Electrical and structural stability of highly strain-sensitive metal-carbon thin films

金属-碳薄膜(Me：A-C：H或Me：A-C)对应变的敏感度(量规系数高达30)比通常用于传统应变计的材料(量规系数为2)高15倍。电阻的应变灵敏度描述了作为施加应变的函数的线性行为，此外，电阻的温度系数可以根据测试体的材料进行调整。因此，Me：A-C：H薄膜有可能作为应变计的敏感元件。它们对于需要检测小菌株的应用特别有意义。当使用镍作为金属(Ni：A-C：H)时，获得了最高的应变灵敏度(量规系数为30)。然而，对于这种薄膜类型，我们注意到随着温度的升高，本征蠕变增加：即在变形后，在恒定应变下，电阻随时间发生系统变化。此外，更高温度下的电阻在没有负载的情况下漂移。这些不稳定性限制了Ni：A-C：H在传感器中的应用。当镍被铬(NiCr：A-C：H)部分取代时，不稳定性降低。遗憾的是，NiCr：A-C：H的应变灵敏度仅为传统应变计的5倍。透射电子显微镜对Ni：A-C：H薄膜的研究表明，薄膜是由嵌入在石墨碳洋葱壳结构中的纳米级金属颗粒和介于这些结构之间的非晶态碳区组成的。首先，我们注意到了Ni：A-C：H和NiCr：A-C：H薄膜的形貌差异：在NiCr：A-C：H薄膜中没有观察到石墨碳洋葱壳。我们的目标是确定导致Ni：A-C：H高应变敏感性和不稳定性的薄膜结构和工艺，并通过保持高应变敏感性来降低高温下的蠕变速率。作为工作假设，我们假设石墨碳网络中的滑动过程是导致观察到的电阻变化的原因。我们将首先研究铬是如何改变薄膜的生长和机械-电学性能的。我们将利用原位退火法、电子显微镜和X射线衍射仪等实验手段来识别热致扩散和重排过程。我们计划对金属碳薄膜进行建模和蒙特卡罗模拟，以研究其导电性和形变的影响。设想了不同的方法来修改薄膜组合物，以使石墨碳网络变硬并防止滑动过程。

Metal-carbon thin films (Me:a-C:H or Me:a-C) are able to react up to 15 times more sensitive on strain (gauge factor up to 30) than materials which are normally used in conventional strain gauges (gauge factor of 2). The strain sensitivity of the electrical resistance depicts a linear behavior as a function of the applied strain, furthermore the temperature coefficient of resistance can be adjusted to the material of the test body. Hence Me:a-C:H thin films provide the potential to be used as the sensitive element of strain gauges. They are particularly of interest for applications in which small strains need to be detected. The highest strain sensitivities (gauge factor of 30) are obtained when nickel is used as the metal (Ni:a-C:H). However, with this thin film type we noted an increasing intrinsic creep when temperature is increased: i.e. after deformation a systematic change in the electrical resistance occurs with time at constant strain. In addition, the electrical resistance at higher temperatures is drifting without load. Those instabilities limit the application of Ni:a-C:H in sensors. Instabilities are reduced when nickel is partially replaced by chromium (NiCr:a-C:H). Unfortunately the strain sensitivity of NiCr:a-C:H is only 5 times higher than that of conventional strain gauges. Transmission electron microscopy (TEM) investigations of Ni:a-C:H have shown that the films are constituted of nanoscaled metallic particles, which are embedded in structures of graphitic carbon onion shells, and of regions of amorphous carbon between these structures. First differences in the morphology of Ni:a-C:H and NiCr:a-C:H were noted: no graphitic carbon onion shells were observed in NiCr:a-C:H thin films. We aim to identify the film structures and processes, responsible for the high strain sensitivity and instabilities of Ni:a-C:H, and to decrease the creep rate at higher temperatures by preserving the high strain sensitivity. As a working hypothesis we assume that gliding processes in the graphitic carbon network are responsible for the observed changes of resistance. We will first investigate, how chromium is modifying the film growth and the mechanical-electrical properties. In situ annealing TEM and x-ray-diffraction experiments will be used to identify thermal induced diffusion and rearrangement processes. Modelling the metal carbon thin film and Monte Carlo simulations are planned to investigate the electrical conductivity and the implications of deformation. Different approaches are envisaged to modify the thin film composition in order to harden the graphitic carbon network and to prevent gliding processes.