Transition Temperature Curve Research Articles

Superconductivity in hydrogen-doped YBa2Cu3O7−x

Abstract Resistivity and magnetic susceptibility measurements have been performed on the samples of YBa 2 Cu 3 O 7- x H y ( y =0.18, 0.30) and YBa 2 Cu 3 O 7 -x D 0.22 . The electrical resistivity measurement shows superconductivity even in the y =0.30 sample, but the transition is more gradual. Above the superconducting transition temperature, the resistivity vs. temperature curve is linear, however, the slope decreases and the absolute value of resistivity increases with the H content. The bulk nature of the superconductivity was confirmed by observing the diamagnetism below the transition temperature, but the Meissner effect was weakened by H-doping.

Antiphase Behavior in the 4-n-alkoxybiphenylyl-4′-cyanobenzoates (nOBCB's)

Abstract A number of members of the 4-n-alkoxybiphenylyl-4′-cyanobenzoate (nOBCB) homologous series were prepared and found to exhibit smectic antiphase behavior. The early homologues (n-pentyloxy to n-heptyloxy) possess At phases while the higher members (n-octyloxy to n-dodecyloxy) show_Ad phases. At the n-octyl homologue a tilted C antiphase is introduced. The Ad to [Ctilde] phase change was shown to be strongly first order from differential scanning calorimetry. Later members of the series also exhibit smectic Cˇ phases on cooling the C phase, for example, the decyloxy compound exhibits an Ad, [Ctilde], C phase sequence on cooling. Miscibility studies between the early and later members show reentrant nematic phase behavior which occurs at the junction of the N-A1 and N-Ad transition temperature curves. The analogous materials, the 4-cyanophenyl 4′-n-alkoxybiphenyl-4-carboxylates do not exhibit antiphases and simply show N and A phases.

Exhibition of Non-Linear Behaviour and Smectic and Nematic Mesophases in the Binary Systems Where Both the Components are Non-Mesogenic

Abstract Binary systems of two non-mesogens exhibiting polymesomorphism are rare. In the present study three binary systems of non-mesogens exhibiting smectic and/or nematic mesophases are reported. One of the component is Schiff base and possesses strongly polar nitro end group. The transition temperature curves exhibit non-linear behaviour. In another binary system where one of the component exhibit monotropic nematic phase, induced smectic mesophase is observed and the monotropic nematic phase is converted to enantiotropic phase in certain concentration range. The binary systems of non-mesogens having identical structures (homologues) do not exhibit mesomorphism. Binary systems, where both the components are homologues and one of them is monotropic nematic, then, ideal behaviour is observed in their mixed mesomorphism.

The intralayer order-disorder transition in monoclinic Cr 3± xSe 4

The Cr 3± x Se 4 shows an order-disorder transition from the monoclinic Cr 3Se 4 type to the CdI 2-type structure. This transition originates in the intralayer disordering of metal vacancies in the alternate partially filled metal layers along the c axis. The transition is reversible and of first order. The transition temperature vs composition curve has a maximum (∼900°C) at a stoichiometric composition of Cr 3Se 4. A statistical treatment of this phenomenon was attempted via the Bragg-Williams approximation.

Properties of the liquid crystals formed by certain 2-p-alkoxybenzylideneamino-9,10-dihydrophenanthrenes

The liquid crystal transition temperatures of 11 2-p-alkoxybenzylideneamino-9,10-dihydrophenanthrenes are reported and discussed. Graphical presentation shows a fall to a minimum followed by a rise to a maximum for the N–I transition temperatures of the early homologues and lack of convergence of the rising SA–N and the falling N–I transition temperature curves for the higher homologues, i.e. there is persistence of nematic phases to a late stage in the homologous series. This behaviour is consistent with very low lateral cohesive forces operating throughout the homologous series.

Miscibility Studies Indicating a Low Temperature Smectic A Phase in Biaromatic Liquid Crystals with Re-entrant Behaviour

Recent reports [1] have shown that compounds of the homologous series of the 4(4-n-alkyloxy- benzoyloxy)-4′-cyanoazobenzenes (Cn; n = 1 - 12), all have a low temperature smectic A phase (SA″). The homologues with alkyl chain length n≧9 exhibit in addition a high temperature smectic A phase (SA′), separated from the first one by a re-entrant nematic phase (N(re)). Homol­ogues of this series were used for miscibility studies with some compounds of the biaromatic liquid crystal series of the 4-cyano-4′-n-alkyloxybiphenyls (n̄ CBP) and of the 4-cyanobenzylidene- 4′-n-alkyloxyanilines (CN · n̄). The phase diagrams studied show that the smectic A phases of biaromatic liquid crystalline nitrile compounds correspond to the high temperature smectic SA′ type and are not miscible with the low temperature SA″ phase. Moreover, the shapes of the transition temperature curves to the SA″ phase indicate a low temperature smectic A phase (SA″) for the biaromatic compounds. This suggests that for biaromatic compounds with re-entrant behaviour, the dependence of liquid crystalline phase sequences on the alkyl chain length is very similar to that of triaromatic compounds.

Pure and diluted anisotropic quantum Heisenberg model

Pure and diluted anisotropic quantum Heisenberg models are treated by renormalisation group scaling methods. Detailed discussions are given of the dependence of transition temperature on anisotropy and concentration, and of the dependence of correlation length on anisotropy, concentration and temperature. Particular emphasis is given to the weakly anisotropic system near the percolation limit and to the weakly anisotropic and Ising-like situations near the pure limit. One-, two- and three-dimensional cases are treated in turn. Some comparisons are made with neutron scattering results for the correlation length of quasi one- and two-dimensional systems and with the measured transition temperature curve for a diluted anisotropic layer system.

On the problem of the mechanism of postglacial rises

Summary In this paper one of the possible mechanisms for postglacial uplift is considered. It is based on the known fact of the phase boundary reaction to change in pressure in the medium. Unlike the mathematical model for such mechanisms considered previously in a number of publications, we propose a model that takes into account the supercooling (superheating) on the crystallization (melting) front responsible for a phase transition. The algorithm for a numerical solution to the mathematical model is derived. The solution leads to the following conclusions: 1. Geological data on postglacial uplift are accounted for if the following conditions are observed, (a) There is one phase boundary in the layer subjected to a phase transition. If the layer is bounded by phase boundaries on two sides its thickness should be no less than 200 km. (b) The transition temperature curve and that of the temperature of the medium are close to each other, (c) The latent heat is close to 10 cal g−1. (d) Properties of the mantle rocks are such that the parameter characterizing overheating in the phase transition should satisfy a certain condition. 2. In view of strict limitations set by this work on the phase mechanism, the latter is less probable in a real medium than the mechanism related to viscosity of the mantle.

Dynamic Fracture Initiation: A Comparison of Two Experimental Methods

This paper presents a comparison of dynamic fracture initiation results obtained from two independent experimental methods: (1) the instrumented version of the precracked Charpy test and (2) a new dynamic fracture test developed at Brown University (BU). Dynamic fracture tests were conducted using both methods on 4340 steel at room temperature and on 1018 cold-rolled steel (CRS) at temperatures over the range −157 to 107°C. This range covered temperatures on the lower shelf, the transition regime, and the upper shelf of the transition temperature curve. Stress intensity rates of K˙I = 2.2 × 105 MN-m−3/2/s and K˙I = 2.2 × 106 MN-m−3/2/s were used in the instrumented precracked Charpy and BU tests, respectively. In the analysis of test results, both elastic (KIc) and elastic-plastic (J-integral) fracture parameters were computed. Results of the two test methods were found to be generally in good agreement, except that some differences were observed between 1018 CRS test results on the upper shelf. Possible reasons for these differences are discussed.

Thermotropic Liquid Crystals I: p-n- Aalkoxybenzylidene-p-Bromoanilines

Abstract A homologous series (I) of mesomorphic Schiff bases is synthesized by condensing p-n-alkoxybenzaldehydes with p-bromoaniline. The first five members are non-mesomorphic. Hexyl to octadecyl derivatives are enantiotropic smectic. The nematic mesophase is not observed in any of the compounds. The series is smectogenic in nature. The plot of transition temperatures versus the number of carbon atoms in the alkyl chain does not show odd-even effect. The smectic-isotropic transition temperature curve shows rising tendency as the series is ascended. It rises to the maximum at the octyl derivative and then levels off. The thermal stabilities of the present series are compared with those of the other related homologous series.

Pressure dependence of the superconducting transition temperature in alkali tungsten bronzes

The superconducting transition temperatures have been measured for the tetragonal I phase of Na0.23WO3, the hexagonal phase of K0.33 WO3, and the hexagonal phase of Rb0.33WO3 for pressures up to 20,000 bar. The slope of the curves is + 1.7 × 10−5 K/bar for the sodium bronze, −5.8 × 10−5 K /bar for the potassium bronze, and −1.5 × 10−5 K/bar for the rubidium bronze. A sharp change in slope of the transition temperature vs. pressure curve for the potassium bronze at 4000 bar may indicate a phase change.

A Comparison of Charpy V-Notch, Dynamic Tear, and Precracked Charpy Impact Transition-Temperature Curves for AAR Grades of Cast Steel

Impact tests using Charpy V-notch, Dynamic Tear and Precracked Charpy samples were conducted on AAR-B, AAR-C (several compositions and heat treatments), and AAR-E cast grades of steel and on a high-strength, alloy cast steel. Graphical presentations of the transition-temperature data are given. Differences in the relative shapes of the curves and the locations of the NDT temperatures with respect to the transition temperatures are discussed. Fracture toughness, KId, values are also presented.

Pressure dependence of the helix-coil transition temperature for polynucleic acid helices.

AbstractThermal denaturation of DNA's and the corresponding helix–coil transformation of artificial polyribonucleic and polydeoxyribonucleic acids have been studied extensively both theoretically1–13 and experimentally. 14–30 Much less work has been carried out on the properties of these polynucleic acids at high pressure, and in particular, on the presure dependence of the helix–coil transition temperature.31–33 Light‐scattering techniques have been used in this study to measure the pressure dependence of the helix–coil transition temperature of the two‐ and three‐stranded helices of polyriboadenylic and polyribouridilic acids and of calf thymus DNA. From the slopes of the transition temperature vs. pressure curves and heats of transition obtained from the literature,20,34 the following volume changes from these helix–coil transitions have been obtained: (a) −0.96 cc/mole of nucleotide base pairs for the poly (A + U) transition, (b) +0.35 cc/mole of nucleotide base trios for the poly (A + 2U) transition, and (c) +2.7 cc/mole of nucleotide base pairs for the DNA transition. The relative magnitudes and signs of these volume changes which show that poly (A + U) is destabilized by increased pressure, whereas poly (A + 2U) and calf thymus DNA are stabilized by increased pressure, indicates that further development of the helix–coil transition theory for polynucleotides is needed.

First-Order magnetic phase transitions

The general properties of the first-order magnetic phase transitions between ordered structures have been investigated. The number of possible models has been limited by supposing the sensitivity of the exchange energy to the interatomic distance to be responsible for the phase transition. It is shown that the magnetic phase transition occurs as a result of a signchange in the proper combination of the exchange parameters. The transition may be influenced by the variation of the pressure and the magnetic field. The actual phase transition is, however, hindered in both directions by the elastic energy wall, separating the free energy minima of the different phases. TheT-P phase diagram has been calculated and the conditions for the existence of a triple point in theT-P plane have been investigated in the case of antiferromagnetic ⇆ ferromagnetic transitions. The temperature hysteresis, i.e. the difference between the upper (T sup) and lower (T inf) transition temperatures has been determined at different pressures and for both temperatures a linear dependence has been obtained. The character of the disappearance of the magnetic order has been also studied and it has been found that under certain conditions the order of the ferromagnetic-paramagnetic transition may be different from that of the antiferromagnetic-paramagnetic one. TheT-H coexistence curve between the ferromagnetic and the spin-flop antiferromagnetic phases has been determined. The upper and the lower phase transition temperature versus magnetic field curves are calculated and it is found that with increasing magnetic field and decreasing temperature the difference between these two curves (i. e. the hysteresis width) becomes increasingly small until at a critical field (H t) and temperature (T t) they end in a common point with the coexistence curve. BelowT t the variation of the magnetic field does not lead to a first-order antiferromagnetic ⇆ ferromagnetic transformation. The ferromagnetic phase is nothing, but the antiferromagnetic phase in a magnetic field strong enough to turn the moments parallel to the field direction. AboveH t but belowH c the variation of the temperature brings about also a second-order ferromagnetic ⇆ antiferromagnetic transition. The discontinuous change of the lattice parameter and that of the sublattice magnetization have been calculated at the coexistence temperature and at both the upper and lower transition temperatures. It has been shown that the entropy change of the transition has two contributions: one of them is associated with the volume change while the other with the magnetization change. Under certain conditions the order of magnitude of both contributions may be the same.

Nonideal Behavior of Inhomogeneous Superconducting Alloys

An investigation of nonideal superconducting properties of inhomogeneous tin alloys reveals that the behavior of such samples represents a diverse set of phenomena intimately related to the nature of the inhomogeneities. Resistive temperature and field transitions are presented along with critical field data, magnetization curves, and remnant moment measurements. Several inhomogeneous samples are shown to possess very unstable magnetization curves and to show flux penetration at fields less than the bulk critical value. Annealing can make the magnetization more or less ideal, or produce very little change, depending on the nature of the inhomogeneity and the way it is altered in the process. Annealing sharpens the resistive transition temperature curve when more impurity goes into solution. It also can produce a large change in the apparent transition temperature. Samples examined represent both positive and negative surface energy materials containing either predominantly normal state or superconducting state inhomogeneities, or a combination of both. Although negative surface energy materials show more hysteresis than positive surface energy materials under equivalent conditions, the nature of the inhomogeneity and its role in flux retention is more important than the simple difference between positive and negative surface energy. A detailed description of the behavior of each sample and speculations as to the origins of features not heretofore reported are included.

Effect of Hydrostatic Pressure on the Phase Transitions in NaNO2

Temperature dependence of the dielectric constant e 22 of NaNO 2 was measured at various hydrostatic pressures up to about 10 kbar. Both the Curie temperature T f and the Neel temperature T N increase with increasing pressure. The initial slopes of the transition temperature vs pressure curves are given as:

Some properties of the A-15 phase in the V-Ga system

The range of stability and some properties of the A-15 phase in the V-Ga system were investigated. It was found that the A-15 phase forms at temperatures below 1100°C and exists over a composition range of approximately 3.5 V : Ga to 2.1 V : Ga at the maximum temperature of formation investigated. The width of the phase field decreases with decreasing temperature. Accurate values of the composition range could not be established because of the presence of a second phase called the γ phase which forms in the temperature range of 1100°C to 1500°C. The structure of the γ phase has been tentatively identified as hexagonal. Above 1500°C to the melting point, the bcc solid solution of Ga in V is the stable phase and the solubility of Ga in V in this temperature range is at least 33 a/ O Ga in V. The solid solution of Ga in V could be retained at room temperature by cooling from the melt at vanadium compositions of 80% and higher. The A-15 phase could then be precipitated from solid solution by heat treating at temperatures of 700°C to 1050°C. When formed in this manner, the A-15 phase had a superconducting transition temperature comparable to the as-cast stoichiometric material, but the slope of the critical magnetic field versus transition temperature curve was found to be approximately one-half the value of the as-cast stoichiometric material. It is suggested that the presence of the γ phase in the microstructural morphology characteristic of as-cast material is responsible for the observed high value of the slope of the critical field versus transition temperature curves. In fact, the critical field values obtained experimentally for stoichiometric as-cast material may be those for H c3 while for the case of the A-15 phase precipitated from solid solution the critical field values measured are H c2 . The degree of order of the A-15 phase as a function of composition was investigated and the manner of substitution of the excess atoms in off-stoichiometric compositions is described.

Effects of Pressure on the Superconducting Transition Temperatures of Sn, In, Ta, Tl, and Hg

Solid parahydrogen has been used as a transmitter of approximately hydrostatic pressure to study the effects of pressures up to 10 000 atmos on the superconducting transition temperatures of polycrystalline tin, indium, tantalum, thallium, and mercury. The technique which was used allowed an approximate evaluation of the effects of sample deformation and pressure gradients, and the results are considerably more accurate than the high-pressure data previously available. The transition temperature data for tin and indium showed considerable curvature when plotted vs pressure, but gave a roughly linear relationship when plotted against volume. No curvature was found for tantalum. The thallium data agree qualitatively with previous work, and show a maximum in the transition temperature vs pressure curve at about 2000 atmos. The mercury results were anomalous in that two distinct transition temperature vs pressure curves (with different zero-pressure transition temperatures) were found; one when the pressure was kept below 4000 atmos, and the other when the sample was cycled from zero to 10 000 atmos. Other experiments have shown that these results are due to two different modifications of solid mercury, each of which can exist at liquid helium temperatures.

The Structure of Borosilicate Glasses

In the author's previous paper, it was stated that due to the disposition that the cations of the modifying oxide do not approach to each other, atomic groups denominated as xy4 are formed in glasses containing B2O3 at the viscosity of about 1.5×1012 poises, i.e., near the contraction temperature of the glass. In this paper, further considerations concerning the xy4-group formation in borosilicate glasses are given from the point of view of the ionic refraction of various oxygen in sodium borate glasses.It is deduced that the B-O bonds in the BO4 tetrahedron which is located in the central part of the xy4-group is very strong, and this tetrahedron is formed at the expense of the loosening of the B-O bonds in the BO3 triangles around it, i.e., these BO3 triangles are unstable in comparison with those in the normal state. Accordingly, the predominance in number of the atomic groups xy4 in comparison with other groups such as xy3z, xy2z2, xyz3, or xz4, where x, y, and z represent BO4 tetrahedron, BO3 triangle and SiO4 tetrahedron, respectively, is explained.When the Na2O content in the sodium borate glass is increased above 0.167, Na ions begin to cut the B-O bonds around xy4-groups, and as the result, the unstable BO3 triangles around BO4 tetrahedron must be stabilized to some degree. It may be supposed that these Na ions should then be trapped in deep potential holes on cutting off the B-O bonds. These Na ions should not be so free as those added later, and they may be regarded as the so-called “enclosed” Na ions.It may also be supposed that for each one xy4-group, one Na ion could be the “enclosed” ion, i.e., free Na ions should begin to be produced at the composition 2Na2O⋅5B2O3 or 28.6 mol% Na2O. Thus the location of the maximum transition temperature curve in the ternary diagram Na2O-B2O3-SiO2, and the stability of the network of borosilicate glasses are discussed.