Alpha Phase Transition Research Articles

Martensitic-like microstructures across the isostructural phase transitions in Cerium

The isostructural gamma–alpha phase transition in elemental cerium is an electronic transition caused by a delocalization of the 4f electrons. This affects the bonding properties of Ce atoms and leads to a large volume collapse reaching ∼17% in the low pressure regime (¡ 2 GPa). While great attention has been drawn on the electronic description of this transition, attempts to understand the mesoscale mechanisms of this structural transition and their consequences in terms of microstructure remain scarce. We have investigated this transition by means of combined X-ray Computed Tomography and Energy Dispersive X-ray Diffraction on polycrystalline samples. Our experimental observations reveal a platelet-shape microstructure across the transition and up to the critical point, which have been associated to displacive mechanisms. Based on continuum mechanics modeling and ab initio calculations, we propose that here, this microstructure is initiated by elastic instability and shear anisotropy in the ¡100¿ directions.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Effect of Mg2+ doping on beta–alpha phase transition in tricalcium phosphate (TCP) bioceramics

The beta to alpha transition in tricalcium phosphate (TCP) bioceramics containing different amount of magnesium was studied in the present work. Mg-doped TCP powder was obtained by solid-state reaction starting from pure calcium carbonate, ammonium phosphate dibasic and magnesium oxide powders. The β to α transformation temperature was identified by dilatometric and thermo-differential analyses. Small pellets produced by uniaxial pressing samples were employed to study the influence of Mg2+ on the transition kinetic, after sintering at 1550°C and subsequent slow or fast cooling down to room temperature. The evolution of β- and α-TCP crystalline phases during each thermal treatment was determined by X-ray powder diffraction analysis combined with Rietveld method-based software An annealing treatment, suitable to reconvert metastable α phase to the more clinically suitable β phase, was also investigated.It is shown that the presence of magnesium within the TCP lattice strongly influences the kinetic of the β⇆α phase transition, promoting the spontaneous α→β reconversion even upon fast cooling, or slowing down the β→α transition during heating. Similarly, it allows the α→β transformation in TCP sintered components by optimized annealing treatment at 850°C. Statement of SignificanceThis work concerns the effect of Mg2+ doping on the β→α phase reconstructive transition in tricalcium phosphate (TCP), one of the most important bio-resorbable materials for bone tissue regeneration. The transition occurs upon the sintering process and is has been shown to be strongly irreversible upon cooling, leading to technological issues such as poor mechanical properties and excessive solubility due to the presence of metastable α-phase.This paper points out the kinetic contribution of Mg2+ on the spontaneous α→β reconversion also upon fast cooling (i.e. quenching). Moreover, an annealing treatment has been shown to be beneficial to remove the retained α-phase in sintered TCP components, the presence of Mg promoting the reconversion process.

The effect of chromium on the gamma to alpha phase transition of alumina coating formed on 316L SS by a cathodic micro arc deposition (CMAD) process

Alumina ceramic coatings are widely used because of their favorable physical and chemical stability. However, the formation of stable alpha phase alumina on metallic substrates at lower temperatures still presents technical challenges. In this work, an Cr doped Al2O3 ceramic coating was successfully fabricated on 316L stainless steel by a new cathodic micro-arc deposition (CMAD) technique using Al(NO3)3 as the precursor. We studied the effect of chromium on its phase transition from gamma to alpha as well as the underlying mechanism. Phase and chemical composition distribution, surface morphology, and photoluminescence data were characterized by X-ray diffraction (XRD), electron probe microanalysis (EPMA), a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS), and a microscopic confocal Raman spectrometer (MCRS). The results showed that the ceramic coatings have a porous microstructure and were mainly composed of Cr doped α-Al2O3 and γ-Al2O3. This suggests that the introduction of Cr in the electrolyte solution can promote the nucleation and growth of the corundum phase of α-Al2O3. Based on the comparison of conventional heat treatment of alumina precipitates with and without Cr dopant via a sol–gel process, we proposed that the reducing atmosphere and local high sparking temperature that subsequently enhances the diffusion of defects in oxides may be contributed to this transition. This finding might provide a simple, cost efficient way to fabricate corundum alumina coatings on metallic substrates under mild temperatures without sacrifice of its mechanical properties.

Phospholipid Bilayer Free Volume Analysis Employing the Thermal Ring-Closing Reaction of Merocyanine Molecular Switches

The free volume properties of phospholipid bilayers have been determined using a new assay that applies the photochromic and solvatochromic properties of merocyanines. The orientation and embedding depth of the merocyanines in the bilayer are controlled using substitution on the merocyanine indole moiety. The free volume changes at the aqueous interface (region 1), the phospholipid headgroup (region 2), and the aliphatic interior (region 3) of the bilayer are compared by analyzing the rate constants for the merocyanine ring-closing reaction. Free volume variations during the P(beta)(')(gel) <--> L(alpha)(liquid) phase transition are observed in region 1, in accordance with large structural rearrangements between the gel and the liquid phases in this region. The largest free volume is found in region 3, and the smallest is found in region 2. This distribution of free volume in the bilayer agrees with computational studies of these systems. Comparison of the free volume in region 2 of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids shows that this method is sensitive to small structural differences between lipids. In region 2, the free volume is found to be approximately 2 times larger in DPPC bilayers, which could be related to different merocyanine interactions with the two phosphatidylcholines. Free volume properties determined on picosecond and second time scales are compared based on an analysis of merocyanine formation and decoloration reactions.

Magnetic Form Factor ofα-Ce: Towards Understanding the Magnetism of Cerium

A favored interpretation of the gamma <--> alpha phase transition in cerium postulates the transformation of the localized 4f state in gamma-Ce to a weakly correlated itinerant 4f band in alpha-Ce. However, results of high-energy neutron inelastic scattering measurements, presented here, show clearly that the magnetic susceptibility response from alpha-Ce follows the Ce3+ form factor despite the large, 30-fold, increase in its spectral width relative to that in gamma-Ce. This observation provides, for the first time, indisputable evidence for the localized character of the 4f state in the alpha phase. The present findings appear consistent with recent calculations combining dynamical mean-field theory with the local density approximation that indicate a strongly correlated 4f state in alpha-Ce. The localized 4f state is also fundamental to the Kondo volume collapse theories for the gamma <--> alpha phase transition in cerium.

Phase behavior of poly(oxyethylene) cholesteryl ether/novel alkanolamide/water systems.

The phase behavior and microstructure of mixed nonionic surfactant systems containing poly(oxyethylene) cholesteryl ether (ChEOn, n=15 and 10), a new alkanolamide-type foam booster, dodecanoyl N -methylethanolamide (NMEA-12), and water, were investigated at 25 degrees C by means of visual observation and small-angle X-ray scattering. In the ChEO(15)/water binary system, aqueous micellar (W(m)), discontinuous cubic liquid crystal (I(1)), hexagonal (H(1)), rectangular ribbon (R(1)), lamellar (L(alpha)), and solid (S) phases are successively formed with increasing surfactant concentration. Although the R(1) phase is an intermediate phase formed in a very narrow composition range in conventional surfactant systems, its domain is unusually wider than that of H(1), which may be attributed to the packing constraint caused by the bulky cholesteric group in the lipophilic core of the aggregate. Upon addition of lipophilic NMEA-12 to the ChEO(15)/water binary system, the interfacial curvature of the aggregates decreases, and the micellar or liquid crystal phases formed in the binary system transform to the reverse micellar (O(m)) phase via the L(alpha) phase existing over a wide concentration range. The SAXS results establish an epitaxial relationship between the (11) plane of the R(1) phase and the (10) plane of the L(alpha) phase. The ChEO(10)/NMEA-12/water system shows a phase diagram of similar general appearance, except that the W(m) to R(1) phase transformation occurs via an optically anisotropic liquid crystal phase of unknown structure and the R(1) to L(alpha) phase transition occurs through a narrow intermediate defected lamellar (L(alpha)(H)) phase. The variation in the aggregate size and shape and the unit cell of the R(1) phase formed in ChEOn/NMEA-12/water systems is also discussed.

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.

Specific heat calculated from the Raman frequency shifts close to the alpha phase transition in NH4Br

We calculated the specific heat C V I close to the λ-phase transition in NH 4 Br on the basis of predictions of the Ising model. We have used our observed Raman frequencies of the ν 5 (134 cm - 1 ) lattice mode to extract the critical exponent for the specific heat. The C V I values calculated by us with this exponent value agree with the measured Cp data reported in the literature. This agreement is particularly good above T λ . This indicates that the observed behaviour of NH 4 Br can be satisfactorily explained close to the λ-phase transition in this crystal.

Observation of the soft-mode condensation in the Sm-A-Sm-C(*)(alpha) phase transition by nonlinear dielectric spectroscopy.

We have theoretically and experimentally studied the nonlinear dielectric response under large orientational fluctuations near the Sm-A-Sm-C(*)(alpha) phase transition point in a chiral smectic liquid crystal. The third-order nonlinear dielectric response due to the fluctuations was observed in the vicinity of the transition point. By using it, we have successfully observed the soft-mode condensation inducing the Sm-A-Sm-C(*)(alpha) phase transition.

Nonlinear dielectric spectroscopy in the smectic-A-smectic-C(*)(alpha) phase transition.

We performed nonlinear dielectric spectroscopy to study the dynamic properties near the smectic-A-smectic-C(*)(alpha) phase transition point of an antiferroelectric liquid crystal 4-(1-methyl-heptyloxycar- bonyl)phenyl 4-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC). A Landau-type theory was developed in order to analyze the experimentally obtained frequency dispersions. From the experimental and theoretical results we successfully obtained the temperature dependence of the relaxation frequency of the soft mode in both the smectic-A and smectic-C(*)(alpha) phases, which gave direct evidence that the transition is brought about by soft mode condensation. Significant features of this transition are also discussed.

Critical behavior of nonlinear permittivity in the smectic-A phase of chiral liquid crystals.

We study the nonlinear permittivity of chiral liquid crystals in the smectic-A (Sm-A) phase near the ferroelectric smectic-C* (Sm-C*) phase and the smectic-C(*)(alpha) (Sm-C(*)(alpha)) phase theoretically and experimentally. The third-order nonlinear permittivity epsilon(3) shows the critical behavior with the exponent of four near the Sm-C* phase and its sign depends on the order of the phase transition. In the case of the Sm-A-Sm-C(*)(alpha) phase transition, the sign inversion of epsilon(3), presumably due to the large fluctuation of order parameter, is observed near the transition temperature.

Field-induced behaviors near the smectic-A-smectic-C * alpha phase transition of an antiferroelectric liquid crystal.

We have experimentally and theoretically investigated the electric-field effect on the smectic-A-smectic-C(*)(alpha) phase transition of an antiferroelectric liquid crystal4-(1-methyl-heptyloxycarbonyl)phenyl 4(')-octylcarbonyloxybiphenil-4-carboxylate. From the microscopic observations under electric fields and the measurements of the D-E hysteresis loop and apparent tilt angle, the E (electric field)-T (temperature) phase diagram was established, in which a tricritical point was found. From the dielectric measurements under dc electric fields, a soft mode inducing the transition was observed. Furthermore, we have developed a phenomenological theory under dc fields to analyze the experimental results. The origin of the soft mode observed under fields was clarified and the temperature dependence of the relaxation frequency was obtained by the analyses.

Observation of the soft mode in the Sm-C(*)(alpha) phase

We have performed the electro-optical measurements in an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4(')-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC) and observed the soft mode in the chiral Sm-C(*)(alpha) phase. This is a direct consequence that the Sm-C(*)(alpha) phase is induced by the condensation of a tilting mode. A Landau-type theory was developed to analyze frequency dispersions in the second-order response and it was found that the Sm-A-Sm-C(*)(alpha) phase transition is close to a tricritical point.

Critical Behavior of Layer Compression Modulus near the Smectic-A–Smectic-Cα*Transition

The layer compression modulus was measured near the smectic- A-smectic- C(*)(alpha) phase transition in a chiral smectic liquid crystal. The layer compression modulus B shows marked pretransitional effects above the phase transition. The critical softening of B can be fit to a simple power law. It is described by the theoretical description based on the renormalization-group method and seems to suggest a crossover behavior. These effects clearly show that the transition is not of the Landau mean-field type.

Electric quadrupole interactions and the - phase transition in Ce: the role of conduction electrons

We present a theoretical model of the "isostructural" \gamma-\alpha phase transition in Ce which is based on quadrupolar interactions due to coupled charge density fluctuations of 4f electrons and of conduction electrons. Conduction electrons are treated in tight-binding approximation. The \gamma-\alpha transition is described as an orientational ordering of quadrupolar electronic densities in a Pa3 structure. The quadrupolar order of the conduction electron densities is complementary to the quadrupolar order of 4f electron densities. The inclusion of conduction electrons leads to an increase of the lattice contraction at the \gamma-\alpha transition in comparison to the sole effect of 4f electrons. We calculate the Bragg scattering law and suggest synchrotron radiation experiments in order to check the Pa3 structure. The theory is capable of accounting for transitions to phases of non-cubic symmetry, but it is not sufficient to describe the magnetic phenomena which we ascribe to the Kondo mechanism. We also present a microscopic derivation of multipolar interactions and discuss the crystal field of \gamma-Ce.

Slow mode of the smectic-A–smectic-Cα*phase transition

Unusual slow fluctuations as revealed recently by dynamic light scattering close to the Sm-A-Sm-C(alpha)(*) phase transition in the antiferroelectric liquid crystal 4-(1-methylheptyloxy-carbonyl)phenyl 4(')-octyloxy biphenyl-4-carboxylate can be explained by the electrostatic coupling between impurity ions and director fluctuations. Within the vicinity of Sm-A-Sm-C(alpha)(*) transition, the relaxation rate of the slow mode depends linearly on temperature, but with a different slope in each phase. The square root of its intensity shows a clear Curie-Weiss divergence at the phase transition, which is a direct confirmation of the electrostatic coupling mechanism.

Effect of compact structures on the phase transition, subsequent densification and microstructure evolution during sintering of ultrafine gamma alumina powder

The effect of compact structures on the phase transition from gamma alumina (γ-Al 2O 3) to alpha alumina (α-Al 2O 3) and densification during sintering of the compacts of ultrafine gamma alumina powder (n-Al 2O 3) has been studied. The onset temperature of gamma→alpha phase transition during the sintering of n-Al 2O 3 compacts is affected significantly by the compact density as well as by the population of α-Al 2O 3 seed particles in the γ-Al 2O 3 matrix. The nucleation and growth of alpha phase is considered to be facilitated by the mass transfer around the particle contacts and by the imbedded α-Al 2O 3 particles. The gamma→alpha phase transition is proved to be detrimental to full densification of the n-Al 2O 3 compacts of gamma phase. The densification of high density compacts seems to be affected detrimentally by the phase transition in the range of 1050°C to 1150°C but finally reaches about 99% of theoretical density (TD) with fine grain structure. The unseeded compacts made by slurry casting are not densified above 73% TD even at 1400°C, whereas the α-Al 2O 3-seeded compacts are densified up to 90% TD at 1400°C and they have very large grain size of about 1 μm in diameter. High density compaction technology is considered a useful method required to achieve fully dense sintered compacts with fine grains below 100nm even in the sintering of ultrafine powder compacts with a metastable phase.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 1. DMPC/cholesterol membranes

The intramolecular dynamics of the excimer forming dipyrenyl lipids (DipynPC) of different chain lengths (n) in ethanol and in dimyristoylphosphatidycholine (DMPC) membranes was investigated by the use of frequency-domain fluorescence intensity decay technique. Based on a 3-state model, the extent of aggregation and rotational rate of the two intralipid pyrene moieties in the dipyrenyl lipids were estimated from the frequency-domain data. In ethanol (20 degrees C), the rotational rate for DipynPC increased progressively as n was varied from 4 to 12. At the gel (L beta)-to-liquid crystalline (L alpha) phase transition of DMPC (approximately 23 degrees C), the rotational rate increased and aggregation decreased significantly for Dipy10PC, whereas only the rotational rate was changed for Dipy4PC. In the presence of 30 mol% cholesterol, significant increases in both the rotational rate and aggregation were observed for Dipy10PC in both L beta and L alpha phases. However, for the case of Dipy4PC, an increase in the rotational rate but a decrease in the aggregation were noticed only in the L beta phase, and no similar changes were detected in the L alpha phase. Our results indicate differential effects of cholesterol on the conformational dynamics of acyl chains at different depths of the membranes.

Kinetics and mechanism of the barotropic lamellar gel/lamellar liquid crystal phase transition in fully hydrated dihexadecylphosphatidylethanolamine: a time-resolved x-ray diffraction study using pressure jump

The kinetics and mechanism of the barotropic lamellar gel (L beta')/lamellar liquid crystal (L alpha) phase transition in fully hydrated 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) has been studied using time-resolved x-ray diffraction (TRXRD). The phase transition was induced by pressure jumps of varying amplitudes in both the pressurization and depressurization directions at controlled temperature (78 degrees C). Both low- and wide-angle diffracted x rays were recorded simultaneously in live time using an x-ray-sensitive image intensifier coupled to a CCD camera and Super-VHS videotape recorder. Such an arrangement allowed for the direct and quantitative characterization of the long- (lamellar repeat spacing) and short-range order (chain packing) during a kinetic experiment. The image-processed live-time x-ray diffraction data were fitted using a nonlinear least-squares model, and the parameters of the fits were monitored continuously throughout the transition. The pressure-induced transitions from the L alpha to the L beta' phase and from the L beta' to the L alpha phase was two-state (no formation of intermediates apparent during the transition) to within the sensitivity limits of the method. The corresponding transit time (the time during which both phases coexist) associated with the long- and short-range order of the pressurization-induced L alpha-to-L beta' phase transition decreased to a limiting value of approximately 50 ms with increasing pressure jump amplitude. This limiting value was close to the response time of the detector/recording system. Thus, the intrinsic transit time of this transition in fully hydrated DHPE at 78 degrees C was less than or equal to 50 ms. In contrast, the depressurization-induced L beta'-to-L alpha phase transition was slower, taking approximately 1 s to complete, and occurred with no obvious dependence of the transit time on pressure jump amplitude. In the depressurization jump experiment, the lipid responded rapidly to the pressure jump in the L beta' phase up to the rate-determining L beta'-to-L alpha transition. Such behavior was examined carefully, as it could complicate the interpretation of phase transition kinetic measurements.