Heat capacities, standard entropies, and magnetic phase transitions in iron-bearing mantle minerals  are poorly known because conventional adiabatic calorimetry requires samples too large to be synthesized at very high pressure. Specific heat measurements on microgram sized samples are based on a Si-micromachined calorimetry device. These devices have been in use for a decade in the physics and materials science community for measurements on metals and on selected oxides such as CoO. These calorimeters on a chip have been used for both thin films and small bulk samples.  Different designs have been optimized for different uses, but the heart of the device is a thin (1800 Å) 0.5 x 0.5 cm2 amorphous Si3N4 membrane supported by a 1 x 1 cm2 Si frame.  On one side of this membrane, we deposit and pattern thin film heater, thermometers, and electrical leads of appropriate resistance and temperature coefficient.  On the other side, in a 0.25 x 0.25 cm2 area at the center, we deposit the sample and a thermally conducting material such as gold or copper.  This thin membrane provides the necessary thermal isolation of sample from environment, while still providing a sample/thermometer platform.  On the frame are matching thermometers to those on the membrane to permit a high sensitivity differential temperature measurement.  We have made thousands of these devices and have measured hundreds.  The devices are metallurgically stable and physically robust under cycling between 4.2 K and 1000 K, and can withstand photolithographic processing.  Because of the nature of the fabrication process, reproducibility of specific heat addenda and of thermal link between sample and environment is very good, varying from device to device by less than 5%.  Samples are measured using the relaxation method, requiring a separate determination of the thermal link between sample and environment (the steady state increase of the sample temperature with the sample heater turned on) and the time constant of the relaxation of the sample temperature after turning off the sample heater. 

Using these devices we have measured thin film samples less than 1000 Å thick (weighing < 10 mg) below 20 K and 1000-5000 Å thick (weighing 10-50 mg) up to 525 K in magnetic fields from 0 to 8T to date to approximately 2% absolute accuracy. In most cases, the limit on accuracy is the uncertainty in the thickness of the thin film sample.  Relative measurements such as at a phase transition are made with precision better than 1%. We have also successfully measured small single crystal or other bulk samples (200-500 mg) thermally anchored to the device by conducting grease, In, or Ga, and powder samples dissolved in a solvent and dropped onto devices. 

Picture of microcalorimeter (“calorimeter-on-a-chip”)

We are exploring this methodology for application to high pressure samples synthesized in the multianvil or even in the diamond anvil cell. Our initial goal is to reproduce the known heat capacity and magnetic transition data in Fe₂SiO₄ fayalite and to make the first measurements on its spinel polymorph. Preliminary results are presented.

[ return to abstract list ]