Diamond working window

The current standard material for optical elements in CO2 lasers is ZnSe [27] because of its very low intrinsic absorption at 10.6 pm [14,15,34]. Table 3 compares the critical parameters of CVD diamond and ZnSe showing that CVD diamond, for the reasons discussed above, has the potential to handle much greater beam powers. This has been long recognized [51] and the thermal effects in diamond laser windows have been theoretically modelled in previous work [27,51-53]. Some of the earlier results [51-53] were derived before reliable data were available on the properties of polycrystalline diamond and were therefore very speculative. The following is an up-date of the predicted thermal effects in optical grade CVD diamond and ZnSe windows where some of the earlier calculations are revisited. 

The opposed anvil cell consists of two optical anvils and a gasket, located between the parallel faces of the two opposing anvils. Samples are placed in the hole of the gasket and are pressurized when the opposed anvils are pushed towards each other. The most common material for anvils is diamond. For mid and far infrared spectra, type Ila diamonds are used, while low-fluorescent type la diamonds are used for Raman spectroscopic measurements [5]. We have also devised a glass anvil cell for Raman spectroscopic measurements [6], and a calcium fluoride anvil cell for infrared spectroscopic measurements [7] with attainable working pressures of 13 and 6 kbar, respectively. Diagrams, for the interested reader, of the window and opposed anvil cells can be found in reference 1. 

To conclude with the primary electrode characteristics, we describe briefly the DLC electrodes. The data are scarce and partly contradictory, probably due to the differences in film preparation methods. According to Howe [60], even films as thin as 50 nm are quite stable against corrosion. However, in later works [61, 62] such thin films turned permeable for electrolytes. The penetration of the electrolyte to a substrate metal resulted in its corrosion and, ultimately, in film peeling. Thicker films (0.1 to 1 pm) were less subjected to damage. The current-potential curves in supporting electrolytes resemble those for crystalline diamond electrodes (see Figs. 7, 8) the potential window is narrower, however [63], Fluorination of a-C H enhances corrosion resistance of the films significantly [64],... 

In the 1990s, research works had evidenced that diamond was an exceptional material. It shows high overpotentials for water electrolysis (hydrogen and oxygen evolution) that offers an electrochemical window which has never been observed with other materials. The wideness of this window depends on the diamond purity and doping level. 

Electrically conductive diamond electrodes possess several properties that clearly distinguish them from conventional sp carbon electrodes, like GC, and make them attractive for electrochemical use [2-5, 21] (1) background current densities approximately 5-10 times lower than freshly polished GC leading to enhanced S/B ratios, (2) a working potential window of 3-4 V in aqueous media, which is over 1 V wider than GC, (3) superb response... 

The importance of the large working potential window for diamond is that electrochemical reactions can be studied over a much wider potential range than is possible with other carbon electrodes. Similarly, wide working potential windows are also observed in nonaqueous media (e.g., TBACIO4/CH3CN) [108]. 

The use of electrically conductive diamond as an optically transparent electrode is a new field of research [50,52,117,118]. Diamond possesses attractive qualities as both an electrode and an optically transparent material, making it an obvious choice for use as an OTE in spectroelectro-chemical measurements. Diamond OTEs exhibit several technologically useful properties (1) the possibility of transmission measurements from the near-UV to the far-IR (0.225-100 pm) (2) low background current (3) wide working potential window (4) good responsiveness for many... 

Window materials for microscopy Typical window materials for microscopy include barium fluoride (BaF2) for use with polar solvents (including water), potassium bromide (KBr) for solids and nonpolar solvent use, zinc selenide (ZnSe) with its high refractive index for use in diamond cell filler (background measurement), and diamond for compression cell work in which higher pressures are required. 

Electrically conducting diamond is a new type of carlxm electrode material that is beginning to find widespread use in electroanalysis (86-88). The material possesses properties superior to other forms of carbon that include (i) low and stable background current over a wide potential range, (ii) wide working potential window in aqueous media, (iii) relatively rapid electron-transfer kinetics for several redox systems without conventional pretreatment, (iv) weak molecular adsorption, (v) dimensional stability and corrosion resistance, and (vi) optical transparency. The material is now available from several commercial sources and is not overly expensive, as commonly perceived (89). 

In the past decade or so, extensive work by Diamond and co-workers [312-317] has shown that exposed outdoor surfaces in urban areas rapidly become coated with a complex mixture of chemical compounds ( urban sxuface film ), most readily encountered as window grime. This film grows via accretion from the atmosphere and is removed by rain wash-off, or revolatilization processes, yielding an (estimated) steady-state thickness of several tens to hundreds of nanometers. Chemical analysis of these films has been carried out both in a broad brush approach [312, 313], which identified the compound classes present, and by more detailed studies [315-317] that determined the specific compounds within these classes. 

In recent years, designs for UMEs and UME arrays continue to evolve. For example, works based on microfabricated diamond UMEs and arrays are increasingly common, motivated by this material s attractive properties as an electrode that include mechanical stability, chemical inertness, low background currents, wide potential window, and resistance to electrode fouling. Individual electrodes fabricated with focused ion beam (E1B) ° and arrays fabricated with thin-film technol-ogy2i,22 gj.g demonstrated. 

Fig. 6.1 shows the cyclic voltammograms (CVs) obtained for an as-deposited diamond electrode in several non-aqueous electrolytes and an aqueous electrolyte (l M H2SO4). In this study, the working potential window is defined as the range between the potentials at which the anodic and cathodic current densities reach 2 mA cm 2 [1] The non-aqueous electrolytes examined exhibit potential windows (Fig. 6.1 (a)-(e)) that are 1.5-2.5 times wider than that obtained in the aqueous acid electrolyte (Fig. 6.1 (0). In aqueous electrolytes, the potential window observed for diamond electrodes is wider than that for the other carbon-based electrodes. In non-aqueous electrolytes, however, those values are very similar to those reported for the other carbon-based electrodes [4].

Cyclic voltammetry - Because the advantage of diamond in the double-layer capacitor application is its wide working potential window, we have examined the current-potential behavior for the honeycomb films (Figure 19.2A). Interestingly, the working potential window for the honeycomb films remained essentially the same as that for the as-deposited film, even after extended oxygen plasma treatment. 

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