Micro-differential scanning calorimeter for combustible gas sensing

A micron-scale differential scanning calorimeter (muDSC) has been produced on a silicon chip allowing for microscopic differential scanning calorimetry (DSC) measurements on small samples. The device consists of a suspended rectangular microhotplate with sample and reference zones at either end, each with a polysilicon microheater for temperature control. The temperature difference between the two zones is measured with a thermopile consisting of a series of successive polysilicon/metal junctions which alternate between the two zones. In a scanning differential calorimetry measurement, the two elements are heated simultaneously with a ramped temperature profile. A thermal process zone is defined on one of the elements, for example, a catalyst for chemical sensing, a material which exhibits a phase transition, or a chemically selective reactive material. When temperature is scanned the loss or gain of heat associated with the reaction or phase transition on the sample zone produces a difference signal on the thermopile. The device has a temperature range from 20 to 600 degreesC, and can be heated to that temperature in as little as 40 mus, while the cooling time constant is 5 ms. Thermal imaging was used to characterize heat flow across the device in response to a 40 mus voltage pulse applied to one side. At 4 ms after the pulse the heat distribution has become largely uniform across the device, showing that scans shorter than this time-scale will minimize the effects of heat loss from the sample to the reference zone. An example application shows the response to varying concentrations of methanol, ethanol, acetone, benzene, and hydrogen in air, when operated with periodic ramps to 570 degreesC of duration 3.5 s. The thermopile responds with a periodic waveform which is different for different gases, making the use of pattern recognition analytical methods for gas identification possible. (C) 2003 Elsevier B.V. All rights reserved.