordinate aluminum ions.

Journal ArticleDOI
TL;DR: In this paper, a low temperature (450-600 $oC$) route for the synthesis of highly crystalline and homogeneous nanoparticles of lanthanum calcium manganese oxide (LCMO) was reported.

Journal ArticleDOI
TL;DR: In this article, structural and electronic properties of single-phase CrN(001) thin films grown on MgO(001)-strained N plasma-assisted molecular-beam epitaxy are investigated.
Abstract: Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1×1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.

Journal ArticleDOI
TL;DR: In this paper, an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal-insulator transition temperature (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition is presented.
Abstract: In this paper, we illustrate an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal–insulator transition temperature (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition. Using this approach, we have estimated the effect of the biaxial strain on TP. Ultrathin films, independent of epitaxial strain, do not show any metal–insulator transition over the full temperature range. This finding confirms the existence of an interface dead layer. The strain-independent decrease in TP, relative to its bulk value, observed for a much wider thickness range (up to about 1000A) can most likely be attributed to oxygen deficiency.

Journal ArticleDOI
TL;DR: In this paper, the parameters of reactive pulsed laser deposition were successfully optimized for fabrication of vanadium dioxide thin films, and the structural properties of the deposited films were analyzed by x-ray diffraction, while their semiconductor-to-metal phase transitions were studied by electrical resistivity using the four-point technique and infrared transmittance from room temperature up to 100'°C.
Abstract: The parameters of reactive pulsed laser deposition were successfully optimized for fabrication of vanadium dioxide thin films. It is observed that the O2 concentration in Ar gas and the total deposition pressure are critical in stabilizing the single VO2 phase. Thermochromic VO2 and V1−xWxO2 (x=0.014) thin films were synthesized on various substrates (silicon, quartz, and sapphire) at 5% of O2/Ar ratio gas and total pressure of 90 mTorr. The structural properties of the deposited films were analyzed by x-ray diffraction, while their semiconductor-to-metal phase transitions were studied by electrical resistivity using the four-point technique and infrared transmittance from room temperature up to 100 °C. The observed transition temperature was about 36 °C for W-doped VO2 compared to 68 °C for VO2 films. This transition temperature was then lowered by about 22.85 °C per 1 at. % of W added. The temperature coefficient of resistance was about 1.78%/°C for VO2 and about 1.90%/°C for W-doped VO2. Using the pump...

Journal ArticleDOI
TL;DR: In this paper, a powder of ZnO and TiO 2 in a molar ratio of 1:1 was mixed in a ball mill and then heated at temperatures from 700 to 1000°C for various time periods in air.

Journal ArticleDOI
TL;DR: In this article, magnetization measurements for the Co3O4 nanoparticles were performed and their diameters were estimated to be 2 or 3 µm by using X-ray diffraction patterns.

Journal ArticleDOI
TL;DR: In this paper, the formation of lamella mesophases by cooling a fully disordered system composed of symmetric (A7B7) rod-coil diblock copolymers is shown.
Abstract: Using mesoscale dissipative particle dynamics (DPD) simulations, which ignore all atomistic details, we show the formation of lamella mesophases by cooling a fully disordered system composed of symmetric (A7B7) rod–coil diblock copolymers Equilibration is achieved very rapidly using DPD, and isotropic, smectic A and crystalline phases of the rod–like blocks can be observed either by heating or cooling An interesting pseudo–smectic phase can be characterized when the order–disorder transition temperature is above the clearing temperature This phase gradually fades into a normal microphase–separated structure as the system is heated through the clearing temperature Simulations of pure rods, however, show the formation of isotropic, nematic, smectic A and crystalline phases

Journal ArticleDOI
TL;DR: The structural properties of the spin crossover compound [Fe(btr)2(NCS)2](H2O), where btr stands for 4,4'-bis-1,2,4-triazole, are investigated by single crystal X-ray diffraction at different temperatures in the thermal spin transition regime as mentioned in this paper.
Abstract: The structural properties of the spin crossover compound [Fe(btr)2(NCS)2](H2O), where btr stands for 4,4’-bis-1,2,4-triazole, are investigated by single crystal X-ray diffraction at different temperatures in the thermal spin transition regime. The 104.0(5) K low spin (LS) crystal structure is compared to the room temperature high spin (HS) crystal structure. The C2/c space group is retained in the LS state with an abrupt anisotropic shortening of the b and c cell parameters and a lengthening of a at the transition temperature. The major structural modifications related to the spin transition are a shortening of the Fe-N bond lengths ( $\Delta d_{{\rm Fe}-NCS} = -0.175$ (4) A, $\Delta d_{{\rm Fe-N}(btr)} = -0.213$ (3) A) and a reorientation of the NCS groups with a more linear Fe-N-C-S geometry on going from HS to LS. Diffraction measurements have also been performed at 124 K on a trapped mixed spin state. The observed diffraction pattern shows the coexistence of two crystal lattices corresponding to ordered LS and HS species, which is a direct evidence of spin-like domain formation during the transition. The corresponding fraction of HS species ( $\gamma_{\it HS}\approx 0.10$ ) has been determined by structural refinement using several reference temperature measurements. To investigate dynamical aspects, X-ray data were collected versus time during the spin transition at constant temperature (T = 117.2(2) K). No evidence has been found for any putative presence of an intermediate structural state during the spin transition.

Journal ArticleDOI
TL;DR: In this paper, the transport and magnetic properties have been studied in two sets of sol-gel prepared nanoparticles having average particle size of 30 nm and 45 nm, which suggest that the formation of charge ordered state is largely affected due to lowering of particle size, but the ferromagnetic transition temperature remains unaffected.
Abstract: Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr$_{1/2}$Sr$_{1/2}$MnO$_{3}$ nanoparticles having average particle size of 30 nm and 45 nm. Our measurements suggest that the formation of charge ordered state is largely affected due to lowering of particle size, but the ferromagnetic transition temperature ($T_{C}$) remains unaffected.

Journal ArticleDOI
TL;DR: In this article, a collinear model of the magnetic ordering of α-Mn2O3 at 10 K was presented, where the temperature dependence of magnetization near 0 K for these peaks differs from Bloch's T3/2 law.

Journal ArticleDOI
TL;DR: In this paper, an array of crystalline nanowires (average diameter of 65 nm) of colossal magnetoresistive oxide La0.67Ca0.33MnO3 (LCMO, x50.33) within anodized alumina templates by filling the pores with a sol that allows formation of LCMO phase at a relatively low temperature of 600 °C.
Abstract: We report fabrication of array of crystalline nanowires (average diameter of 65 nm) of colossal magnetoresistive oxide La0.67Ca0.33MnO3 (LCMO, x50.33) within anodized alumina templates by filling the pores with a sol that allows formation of LCMO phase at the relatively low temperature of 600 °C. The crystalline nanowires with correct stoichiometry stabilize in the orthorhombic phase at room temperature. The nanowires are ferromagnetic at room temperature and exhibit enhanced ferromagnetic transition temperature well in excess of 300 K, which is substantially higher than that of single crystalline LCMO. This enhancement we attribute to the size induced lattice contraction in the nanowires.

Journal ArticleDOI
TL;DR: In this paper, a systematic study of the weak link behavior for (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques.
Abstract: A systematic study of the weak link behavior for (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques. In this project, we have prepared a series of Bi-2223 samples with different fractions of Bi-2212 phase and investigated its effect on the intergranular properties. It was found that the Bi-2212 phase exist within the grain boundaries, play a role of weak links and consequently reduces the intergranular coupling. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (Tc) has been done employing Bean’s critical state model. The observed variation of intergranular critical current densities (Jc) with temperature indicates that the Jc decreases with the amount of Bi-2212 phase.

Journal ArticleDOI
TL;DR: In this paper, the effect of monovalent cation addition on the γ-Al2O3-to-α-Al 2O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific surface-area measurements.
Abstract: The effect of monovalent cation addition on the γ-Al2O3-to-α-Al2O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li+, Na+, Ag+, K+, Rb+, and Cs+ were added by an impregnation method, using the appropriate nitrate solution. β-Al2O3 was the crystalline aluminate phase that formed by reaction between these additives and Al2O3 in the vicinity of the γ-to-α-Al2O3 transition temperature, with the exception of Li+. The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs+ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al2O3. Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al2O3 in these samples, suppression of ionic diffusion in γ-Al2O3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al2O3. Larger cations suppressed the diffusion reaction more effectively.

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis of the magnetization reversal process in a bi-layer structure with hard and soft (with metamagnetic transition) magnetic layers on an example of an $\mathrm{Fe}Mathrm{Pt}∕\mathrm {Fe}mathrm    
Abstract: We present a theoretical analysis of the magnetization reversal process in a bi-layer structure with hard and soft (with metamagnetic transition) magnetic layers on an example of an $\mathrm{Fe}\mathrm{Pt}∕\mathrm{Fe}\mathrm{Rh}$ bi-layer. The latter leads to the formation of a new type of exchange spring which results in a significant reduction of the switching field in the temperature range of the metamagnetic (from antiferromagnetic to ferromagnetic state) transition in an FeRh layer. Analytic expressions for nucleation and switching fields are presented along with results of numerical micromagnetic simulations. The reduction of the switching field due to the metamagnetic transition is controlled by the following microscopic parameters: (i) the interfacial exchange coupling parameter ${J}_{12}$; (ii) saturation magnetization of the FeRh layer in a ferromagnetic phase; (iii) the metamagnetic transition temperature. The switching field dependence on the ${J}_{12}$ parameter is shown to saturate quickly as it approaches the bulk exchange interaction value which has been evaluated using first-principles method used also to verify the electronic nature of the metamagnetic transition. Theoretical results are discussed in the context of recent experimental observations.

01 Mar 2004
TL;DR: In this paper, a dome-shaped relation between the transition temperature into the low-T magnetic state and the composition x for NaxCoO2 and/or the high-temperature asymptotic limit of thermopower in the more complex 3-and 4-layer cobaltites was reported.
Abstract: Using muon spin spectroscopy we have found that, for both NaxCoO2 (0.6 ≤ x ≤ 0.9) and 3- and 4-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO2 planes. Here we summarize those results and report a dome-shaped relation between the transition temperature into the low-T magnetic state and the composition x for NaxCoO2 and/or the high-temperature asymptotic limit of thermopower in the more complex 3- and 4-layer cobaltites. This behavior is explained using the Hubbard model on two-dimensional triangular lattice in the CoO2 plane.

Journal ArticleDOI
TL;DR: In this paper, single-crystal Brillouin scattering measurements on natural orthoenstatite [OEN] to 1350 °C at 1 atm were used to detect acoustic mode-softening in orthenstatite at high temperature and room pressure.
Abstract: Single-crystal Brillouin scattering measurements on natural orthoenstatite [OEN] to 1350 °C at 1 atm show significant softening of the elastic moduli C_33 and C_55 ahead of a phase transition. To our knowledge, these are the first observations of acoustic mode-softening in orthoenstatite at high temperature and room pressure and could have important implications for Earth's mantle. The phase transition is rapid and shows some hysteresis in the observed transition temperature, T_tr. Experiments performed on increasing and decreasing temperature bracket the transition temperature between 1090(10) °C ≤ T_tr ≤ 1175(10) °C, and pronounced acoustic mode-softening is evident at temperatures above 900 °C. Backscattering measurements to T = 1350 °C show no evidence for additional transitions. OEN was recovered at room temperature. Our results are interpreted in terms of elastic softening ahead of a displacive phase transition. Before the displacive transition can occur, however, the elastic softening appears to trigger the observed reconstructive transition to the more-stable protoenstatite (or high clinoenstatite) structure. We suggest that the displacive phase transition would lead to a previously unreported pyroxene structure with Cmca symmetry.