Showing papers on "Phase (matter) published in 1989"

Olivine-modified spinel-spinel transitions in the system Mg2SiO4-Fe2SiO4: Calorimetric measurements, thermochemical calculation, and geophysical application

Abstract: The olivine(α)-modified spinel(β)-spinel (γ) transitions in the system Mg2SiO4-Fe2SiO4 were studied by high-temperature solution calorimetry. Enthalpies of the β-γ and a α-γ transitions in Mg2SiO4 at 975 K and of the α-γ transition in Fe2SiO4 at 298 K were measured. The γ solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and γ solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α − (Mg0.89, Fe0.11)2SiO4 transforms to β through a region of α+β without passing through the α+γ phase field at around 400 km depth in the mantle with an interval of about 18(±5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm.

Insitu photopolymerization of oriented liquid-crystalline acrylates .3. oriented polymer networks from a mesogenic diacrylate

Abstract: Synthesis, mesomorphism, orientation and photo-initiated chain crosslinking of the liquid-crystalline diacrylate 1,4-phenylene bis{4-[6-(acryloyloxy)hexyloxy]benzoate} (1) are studied. Monomer 1 exhibits a broad nematic phase between 108 and 155°C and a monotropic smectic phase below 88°C. The monomer is uniaxially oriented in its nematic phase at a substrate which has been coated with polyimide and unidirectionally rubbed with tissue. At the transition temperature to the smectic phase the order parameter is measured to be 0,7. During polymerization, the ordering of the mesogens is frozen-in, yielding a uniaxially crosslinked network. The clear films of oriented poly(1) exhibit a birefringence Δn between 0,12 and 0,15, depending on the polymerization temperature. In the highest oriented state of 1 a small reduction of the degree of order is observed during the crosslinking reaction, whereas at higher temperatures and lower ordering of 1, the uniaxially orientation increases upon reaction. A special feature of the oriented networks is that the ordering is maintained while heating at high temperatures. The polymerization of the acrylate groups in the mesomorphic phases proceeds fast and to high conversion. Below 90°C the polymerization behaviour is similar to that of conventional isotropic diacrylates. Above 90°C the polymerization reaction of the liquid-crystalline diacrylate proceeds faster than that of an isotropic diacrylate.

Stabilization of a liquid-crystalline phase through noncovalent interaction with a polymer side chain

Abstract: Le melange equimolaire d'un polyacrylate de carboxy-4p phenoxy-5 pentyle mesomorphe avec un benzoate de transstilbazolyle mesomorphe est miscible et presente une mesophase tres stable grâce a la formation d'un motif mesogene plus long via une liaison hydrogene entre les 2 molecules

Characterization of a new helical smectic liquid crystal

Abstract: The discovery of the first liquid-crystalline material in 18881,2 also heralded an age of fascination with chirality and optical activity in ordered fluids. The cholesteric mesophase, which was the first liquid crystal to be found, exhibits form optical activity by virtue of a helical arrangement of its constituent molecules. One hundred years after the discovery of this first liquid crystal, we report the discovery of a new helical smectic liquid crystal, the smectic-A* phase. In this phase the lath-like molecules are arranged in layers with their long axes on average normal to the layer planes. Parallel to the layers there is a helical ordering of the molecules (Fig. 1). We suggest that this phase may be described by a model in which grain boundaries of screw dislocations rotate blocks of layers with respect to each other.

Hydrophobic effect in protein folding and other noncovalent processes involving proteins

Abstract: Large negative standard heat capacity changes (delta CP degree much less than 0) are the hallmark of processes that remove nonpolar surface from water, including the transfer of nonpolar solutes from water to a nonaqueous phase and the folding, aggregation/association, and ligand-binding reactions of proteins [Sturtevant, J. M. (1977) Proc. Natl. Acad. Sci. USA 74, 2236-2240]. More recently, Baldwin [Baldwin, R. L. (1986) Proc. Natl. Acad. Sci. USA 83, 8069-8072] proposed that the delta CP degree of protein folding could be used to quantify the contribution of the burial of nonpolar surface (the hydrophobic effect) to the stability of a globular protein. We demonstrate that identical correlations between the delta CP degree and the change in water-accessible nonpolar surface area (delta Anp) are obtained for both the transfer of nonpolar solutes from water to the pure liquid phase and the folding of small globular proteins: delta CP degree/delta Anp = -(0.28 +/- 0.05) (where delta Anp is expressed in A2 and delta CP degree is expressed in cal.mol-1.K-1; 1 cal = 4.184 J). The fact that these correlations are identical validates the proposals by both Sturtevant and Baldwin that the hydrophobic effect is in general the dominant contributor to delta CP degree and provides a straightforward means of estimating the contribution of the hydrophobic driving force (delta Ghyd degree) to the standard free energy change of a noncovalent process characterized by a large negative delta CP degree in the physiological temperature range: delta Ghyd degree congruent to (80 +/- 10)delta CP degree.

Modeling the transport of volatile organics in variably saturated media

Abstract: The understanding of the processes of dissolution, volatilization, and gas-liquid partitioning in porous media is very limited. The few models which attempt to characterize the transport of volatile organics such as petroleum products and halogenated hydrocarbon solvents in variably saturated media all assume that mass transfer processes are at equilibrium. In addition, gas phase advection is neglected by assuming that gas phase pressures are uniformly atmospheric and that density gradients are negligible. In this study a model was developed to solve for water phase flow and transport and density dependent gas phase flow and transport. Simple expressions for dissolution, volatilization, and gas-liquid partitioning, employing the concept of an overall mass transfer coefficient, were incorporated into the model. The transport of trichloroethylene in a variably saturated vertical cross section, under a variety of conditions, was simulated. Results of the simulations appeared qualitatively correct. The importance of gas phase processes in increasing subsurface contamination from volatile organics, and in dissipating residual amounts of these substances, was demonstrated. The lack of similar analytical and/or numerical models, or suitable experimental studies, excluded the possibility of validating, or verifying, the model.

Complete phase diagram of a charged colloidal system: A synchro- tron x-ray scattering study.

Abstract: High-resolution, small-angle, synchrotron x-ray-scattering techniques were used to determine the phase diagram, structure factor, and pair distribution function for a charged colloidal suspension from 6% to 30% volume fraction. The expected correlated liquid and fcc and bcc solid phases were observed along with a glass phase at high concentration with structure similar to metallic glasses. At high volume fractions the finite core size leads to substantial deviation from predictions resulting from a screened Coulomb interaction.

The highly concentrated liquid-crystalline phase of DNA is columnar hexagonal

Abstract: The DNA molecule is extremely compacted in bacteria, in cell nuclei, sperm heads and virus capsids. These interactions between DNA molecules are important to our understanding of chromatin condensation. DNA forms multiple liquid-crystalline phases whose nature depends on the polymer concentration, and it has been suggested that the highly concentrated phase of 50-nm DNA molecules is two-dimensionally ordered and smectic-like. We rule out this smectic hypothesis and demonstrate by polarizing microscopy, electron microscopy and X-ray diffraction that this phase is characterized by a columnar longitudinal order and a hexagonal lateral order, with intermolecular distances ranging from 2.8 to 4.0 nm depending on the DNA concentration.

A new molecular ordering in helical liquid crystals

Abstract: In the pursuit of new ferroelectric liquid crystals, a novel intermediary state of matter, the helical smectic A* mesophase, was discovered. This layered phase was found between the isotropic liquid and the smectic C* phase in a variety of optically active (R)- and (S)-1-methylheptyl 4{prime}-(((4{double prime}-n-alkoxyphenyl)propioloyl)oxy)biphenyl-4-carboxylates. In these materials, the temperature at which the transition to the ferroelectric smectic C* phase occurs is relatively close to that of the clearing point. Moreover, the materials show a high degree of chirality. These two properties ensure that the A* phase possesses relatively large molecular fluctuations, which give rise to twist and bend distortions resulting in the stabilization of a helical structure. This new phase is apparently miscible with the classical smectic A phase, thereby classifying it as the chiral modification of the A phase.

Self‐assembly of surfactant liquid crystalline phases by Monte Carlo simulation

Abstract: Three‐dimensional microstructures of surfactant–water–oil systems self‐assemble in a Monte Carlo lattice model, as shown here. The microstructures that form depend on the volume ratios of oil, water, and surfactant, and on the length of the surfactant, and on the ratio R of the length of the oil‐loving to the water‐loving portion. For R=1 we find lamellar phases when the surfactant is mixed with equal amounts of oil and water. The lamellar spacing increases as the surfactant concentration is lowered. In the presence of water only, as the surfactant concentration is lowered the microstructure evolves from lamellar to broken lamellar to cylinders to spheroids. This progression is found to be independent of lattice size for lattices as large as 40×40×40. For R=3, the progression seems to be replaced by a progression from lamellae to regular bicontinuous structures to cylinders, although we are not yet confident that this latter progression is independent of lattice size effects. The Monte Carlo technique can be used to study a wide variety of interesting phenomena, from micelle size and shape transitions to packing transitions and phase behavior to interfacial properties in the presence of surfactant.

Aggregation of amylose in aqueous systems: the effect of chain length on phase behavior and aggregation kinetics

Abstract: Nearly monodisperse amyloses having a wide range of chain lengths have been enzymically synthesized in vitro. The rate of aggregation from aqueous 0.2-5.0% solution and the physical form of extensively aggregated material have been studied as a function of chain length. Aggregation (monitored by turbidity) is found to be most rapid for chain lengths of ∼100 residues in agreement with previous results (Pfannemuller, B.; Mayerhofer, H.; Schulz, R. C. Biopolymers 1971, 10, 243-261) for initial aggregation rates in 0.1% solution. Amyloses having chain lengths of 1100 residues) gelation is found to predominate over precipitation. The variation of phase behavior and aggregation rate with chain length is rationalized on the basis of ordered polymer-polymer associations involving chain segments of typically less than 100 residues.

The synthesis and transition temperatures of some 4,4″-dialkyl- and 4,4″-alkoxyalkyl-1,1′ :4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and of their biphenyl analogues

Abstract: The tetrakis(triphenylphosphine) palladium (0)-catalysed coupling of arylboronic acids with aryl halides is used to prepare several 4,4″-dialkyl- and 4,4″-alkoxyalkyl-1,1′:4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and their related biphenyl systems. Lithiation ortho to a 1,2-difluoroaromatic unit provides the route to the 2,3-difluoroarylboronic acids.The 2,3-difluoro substituted terphenyls are low-melting liquid crystals with wide-range Sc phases and no underlying smectic phase; these compounds are excellent hosts for ferroelectric systems. The compounds with widest Sc ranges are those with the difluoro substituents in an end ring and the compounds with difluoro substituents in the central ring show more nematic character and so are useful for ECB devices.

Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa2Cu3Ox

Abstract: A PLOT of the superconducting transition temperature (Tc) against oxygen content (x) for YBa2Cu3Ox exhibits two 'plateaus' when oxygen is removed from the material at low temperatures. Tc remains nearly constant at ∼60K for x= 6.6–6.7 and nearly constant at ∼90 K for x= 6.8–7.0. It is now common to assume that there are two distinct superconducting phases in YBa2Cu3Ox, the '60-K' and '90-K' phases, and that the two plateaus correspond to single-phase regions of the respective phases1,2. YBa2Cu3Ox samples prepared at low temperatures contain a variety of ordered oxygen superstructures3–5. Several theoretical studies have tried to predict the phase equilibria between these ordered structures6,7 and to explain how oxygen ordering leads to the Tc plateaus8,9, but no clear understanding of the interrelationships amongst these factors has yet emerged. This is partly because the techniques used previously to prepare the low-temperature samples do not control or monitor all of the key processing variables, particularly the oxygen partial pressure. Here we report on the ordered oxygen structures and superconducting properties of YBa2Cu3Ox samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. We find no evidence for phase separ-ation between structures that have widely different oxygen content, but we do see electron diffraction evidence for phase separation between distinct phases that differ only slightly in oxygen content, and these regions of phase separation coincide with the Tc plateaus. These results show that the commonly held view that the two plateaus correspond to single-phase regions of respective 60-K and 90-K phases is incorrect: the changes in superconducting properties with oxygen content in YBa2Cu3Ox cannot be explained on this basis.

Insitu photopolymerization of an oriented liquid-crystalline acrylate .2.

Abstract: The photoinitiated polymerization of 4-biphenylyl 4-(6-acryloyloxyhexyloxy)benzoate (1) in its oriented liquid-crystalline state was studied. Monomer 1 has a thermotropic nematic phase between 88°C and 98°C, a monotropic nematic phase between 76°C and 88°C, and a monotropic smectic phase below 76°C. The orientation of 1 was established in a glass cell provided with a rubber polyimide coating and characterized by birefringence measurements. During polymerization both the birefringence and the liquid-crystalline transition temperatures increase, indicating that the orientation was maintained. The oriented poly(1) is polycrystalline at room temperature, becomes smectic at 90°C, nematic at 172°C, and isotropic at 190°C. The order parameter of the nematic poly(1) is lower than that of the nematic monomer1 at equal reduced temperatures.

Synthesis of bulk superconducting YBa2Cu4O8 at one atmosphere oxygen pressure

Abstract: THE well-known 90-K superconductor Y2Ba2Cu3O7 ('123') is the first (n = 0) member of a homologous series of compounds with the general formula Y2Ba4Cu6+nO14+n. These compounds combine layers of copper–oxygen pyramids with single and/or double copper–oxygen chains. The n =2 member, Y2Ba2Cu7O8 ('124'), with double Cu–O chains, and the n = 1 ytterbium analogue, Yb2Ba4Cu7O15 ('247'), which mixes single and double chains, were first observed as intergrowths in bulk 123 by electron microscopy1,11. The 124 phase was then synthesized as a majority phase in thin films2,3,12, and its crystal structure was determined4 and found to be in agreement with the model proposed from microscopy. An important advance in the synthesis of bulk materials, the result of extensive pressure–temperature phase equilibria studies, was the isolation of 124 and 247 at oxygen pressures of >200 atm, and detailed determinations of their crystal structures (see, for example, refs 5–9). High-pressure studies have also shown that the 124 phase could be made with many rare-earth elements13. Here we report the synthesis of the 124 phase in pure bulk form by a novel synthetic route in a flowing oxygen stream at 1 atm pressure. This technique allows YBa2Cu4O8 to be synthesized without specialized equipment, will make it generally available for study of its normal and superconducting properties, and will make possible more extensive comparisons with YBa2Cu3O7 and other high-Tc superconductors. Our results suggest that 124 is the thermodynamically stable phase at low temperatures and 1 atm pressure, under oxidizing conditions; the usual inability to synthesize it in these conditions is probably due to the limitations of reaction kinetics.

An assessment of the Fe-Mn system

Abstract: The thermodynamic properties and the phase diagram of the Fe-Mn system have been reassessed using a revised description of manganese and some new experimental information. Some improvements have been made compared to the previous assessment. The metastable hep phase is also included.

Pressure-temperature phase diagram of elemental carbon

Abstract: Carbon atoms form very strong bonds to each other yielding solid crystalline materials like graphite and diamond. Because of the high bonding energies, the vaporization and melting temperatures are very high. Different kinds of atom-to-atom bonding make many solid forms possible, ranging from pure graphite to pure diamond, as well as many types of molecules in liquid or gaseous carbon. Rigorous conditions of high temperature, high pressure, or both, are required to change a given elemental phase of carbon to another. Currently the vapor-pressure line of graphite, the P, T equilibrium line between graphite and diamond, and the graphite/diamond/liquid triple point are fairly well established. The triple point (or points) for graphite (or carbynes)/vapor/liquid remain controversial. At pressures less than 0.1 GPa liquid carbon seems to be a poor electric conductor while at higher pressures it is a good one. Current experimental and theoretical evidence indicate that diamond is stable against collapse to metallic forms (unlike Si and Ge) up to pressures over 350 GPa, and possibly as high as 2300 GPa. The latest static and shock compression experiments on diamond indicate that it melts to a conducting liquid at about 5000 K at pressures of 15 to 30 GPa, but does not melt at about 6000 K at 125 GPa. This suggests that the melting temperature of diamond increases with pressure, and that at the melting temperature liquid carbon is slightly less dense than diamond.