J. M. Martín-Olalla

Dynamic heat flux experiments in Cu67.64Zn16.71Al15.65: Separating the time scales of fast and ultra-slow kinetic processes in martensitic transformations

Crackling noise and avalanches during the martensite phase transformation of Cu67.64Zn16.71Al15.65 were investigated. Heat flux measurements with extremely slow heating rates of 0.005 Kh−1 allowed sufficient separation between the continuous background and the avalanche jerks. The jerk enthalpy is below 3% of the total transformation enthalpy. The crackling noise follows power law behavior with an energy exponent near ɛ = 1.8. The jerks are almost uncorrelated with approximately a Poisson distribution of the waiting times between jerks. Quantitative analysis showed a scaling behavior with p(wt) ∼ wt(γ−1)exp(−wt/τ)n with γ = 0.7 and n ≈ 1.

Phase transitions in lawsonite a calorimetric study

The specific heat of lawsonite, CaAl 2 Si 2 O 7 (OH) 2 .H 2 O, has been measured in the temperature range 125 K - 325 K. An anomaly is seen at 273 K, which is related to the Cmcm — Pmcn phase transition. The magnitude of the total excess entropy associated with this transition is not reproducible, varying in the range 5.93 — 6.24 J K −1 mol −1 . On heating, the specific heat anomaly is consistent with a tricritical phase transition. However, on cooling, significant hysteresis is observed, and the form of the C P anomaly is quite different. In all measurements extensive pre-transitional effects are observed above T C . Analysis of existing specific heat data in the temperature range 75 K — 175 K shows an anomaly associated with the Pmcn — P2 1 cn phase transition. The excess entropy associated with this transition is 6 (1) J K −1 mol −1 . These data are interpreted as showing that both transitions are caused by the interaction of proton ordering and displacive changes in the aluminosilicate framework. The standard entropy of lawsonite at 298 K is recalculated, incorporating the effects of the two transitions. Two methods are used for this recalculation, giving values of S 0 298 = 233.27 and 234.96 JK −1 mol −1 respectively.

Avalanche correlations in the martensitic transition of a Cu–Zn–Al shape memory alloy: analysis of acoustic emission and calorimetry

The existence of temporal correlations during the intermittent dynamics of a thermally driven structural phase transition is studied in a Cu-Zn-Al alloy. The sequence of avalanches is observed by means of two techniques: acoustic emission and high sensitivity calorimetry. Both methods reveal the existence of event clustering in a way that is equivalent to the Omori correlations between aftershocks in earthquakes as are commonly used in seismology.

The influence of an electric field on the latent heat of the ferroelectric phase transition in KDP

The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystal have been measured simultaneously for various electric fields with a conduction calorimeter. The specific heat presents a strong anomaly, but these simultaneous measurements allow us to evaluate the latent heat accurately. The latent heat decreases with field, and the value of the critical electric field—that where the latent heat disappears—is estimated to be (0.44 ± 0.03) kV cm−1. Incidentally, we have measured simultaneously the dielectric permittivity, which suggests that latent heat is developed as domains are growing.

The order parameter–entropy relation in some universal classes: experimental evidence

Asymptotic behaviour near phase transitions can be suitably characterized by the scaling of Δs/Q2 with = 1 − T/Tc, where Δs is the excess entropy and Q is the order parameter. As Δs is obtained by integration of the experimental excess specific heat of the transition Δc, it displays little experimental noise so that the curve log(Δs/Q2) versus log is better constrained than, say, log Δc versus log . The behaviour of Δs/Q2 for different universality classes is presented and compared. In all cases, it clearly deviates from being a constant. The determination of this function can then be an effective method to distinguish asymptotic critical behaviour. For comparison, experimental data for three very different systems, Rb2CoF4, Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function Δs/Q2 does not deviate within experimental resolution from a straight line so that, although Q can be fitted with a non mean-field exponent, the data can be explained by a classical Landau mean-field behaviour. In contrast, the behaviour of Δs/Q2 for the antiferromagnetic transition in Rb2CoF4 and the normal–incommensurate phase transition in Rb2ZnCl4 is fully consistent with the asymptotic critical behaviour of the universality class corresponding to each case. This analysis therefore supports the claim that incommensurate phase transitions in general, and the A2BX4 compounds in particular, in contrast with most structural phase transitions, have critical regions large enough to be observable.

Evidence of latent heat in the Rb2ZnCl4 commensurate-incommensurate phase transition

The commensurate-incommensurate phase transition of a Rb2 ZnCl4 crystal was studied using a conduction calorimeter. By identifying the contribution of the specific heat to the transition enthalpy it was possible to show, for the first time for a compound of this type, transition latent heat. This was estimated to be Q = 2.3 J mol-1 . The result was compared with theoretical predictions for a domain wall model having repulsive-attractive interaction; satisfactory results were obtained for the first time. The same analysis was applied to the normal-incommensurate phase transition and no latent heat was observed, as expected.

Deuteration effect on tricritical phase transition of triglycine selenate: Calorimetric and dielectric measurements analyzed in the framework of Landau theory

The ferroelectric phase transition of three single crystals of (TGSe)1−x(DTGSe)x has been described by using specific heat, latent heat, and dielectric permittivity measurements. Pure, half-deuterated, and highly-deuterated TGSe single crystals were analyzed. Transition temperature and latent heat increase with increasing deuteration. Irrespective of the degree of deuteration, a 2-4-6 Landau model is suitable to describe the phase transition. The fourth-rank prefactor in Landau potential is found to be very sensitive to deuteration while the second-rank and the sixth-rank prefactors smoothly change with composition. The pyroelectric figure of merit for these materials has also been derived from the theoretical model.

Long Time Permittivity Decay in Commensurate Phase of Rb2ZnCl4 Single Crystal.

The decay of the permittivity of Rb 2 ZnCl 4 is studied over ten days in the commensurate phase. No equilibrium state is actually found after an experience of twelve days. Assuming that the heating branch may represent the equilibrium state a three-component relaxation law is obtained with three different relaxation time. These relaxation times are connected to three different mechanisms of permittivity decay: six domain wall merging, pair annihilation and pinning to defects.

Sleep timing in industrial and pre-industrial societies syncs to the light/dark cycle

Did artificial light reshape human sleep/wake cycle? Most likely the answer is yes. Did artificial light misalign the sleep/wake cycle in industrialized societies relative to the natural cycle of light and dark? For the average person—that is, obviating the tail of the distributions— the answer is probably not. Sleep timing in industrial (data from eight national time use surveys) and pre-industrial, hunter-gatherer/horticulturalist societies (seven data from three previous reports) with and without access to artificial light across a wide range of angular distance to the Equator (0° to 55°) finds a remarkable accommodation in trends dominated by the light/dark cycle. Winter sunrise time—the latest sunrise of the year— triggers risetimes, which delays in the poleward direction. The same trend is observed for bedtimes meaning that they are homeostaticly related to the preeceding risetime. Seasonal variations show that summer sleep timing is more alike across the range of angular distance to the Equator due to the fact that the subsolar point at noon is closer to the observers.

Sleep/wake synchronization across latitude

Analysis of time use surveys in seventeen European countries and two American countries suggest that the winter sunrise —the latest sunrise year round— is a synchronizer for the sleep/wake cycle in standard population below 54° latitude, in competition with the noon synchronizer. When comparing industrialized data to data from hunter/gatherer, pre-industrial, Tropical societies only the late event survives as a synchronizer below 54° latitude. People rise immediately before sunrise —winter sunrise in industrialized mid-latitude societies— and abhor morning darkness. Synchronization propagates through the sleep/wake cycle so that people go to bed with increasing distance to sunset in winter as latitude increases in a scenario dominated by artificial light. This suggests a leading role of the homeostatic sleep pressure in understanding sleep/wake cycle at social level.