MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 77
capacity. Thermal measurements at higher temperatures T>100K are typically
only used to study phase transitions.
3. 3. 1 Measurement
The most straightforward method of measuring heat capacity is adiabatic
calorimetry. Here, the heat capacity C=∆Q/∆T is measured directly by applying
a known ∆Q and measuring ∆T. This technique requires that the thermal
relaxation time constant between the sample and its surroundings be large
compared to the measurement time. In a common Differential Scanning
Calorimeter (DSC) or Differential Thermal Analyzer (DTA) the loss of heat
due to thermal relaxation is accounted for by subtracting the signal of an
empty sample holder measured at the same rate. A DSC actually operates by
recording the heat input required ∆Q /dt to keep the temperature changing at
a constant rate ∆T/dt = constant. The DTA supplies a constant ∆Q and
measures ∆T between the sample and a reference. These commercial
instruments are used mostly to detect and estimate the entropy associated
with a phase transition. Very accurate measurements require a more
complicated apparatus with better thermal isolation.
The relaxation technique and ac methods for measuring heat capacity are
preferred when measuring a small ∆Q or ∆T. The heat ∆Q is actually
calculated in the relaxation technique by measuring the thermal time
constant of the system. The sample is attached to an object with high thermal
mass and known temperature (the surroundings) by means of a weak
thermal link. After heating the sample, the temperature of the sample
exponentially decays to that of the surroundings. The heat capacity is then
the product of the time constant of this decay and the thermal conductance of
the weak link. This thermal conductivity can be measured in the steady state
by applying a fixed power and measuring the resulting temperature of the
sample.