Multistep synthesis of VO2 (M) nanoparticles and their application to thermochromic hybrid films for IR modulation

Vanadium dioxide (VO₂) is widely known as one of the excellent thermochromic materials based on a reversible insulator-to-metal phase transition upon temperature change. In this study, VO₂ (M) powder was initially synthesized through a hydrothermal method and a subsequent post-annealing treatment. Additionally, a particle size of the VO₂ (M) powder was reduced and uniformized by introducing a ball-milling process. The resultant VO₂ (M) nanoparticles (NPs) were dispersed in ethanol with the addition of polyvinylpyrrolidone (PVP). The ethanolic dispersion was then coated on a transparent heater used as a substrate by spin-coating to produce VO₂ (M)/PVP composite films. We have attained an exact temperature control of the films by applying voltages to the heater for the assessment of their thermochromic performance such as the solar and the infrared modulation ability. For example, the film temperature could be raised from room temperature to 85.5°C within 180 s at a low voltage of 11 V, which was enough for inducing the phase transition of the VO₂ (M) NPs showing the infrared modulation ability of 19.3%. The combination of the composite films and the heater was thus proved to be a promising way for realizing transparent thermochromic devices.     

In situ Raman spectroscopy analysis of local structure in dry gel for zeolite beta synthesis

To investigate faster crystallization of zeolite beta by the dry-gel conversion method, the local structure of the dry gel before synthesis was quantitatively evaluated using in situ Raman spectroscopy during the drying process. The dry gel prepared from Si and Al sources, and tetraethylammonium hydroxide solution was crystallized after several hours by the dry-gel conversion method. The conformational change of TEA⁺ cations was observed during the drying process by the deconvolution of the spectrum, and the conformational change was larger than that during the synthesis process. The rate of conformational change was increased with the drying temperature, and the apparent activation energy was estimated to be 68.2 kJ/mol. The generation and transformation of double three-membered silicate rings (D3Rs) and 4-2 type secondary building units (SBUs), which are essential for the crystallization of zeolite beta, were observed during the drying process. The transformation from D3R to 4-2 SBU in the dry gel during drying process could be confirmed quantitatively by the difference of the time variation for the amounts of these silicate building units estimated by in situ observation.     

Gas permeation and thermomechanical properties for macroporous alumina focused on necking size at grain boundaries

The gas permeation and thermomechanical properties of macroporous alumina used as a support substrate for microporous ceramic permselective membranes were investigated. The porosity, pore size, and apparent necking size between grains of macroporous alumina were systematically varied, and the relationships between the porous microstructure and material properties were examined. The grain necking size at alumina grain boundaries was evaluated by microstructural observations. The nitrogen gas permeance of the porous alumina increased with increasing pore size. All the measured thermal and mechanical properties decreased with increasing porosity. The properties of porous alumina samples with extensive grain necking showed higher values even in samples with the largest pore size. The high thermal conductivity of porous alumina with extensive grain necking was due to the low interfacial thermal resistance at grain boundaries. Porous alumina with extensive grain necking had high thermal shock strength due to the higher thermal conductivity. It was demonstrated that a porous structure combining high gas permeability and excellent fracture resistance could be successfully achieved.