Quantification of gas-accessible microporosity in metal-organic framework glasses

Metal-organic framework (MOF) glasses are a new class of microporous glass materials with immense potential for applications ranging from gas separation to optics and solid electrolytes. Due to the inherent difficulty to determine the atomistic structure of amorphous glasses, the intrinsic structural porosity of MOF glasses is only poorly understood. In this work, the porosity features of a series of prototypical MOF glass formers from the family of zeolitic imidazolate frameworks (ZIFs) and their corresponding glasses is investigated comprehensively. CO2 gas sorption at 195 K allows to follow the evolution of microporosity when transforming from the crystalline to the glassy state of these materials. Based on these data, the pore volume and the real density of the ZIF glasses is quantified for the first time. Additional hydrocarbon sorption data (n-butane, propane and propylene) together with X-ray total scattering experiments prove that the porosity features (in particular the pore size and the pore limiting diameter) of the ZIF glasses depend on the types of organic linkers present in the glass network. This allows formulating first design principles for a targeted tuning of the intrinsic microporosity of MOF glasses. Importantly, these principles are counterintuitive and contrary to established porosity design concepts for crystalline MOFs but show similarities to strategies previously developed for porous polymers.

Finite Element Simulation of Micro-Thermoelectric Generators Based on Microporous Glass Template

COMSOL Multiphysics software-based three-dimensional finite element analysis is widely used in the performance simulation of thermoelectric devices. In this study, this software is used to simulate the heat transfer processes and power generation performance of micro-thermoelectric generators based on a microporous glass template. The temperature and electrical potential fields are coupled to each other through the thermoelectric effects during the calculations. The power generation performances of micro-thermoelectric generators with different template heights (d) for various temperature differences between their hot and cold ends (∆Th-c) are calculated. For the micro-thermoelectric generator that included four pairs of TE couples, the temperature difference between the two sides of the TE columns (∆TTE) and the open circuit voltage (Uoc) both increased with increasing d, but the growth rate gradually decreased. When d is greater than 0.2 mm, the increment basically becomes negligible. The maximum output power (Pmax) first increases and then decreases with increasing d, reaching a maximum value when d is 0.2 mm. Therefore, we can optimize the size of device according to the simulation results to ensure that the device produces the optimal output performance during the experiments. A model with the same parameters used in the experiment (i.e., d=0.2 mm) was then established and it generated a Uoc of 35.2 mV and a Pmax of 228.8 μW when ∆Th-c was 107.5 K (∆TTE = 97.55 K). The errors between the simulation and the experimental results are small and thus also verify the accuracy of the power generation performance test results.

Electrodeposition of Bi-Te Thin Films on Silicon Wafer and Micro-Column Arrays on Microporous Glass Template

Electrodeposition is an important method for preparing bismuth telluride (Bi2Te3)-based thermoelectric (TE) thin films and micro-column arrays. When the concentrations of Bi:Te in electrolytes were 3 mM:4 mM, the TE films satisfied the Bi2Te3 stoichiometry and had no dependence on deposition potential. With increasing over-potential, crystal grains changed from lamellar structures with uniform growth directions to large clusters with staggered dendrites, causing a decrease in the deposition density. Meanwhile, the preferred (110) orientation was diminished. The TE film deposited at −35 mV had an optimum conductivity of 2003.6 S/cm and a power factor of 2015.64 μW/mK2 at room temperature due to the (110)-preferred orientation. The electrodeposition of TE micro-columns in the template was recently used to fabricate high-power micro-thermoelectric generators (micro-TEG). Here, microporous glass templates were excellent templates for micro-TEG fabrication because of their low thermal conductivity, high insulation, and easy processing. A three-step pulsed-voltage deposition method was used for the fabrication of micro-columns with large aspect ratios, high filling rates, and high density. The resistance of a single TE micro-column with a 60 μm diameter and a 200 μm height was 6.22 Ω. This work laid the foundation for micro-TEG fabrication and improved performance.