Ultrasensitive, fast, thin-film differential scanning calorimeter

Abstract

The equipment for an ultrasensitive, fast, thin-film differential scanning calorimetry [(TDSC) or nanocalorimetry] technique is described. The calorimetric cell (∼0.30 cm2) operates by applying a short (∼10 ms) dc current pulse (∼10 mA) to a thin (∼50 nm) patterned metal strip, which is supported by a thin (∼50 nm) SiNx membrane. The calorimeter operates at high heating rates (15–200 K/ms) and is very sensitive (30 pJ/K). The design of the calorimeter, the timing/synchronization methods, as well as the choice of key components of the instrument are discussed. Comparisons are made between two dc pulsing circuits that generate the current, a battery powered system and a system based on discharge of an assembly of charged capacitors (recommended). Design concepts for the differential as well as a simplified nondifferential technique are discussed and evaluated via experiments on thin films of indium. The differential design shows an increase in sensitivity, making it suitable for small samples. The custom made electronic circuits are also described, including the design of a preamplifier with low (28×) and high (700×) gain options, which are also compared using experimental data. Noise considerations are critical for the method. Simple models which describe noise levels in the calorimetric data are given and methods for reducing noise are discussed in detail. The sources of noise in the instrument are discussed in terms of both fundamental factors such as Johnson noise of the metal strip, as well as the limiting attributes of the sensing and pulsing circuits and instrumentation. These limiting attributes include spurious signals generated by desorption of ambient gases from the sensor, ground loops, switching regulators, and missing codes in analog-to-digital converter instruments. Examples of the experimental data of heat capacity Cp(T) of various thin films of indium, tin, and polystyrene are presented. A complete data set of raw experimental values is included for a 20 nm sample of Sn which shows the values of current and voltage of both the sample and reference sensors, as well as the differential voltage and the final values of the heat capacity.