We report an experimental investigation of the adsorption properties of two important small-pore metal–organic framework (MOF) materials recently identified for gas separation applications, through the development and use of a high-pressure/high-temperature quartz crystal microbalance (QCM) device. In particular, we characterize in detail the CO2, CH4, and N2 adsorption characteristics of the MOFs Cu(4,4′-(hexafluoroisopropylidene)bisbenzoate)1.5 (referred to as Cu–hfipbb) and zeolitic imidazolate framework-90 (ZIF-90). We first describe the construction of a QCM-based adsorption measurement apparatus. Single-component adsorption isotherms of CO2, CH4, and N2 in the two MOFs were then measured at temperatures ranging from 30 to 70 °C and pressures ranging from 0.3 to 110 psi. In both materials, the order of adsorption strength is CO2 > CH4 > N2. We find that adsorption in the 1-D channels of Cu–hfipbb can be well described by a single-site Langmuir model. On the other hand, adsorption in ZIF-90 follows a more complex behavior, commensurate with its pore structure consisting of large porous cages connected in three dimensions by small windows. The nongravimetric QCM-based measurement techniques are shown to be a valuable microanalytical tool for the study of molecular adsorption in MOFs.

Micro-thermal conductivity detector (µTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The method was implemented for a 100-µm-long and 1-µm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the µTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the mixture composition. The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 µs of sampling time. This is less than the energy and time required for steady-state DC measurements.

This work highlights a new bonding technique for micro-fabrication of an all silicon 3 meter gas chromatography column that could withstand the temperature cycling required for axial temperature programming. Proper separation of a complex gas mixture using a miniaturized GC column is critical in improving the overall performance of a lab on a chip system for environmental monitoring, medical diagnoses, and gas impurity measurement. To improve upon current methodology the column was first fabricated using micro-fabrication processes; experimentally validated using a high performance 3 meter GC column coated with OV-1 stationary phase. This process demonstrates that the bonding quality of the GC column to a 200 μm thick silicon lid was improved when using a new gold eutectic bonding technique. Furthermore, a new quality control technique was developed in order to test the overall bonding quality of the bonded pieces by fixing the bottom column and applying a mechanical shear force to the top lid. This method could ultimately be used for quality control of each individual bonded columns. The gold bonded interface of the gold diffusion bonded area is further characterized using surface analysis. In the end, the utility of the column was demonstrated by separating 21 organic compounds with different Kovats retention index and molecular weights in less than five minutes.

In this work, the effect of gap size on a TCD sensor performance was studied through a comprehensive COMSOL model to test an optimal geometric design. The platinum cantilever-based thermal conductivity was designed and fabricated to satisfy the demands for the 3-omega technique thermal stresses as well as provide stability for corrosive gases such as ammonia. The fabricated sensor in this process was capable of detecting 30 ppm ammonia gas in helium. The effects of temperature and humidity change were also explored to establish a function to compensate for environmental variations.

Abstract:In this work we present a high performance micro gas chromatograph column with a novel two dimensional axial heating technique for faster and more precise temperature programming, resulting in an improved separation performance. Three different axial resistive heater designs were simulated theoretically on a 3.0 m × 300 μm × 50 μm column for the highest temperature gradient on a 22 by 22 μm column. The best design was then micro-fabricated and evaluated experimentally. The simulation results showed that simultaneous temperature gradients in time and distance along the column are possible by geometric optimization of the heater when using forced convection. The gradients along the column continuously refocused eluting bands, offsetting part of the chromatographic band spreading. The utility of this method was further investigated for a test mixture of three hydrocarbons (hexane, octane, and decane).