Temperature Quench Experiments Research Articles

Monte Carlo simulations of vector pseudospins for strains: Microstructures and martensitic conversion times

We present systematic temperature-quench Monte Carlo simulations on discrete-strain pseudospin model Hamiltonians to study microstructural evolutions in ferroelastic transitions with two-component vector order parameters (NOP=2) in 2-spatial dimensions. The zero value pseudospin is the single high-temperature phase while the low-temperature phase has Nv variants. Thus the number of nonzero values of pseudospin are triangle-to-centered rectangle (Nv=3), square-to-oblique (Nv=4) and triangle-to-oblique (Nv=6). The model Hamiltonians contain a transition-specific Landau energy term, a domain wall cost or Ginzburg term, and power-law anisotropic interaction potential, induced from a strain compatibility condition. On quenching below a transition temperature, we find behaviour similar to the previously studied square-to-rectangle transition (NOP=1,Nv=2), showing that the rich behaviour found, is generic. Thus we find for two-component order parameters that the same Hamiltonian can describe both athermal and isothermal martensite regimes for different material parameters. The athermal/isothermal/austenite parameter regimes and temperature–time–transformation diagrams are understood, as previously, through parametrization of effective-droplet energies. In the athermal regime, we find rapid conversions below a spinodal-like temperature and austenite-martensite conversion delays above it, as in the experiment. The delays show early incubation behaviour, and at the transition to austenite the delay times have Vogel–Fulcher divergences and are insensitive to Hamiltonian energy scales, suggesting that entropy barriers are dominant. Systematic temperature quench experiments can look for martensite formation and growth during conversion-incubations, divergences, and distributions close to the transition.

Noble metal nanoclusters and nanoparticles precede mineral formation in magmatic sulphide melts

In low temperature aqueous solutions, it has long been recognized by in situ experiments that many minerals are preceded by crystalline nanometre-sized particles and non-crystalline nanophases. For magmatic systems, nanometre-sized precursors have not yet been demonstrated to exist, although the suggestion has been around for some time. Here we demonstrate by high temperature quench experiments that platinum and arsenic self-organize to nanoparticles, well before the melt has reached a Pt–As concentration at which discrete Pt arsenide minerals become stable phases. If all highly siderophile elements associate to nanophases in undersaturated melts, the distribution of the noble metals between silicate, sulphide and metal melts will be controlled by the surface properties of nano-associations, more so than by the chemical properties of the elements.

In situ high P-T Raman spectroscopy and laser heating of carbon dioxide.

In situ high P-T Raman spectra of solid CO(2) up to 67 GPa and 1,660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1,100 K in the 33-65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase diagram of carbon dioxide. Furthermore, our approach opens a new way to perform quantitative in situ Raman measurements under extremely high pressures and temperatures, providing unique information about phase relations and structural and thermodynamic properties of materials under these conditions.

Universal scaling laws for the anisotropic growth of SmA liquid crystal bâtonnets

We present a first quantitative evaluation of the anisotropy of the temporal development of liquid crystal SmA bâtonnets, formed during temperature quench experiments from the isotropic melt, in terms of universal scaling laws for nucleus growth. Experimental investigations were carried out as a function of quench depth and universal growth laws of the form L( t)∼ t n are confirmed to show the theoretically expected change from n=1/2 for vanishing quench depth towards n=1 for increasing super-cooling. Despite the fact that the smectic bâtonnet nuclei exhibit a pronounced anisotropy in their growth behaviour of approximately 4:1, it is demonstrated that nucleus growth follows the same universal scaling laws for both principal growth directions. This behaviour is shown to be greatly independent of nucleus orientation with respect to the imposed preferred direction by unidirectional boundary conditions.

High Pressure and High Temperature Studies on Manganese Oxides

High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature. The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C. Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure. f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO 2 is identified to be a low-density polymorph f -MnO 2 . This unusual result of formation of low-density f -MnO 2 , having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu's (1976) laser-heated DAC experiment.

Phase Diagram and Evolution of Mesophase Structure in Mixtures of a Low Molar Mass Smectic Liquid Crystal and a Nematic Side-on Side-Chain Liquid Crystalline Polymer

Miscibility phase diagrams in mixtures of side-on side-chain liquid crystalline polymers (s-SCLCP) and low molar mass smectic liquid crystals (K36) have been investigated by means of polarized optical microscopy. K36 is a smectic liquid crystal that exhibits a smectic−isotropic transition at 58 °C and a smectic-to-crystal transition (crystallization) at 27 °C. The nematic/smectic phase diagram of the s-SCLCP/K36 has been analyzed by combining the Flory−Huggins (FH) free energy for isotropic demixing and the Maier−Saupe−McMillan (MSM) free energy for smectic ordering of the mesogens. This combined FH/MSM theory compares reasonably well with the observed nematic/smectic phase diagrams composed of isotropic + nematic, smectic + nematic, smectic + isotropic, pure nematic, and pure smectic regions. Several temperature quench experiments have been conducted to confirm various predicted coexistence regions. However, at present, the combined theory does not include any crystallization processes occurring at a lower temperature.

Off-equilibrium dynamics of a (1 + 1)-dimensional directed polymer in random media

The relaxational dynamics of 1+1 dimensional directed polymer in random potential is studied by Monte Carlo simulation. A series of temperature quench experiments is performed changing waiting times. Clear crossover from quasi-equilibrium behavior to off-equilibrium behavior appears in the dynamical overlap function whose scaling properties are very similar to those found in the 3 dimensional spin-glass model. In the off-equilibrium part, the fluctuation dissipation theorem of the 1st kind which relates the response function to the tilt field with the conjugate correlation function, is found broken. These aging effects are brought about by the very slow growth of quasi-equilibrium domain driven by successive loop-excitations of various sizes, which form complex network structures.

Phase Separation Dynamics of Blends of Telechelic Epoxy Terminated Polybutadiene and Maltene

Abstract Thermally induced phase separation in a mixture of telechelic epoxy terminated polybutadiene (ETPB) and maltene has been studied by means of time-resolved light scattering and optical microscopy. Maltene, consisting of various hydrocarbon derivatives, was extracted from asphalt with n-heptane and isolated by centrifugation. The cloud point studies of the ETPB/maltene mixture showed an upper critical solution temperature (UCST) which is thermally reversible. Several deep temperature quench experiments were conducted at an off-critical composition (27/73 ETPB/maltene) from a single phase (80°C) to a two-phase region (27, 29, 31 and 33 °C). The time-evolution of the structure factor for the late stage of spinodal decomposition (SD) was analyzed in the framework of nonlinear and dynamical scaling laws. The reverse quench experiments were also undertaken to elucidate the phase dissolution process.

Anisotropic phase separation kinetics in a polymer blend solution following cessation of shear studied by light scattering

A light scattering instrument capable of monitoring the scattering from samples under the influence of a simple shear field has been constructed. The apparatus consists of transparent cone-and-plate fixtures and a two-dimensional charge coupled device array detector. The detector unit is also capable of measuring the scattering patterns as a function of time. The phase separation kinetics of an 8% solution of a polystyrene/polybutadiene (50:50) blend in dioctyl phthalate following cessation of a steady shear is monitored with this instrument. The sample is two-phase in the quiescent state and the applied shear is sufficient to suppress all scattering observed from the quiescent sample (shear-induced mixing). The evolution of the scattering profiles following cessation of shear is quite different parallel and perpendicular to the original flow direction. In the normal direction, a spinodal growth and coarsening mechanism similar to that observed in temperature quench experiments is observed. In the parallel direction, a different mechanism is followed. We believe this is the first report of this type of anisotropic behaviour in the phase separation kinetics of a polymer blend.

Phase separation in mixtures of poly(ethylene glycol monomethylether) and poly(propylene glycol) oligomers

AbstractTime‐resolved light scattering was employed to investigate kinetics of phase separation in mixtures of poly (ethylene glycol monomethylether) (PEGE)/poly (propylene glycol) (PPG) oligomers. Phase diagrams for PEGE/PPG of varying molecular weights were established by means of cold point measurements. The oligomer mixtures reveal an upper critical solution temperature (UCST). Several temperature quench experiments were carried out with a 60/40 PEGE/PPG blend by rapidly quenching from a single phase (69°C) to two‐phase temperatures (66–61°C) at 1°C intervals. As is typical for oligomer mixtures, the early stage of spinodal decomposition (SD) was not detected. The kinetics of phase decomposition was found to be dominated by the late stage of SD. Time‐evolution of scattering intensity was analyzed in accordance with nonlinear and dynamical scaling theories. The time dependence of the peak intensity Im and the corresponding peak wavenumber qm was found to follow the power‐law {Im(t)˜ tα, qm(t)˜ t‐β} with the values of α = 3 ± 0.3 and β = 1 ± 0.2, which are very close to the values predicted by Siggia. This process has been attributed to a coarsening mechanism driven by surface tension. In the temporal scaling analysis, the structure function reveals university with time, suggesting self‐similarity. Phase separation dynamics in 60/40 PEGE/PPG resembles the behavior predicted for off‐critical mixtures.

Long-range order near the Cu3Au(001) surface by evanescent x-ray scattering.

We present an experimental study of the surface effects on the order-disorder phase transition in the binary alloy ${\mathrm{Cu}}_{3}$Au. By x-ray scattering under the condition of total external reflection, we obtained depth profiles of the order parameter at the (001) surface when the transition temperature ${\mathit{T}}_{0}$ is approached. The scattered intensity is analyzed in the framework of the distorted-wave Born approximation, which is briefly outlined. The temperature and depth dependence of the evanescent (100) superlattice intensity is consistent with a wetting transition, which is driven by the first-order bulk transition: Close to ${\mathit{T}}_{0}$ the disordered phase appears at the (001) surface and grows according to L(t)=(13.1 \AA{})ln\ensuremath{\Vert}1/t\ensuremath{\Vert} with reduced temperature t. During these measurements the near-surface Debye-Waller factor was monitored via the (200) evanescent Bragg intensity, which, however, showed no significant surface effects at ${\mathit{T}}_{0}$. In several temperature quench experiments, where the system was rapidly cooled from T>${\mathit{T}}_{0}$ to ${\mathit{T}}_{0}$-\ensuremath{\Delta}T, we measured time- and depth-resolved evanescent superlattice intensities. We present depth-resolved near-surface relaxation times that exhibit a very distinct depth dependence. A first quantitative analysis of the relaxation processes is indicated and gives results consistent with the assumption of an ordinary ``surface-induced disorder.''

Phase dissolution in a liquid-crystalline polymer solution

Time-resolved light-scattering studies have been undertaken to elucidate the dynamics of phase dissolution in aqueous HPC (hydroxypropylcellulose) liquid-crystalline solutions. HPC solutions phase separate during heating, but return to a single phase upon cooling. A temperature versus composition phase diagram has been established on the basis of cloud point measurements. The behavior of phase separation resembles a lower critical solution temperature, but there is a discontinuity in the curve without a clear minimum or critical point. In the temperature quench experiments the scattering peak decays with time but its position remains stationary, which is in good accord with the linear theory. The kinetic behavior of phase dissolution of 10 wt% aqueous HPC solutions has been quantitatively analyzed in the context of the linear theory.