Jia Yan Law

Nanoporous Thermochromic VO2 (M) Thin Films: Controlled Porosity, Largely Enhanced Luminous Transmittance and Solar Modulating Ability

Vanadium dioxide is the most widely researched thermochromic material with a phase transition temperature (τ(c)) of around 68 °C, and its thermochromic performance can be enhanced by adding nanoporosity. Freeze-drying has been employed to fabricate nanostructures with different porosities from 16 to 45% by varying the prefreezing temperature and precursor concentration. The luminous transmittance (Tlum) and solar modulating ability (ΔTsol) are greatly enhanced as a result of increasing pore size and pore density. The freeze-dried sample with 7.5 mL of H2O2 precursor dip-coated at 300 mm/min gives the best combination of thermochromic properties (Tlum ≈ 50%, ΔTsol = 14.7%), which surpasses the best combined thermochromic performance reported to date that we are aware of (Tlum ≈ 41%, ΔTsol = 14.1%).

Influence of La and Ce additions on the magnetocaloric effect of Fe–B–Cr-based amorphous alloys.

The magnetic entropy change (ΔSM), temperature of peak ΔSM (Tpk) and refrigerant capacity (RC) in Fe(RE)80B12Cr8 (RE=La, Ce, or Gd) alloys were studied. Increasing La, Ce, and Gd content led to relatively constant, decrease, and increase in Tpk, respectively. Both the phenomenologically constructed universal curve for ΔSM and field dependence power laws demonstrated that these alloys exhibited similar critical exponents at Curie temperature. With 5% Ce added to Fe80B12Cr8, Tpk could be tuned near room temperature with relatively constant peak ΔSM. Fe79B12Cr8La1 exhibited enhanced RC compared to Gd5Si2Ge1.9Fe0.1. The tunable Tpk and enhanced RC are needed in active magnetic regenerators.

VO2/Si-Al gel nanocomposite thermochromic smart foils: Largely enhanced luminous transmittance and solar modulation

VO2 nanoparticles with a dimension of approximately 20 nm were obtained by simple mechanical bead-milling method, which were well dispersed in transparent silica-alumina (Si-Al) gel matrix to form nanocomposites. The VO2/Si-Al gel thermochromic nanocomposite foils were fabricated with various VO2 solid contents and foil thickness. With 10% VO2 loading and 3 μm foil thickness, high luminous transmittance (T(lum(20°C))=63.7% and T(lum(90°C))=54.4%), and large solar modulation ability (ΔTsol=12%) can be obtained which surpasses the best reported results (nanoporous films:T(lum(20°C))=43.3%, T(lum(90°C))=39.9% and ΔTsol=14.1%). This current approach provided a simple and scalable preparation method with the best combined thermochromic performance.

Direct magnetocaloric measurements of Fe-B-Cr-X (X = La, Ce) amorphous ribbons

A procedure has been developed to directly measure the adiabatic temperature change of amorphous melt-spun Fe-based ribbons displaying attractive room temperature magnetocaloric properties. Polycrystalline Gd ribbons are used as a reference material to compensate for the contribution of the sample holder to the experimental values. Fe78B12Cr8Ce2 and Fe75B12Cr8Ce5 melt-spun ribbons exhibited a peak adiabatic temperature change (ΔTadpk)u2009∼u200958% larger than Co82.9Si5.9Fe4.5Cr4B2.7 amorphous ribbons. The ΔTadpk in Fe78B12Cr8Ce2, Fe75B12Cr8Ce5, and Fe79B12Cr8La1 ribbons displayedu2009∼u200918-33% enhanced ΔTadpk compared to a GdAl2 alloy.

Comparison of the crystallization behavior of Fe-Si-B-Cu and Fe-Si-B-Cu-Nb-based amorphous soft magnetic alloys

The role of the solute elements, copper, and niobium, on the different stages of de-vitrification or crystallization of two amorphous soft magnetic alloys, Fe73.5Si13.5B9Nb3Cu1, also referred to as FINEMET, and a Fe76.5Si13.5B9Cu1 alloy, a model composition without Nb, has been investigated in detail by coupling atom probe tomography and transmission electron microscopy. The effects of copper clustering and niobium pile-up at the propagating interface between the α-Fe3Si nanocrystals and the amorphous matrix, on the nucleation and growth kinetics have been addressed. The results demonstrate that while Cu clustering takes place in both alloys in the early stages, the added presence of Nb in FINEMET severely restricts the diffusivity of solute elements such as Cu, Si, and B. Therefore, the kinetics of solute partitioning and mobility of the nanocrystal/amorphous matrix interface is substantially slower in FINEMET as compared to the Fe76.5Si13.5B9Cu1 alloy. Consequently, the presence of Nb limits the growth rate of the α-Fe3Si nanocrystals in FINEMET and results in the activation of a larger no. of nucleation sites, leading to a substantially more refined microstructure as compared to the Fe76.5Si13.5B9Cu1 alloy.

The magnetocaloric effect of partially crystalline Fe-B-Cr-Gd alloys.

The influence of annealing temperature and crystallization on the magnetocaloric effect (MCE) of Fe-B-Cr-Gd partially crystalline alloys was studied. Although the alloys exhibited dissimilar devitrification behavior, all the alloys exhibited MCE behavior consistent with a phenomenological universal curve and theoretical power law expressions of the magnetic field dependence of MCE. The TC of partially crystalline Fe75B12Cr8Gd5 alloys increased with increasing annealing temperatures. However, peak magnetic entropy change and refrigerant capacity values remained relatively constant, suggesting that these alloys are promising for active magnetic regenerator applications.

Magnetocaloric effect in heavy rare-earth elements doped Fe-based bulk metallic glasses with tunable Curie temperature

The effects of heavy rare earth (RE) additions on the Curie temperature (T-C) and magnetocaloric effect of the Fe-RE-B-Nb (RE-Gd, Dy and Ho) bulk metallic glasses were studied. The type of dopping RE element and its concentration can easily tune T-C in a large temperature range of 120K without significantly decreasing the magnetic entropy change (Delta S-M) and refrigerant capacity (RC) of the alloys. The observed values of Delta S-M and RC of these alloys compare favorably with those of recently reported Fe-based metallic glasses with enhanced RC compared to Gd5Ge1.9Si2Fe0.1. The tunable T-C and large glass-forming ability of these RE doped Fe-based bulk metallic glasses can be used in a wide temperature range with the final required shapes.

Optimal temperature range for determining magnetocaloric magnitudes from heat capacity

The determination of the magnetocaloric magnitudes (specific magnetic entropy change, Δs M, and adiabatic temperature change, ΔT ad) from heat capacity (c H) measurements requires measurements performed at very low temperatures (~0 K) or data extrapolation when the low temperature range is unavailable. In this work we analyze the influence on the calculated Δs M and ΔT ad of the usually employed linear extrapolation of c H from the initial measured temperature down to 0 K. Numerical simulations have been performed using the Brillouin equation of state, the Debye model and the Fermi electron statistics to reproduce the magnetic, lattice and electronic subsystems, respectively. It is demonstrated that it is not necessary to reach experimentally temperatures very close to 0 K due to the existence of certain starting temperatures of the experiments, the same for Δs M and ΔT ad, that minimize the error of the results. A procedure is proposed to obtain the experimental magnitudes of Δs M and ΔT ad with a minimum error from c H data limited in temperature. It has been successfully applied to a GdZn alloy and results are compared to those derived from magnetization measurements.

A quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect

The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for both applied magnetocaloric research as well as the characterization of other phase change materials. Although Ehrenfest provided a conceptually simple definition of the order of a phase transition, the known techniques for its determination based on magnetic measurements either provide erroneous results for specific cases or require extensive data analysis that depends on subjective appreciations of qualitative features of the data. Here we report a quantitative fingerprint of first-order thermomagnetic phase transitions: the exponent n from field dependence of magnetic entropy change presents a maximum of nu2009>u20092 only for first-order thermomagnetic phase transitions. This model-independent parameter allows evaluating the order of phase transition without any subjective interpretations, as we show for different types of materials and for the Bean–Rodbell model.Magnetocaloric materials often perform best when their magnetic transitions are at the boundary between first- and second-order behavior. Here the authors propose a simple criterion to determine the order of a transition, which may accelerate future magnetocaloric material searches.

Modification of the field dependence and scaling of the magnetocaloric effect in LaFeSi across the tricritical point

The most straightforward classification of magnetocaloric materials is according to the order of the phase transition that they undergo: either second order (continuous) phase transitions (SOPT), like Gd, or first order magneto-structural phase transitions (FOPT) like the giant magnetocaloric material Gd5Si2Ge2.