Arndt Nicolai, Bolwien Carsten, Sulz Gerd, Kühnemann Frank, Lambrecht Armin
Fraunhofer IPM, Georges-Köhler-Allee 301, D-79110 Freiburg, Germany.
Sensors (Basel). 2021 Sep 27;21(19):6442. doi: 10.3390/s21196442.
Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids or solid samples. However, ATR spectroscopy is rarely found in industrial processes, where inline measurement, continuous operation, and minimal maintenance are important issues. Most materials for mid-infrared (MIR) spectroscopy and specifically for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, and aggressive cleaning agents. Sapphire is the usual choice for infrared wavelengths below 5 µm, and beyond that, only diamond is an established material. The use of diamond coatings on other ATR materials such as silicon will increase the stability of the sensor and will enable the use of larger ATR elements with increased sensitivity at lower cost for wavelengths above 5 µm. Theoretical and experimental investigations of the dependence of ATR absorbances on the incidence angle and thickness of nanocrystalline diamond (NCD) coatings on silicon were performed. By optimizing the coating thickness, a substantial amplification of the ATR absorbance can be achieved compared to an uncoated silicon element. Using a compact FTIR instrument, ATR spectra of water, acetonitrile, and propylene carbonate were measured with planar ATR elements made of coated and uncoated silicon. Compared to sapphire, the long wavelength extreme of the spectral range is extended to approximately 8 μm. With effectively nine ATR reflections, the sensitivity is expected to exceed the performance of typical diamond tip probes.
红外衰减全反射（ATR）光谱法是分析高吸收性液体或固体样品的常用实验室技术。然而，ATR光谱法在工业过程中很少见，在工业过程中，在线测量、连续运行和最小维护是重要问题。大多数用于中红外（MIR）光谱法，特别是用于ATR元件的材料，要么没有足够高的红外透射率，要么没有足够的机械和化学稳定性来暴露于过程流体、磨蚀性成分和腐蚀性清洁剂中。对于低于5μm的红外波长，蓝宝石是通常的选择，除此之外，只有金刚石是一种成熟的材料。在其他ATR材料（如硅）上使用金刚石涂层将提高传感器的稳定性，并将能够使用更大的ATR元件，在高于5μm的波长下以更低的成本提高灵敏度。对ATR吸光度与纳米晶金刚石（NCD）涂层的入射角和厚度之间的关系进行了理论和实验研究。通过优化涂层厚度，与未涂层的硅元件相比，可以实现ATR吸光度的大幅放大。使用紧凑型傅里叶变换红外光谱仪，用涂覆和未涂覆硅制成的平面ATR元件测量了水、乙腈和碳酸丙烯酯的ATR光谱。与蓝宝石相比，光谱范围的长波长极限扩展到约8μm。通过有效进行九次ATR反射，预计灵敏度将超过典型金刚石尖端探头的性能。
