Tagliati S, Krasnov V M, Rydh A
Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden.
Rev Sci Instrum. 2012 May;83(5):055107. doi: 10.1063/1.4717676.
A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-μg in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1 K. The device has several distinctive features: (i) The resistive thermometer, made of a Ge(1 - x)Au(x) alloy, displays a high dimensionless sensitivity ∣dlnR∕dlnT∣ ≳ 1 over the entire temperature range. (ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. (iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. (iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 μg piece of superconducting Pb in various magnetic fields.
本文描述了一种基于差分膜的纳米量热计，用于对质量范围从0.5毫克到亚微克的非常小的样品进行一般比热研究。该量热计在从室温以上到0.5 K的温度范围内运行。它由一对单元组成，每个单元是位于氮化硅膜中心的加热器和温度计的堆叠，在300 K时总的背景热容量小于100 nJ/K，在1 K时降至10 pJ/K。该装置具有几个显著特点：（i）由Ge(1 - x)Au(x)合金制成的电阻温度计在整个温度范围内显示出高的无量纲灵敏度∣dlnR∕dlnT∣≳1。（ii）样品直接与温度计接触，温度计可自热。因此，温度计可以在高直流电流下运行以提高分辨率。（iii）数据通过一组八个同步锁相放大器采集，测量加热器和温度计的直流、基波和二次谐波信号。这提供了高分辨率，并允许在无额外噪声的情况下进行连续输出调整。（iv）通过变频固定相位技术实现绝对精度，在测量过程中自动调整测量频率，以考虑样品热容量的温度变化和装置的热导率。通过研究小金样品的热容量和2.6微克超导铅片在各种磁场中的比热，展示了该量热计的性能。
