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）对应变的反应灵敏度（应变系数高达30）比常规应变计中通常使用的材料（应变系数为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薄膜具有作为应变片敏感元件的潜力。它们对于需要检测小菌株的应用特别感兴趣。当使用镍作为金属（Ni：a-C：H）时，获得最高的应变灵敏度（应变计因子为30）。然而，对于这种薄膜类型，我们注意到当温度增加时增加的固有蠕变：即在变形后，在恒定应变下，电阻随时间发生系统性变化。此外，较高温度下的电阻在无负载的情况下漂移。这些不稳定性限制了Ni：a-C：H在传感器中的应用。当镍部分被铬（NiCr：a-C：H）取代时，不稳定性降低。不幸的是，NiCr：a-C：H的应变灵敏度仅为传统应变片的5倍。Ni：a-C：H的透射电子显微镜（TEM）研究表明，薄膜是由纳米级的金属颗粒组成的，这些颗粒嵌入在石墨碳洋葱壳的结构中，以及这些结构之间的非晶碳区域。注意到Ni：a-C：H和NiCr：a-C：H的形貌的第一差异：在NiCr：a-C：H薄膜中没有观察到石墨碳洋葱壳。我们的目标是确定膜结构和过程，负责高应变灵敏度和不稳定性的Ni：a-C：H，并通过保持高应变灵敏度降低蠕变速率在较高的温度。作为一个工作的假设，我们假设在石墨碳网络的滑动过程是负责所观察到的电阻的变化。我们将首先调查，铬是如何修改的薄膜生长和机电性能。原位退火TEM和X射线衍射实验将用于确定热诱导扩散和重排过程。金属碳薄膜的建模和蒙特卡罗模拟计划调查的导电性和变形的影响。设想不同的方法来改变薄膜组成，以便硬化石墨碳网络并防止滑动过程。

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.