Window materials diamond

A critical component of the cell is the X-ray-transparent window that allows the X-ray beam to impinge on the sample and the transmitted or fluorescent X-rays to be detected. Typical window materials that have been used are polyimide (Kapton ), beryllium, quartz, diamond, polyester (Mylar ), and titanium. Table 3 shows estimates of the thicknesses of window materials for various X-ray energies from 5 to 25 keV, determined on the basis of the assumption that 25% of the X-rays are absorbed by the window material. 

Due to its transparency to a wide range of wavelengths from the infrared to the ultraviolet region, as well as to x-rays, combined with its radiation hardness and mechanical sturdiness, diamond is an ideal choice as a window material for lasers, x-ray sources, etc. A systematic evaluation of properties such as IR absorption, elastic properties, mechanical strength and thermal properties has been carried out to reveal that CVD diamond has significant... 

D. C. Harris, Diamond the ultimate durable infrared window material. Naval Res. Rev., 3 3-16 (1992)... 

In this section, applications will be discussed which illustrate the versatility and advantages of CVD diamond as an infrared and multi-spectral window material. We describe the use of CVD diamond optical elements including CVD diamond domes and flat plates as windows for IR seekers or imaging systems in high-speed flight or other mechanically aggressive environments. Then we describe the use of CVD diamond windows for the transmission of high-power IR laser beams. 

Table 2 Property comparison between high quality CVD diamond and alternative window materials that transmit in the long wave (8-14 pm) thermal imaging region...

Measurements of dielectric loss in high quality CVD diamond at millimetre wave microwave frequencies, reported in 1993 [2], furnished for the first time clear evidence that the loss tangent of CVD diamond could be comparable or lower than conventional dielectrics such as sapphire or boron nitride. This initial data and subsequent first dedicated dielectric property studies [61] intensified the considerable amount of interest that had already existed in the nuclear fusion community for CVD diamond as a high power window material [62] especially for the development... 

The detailed requirements for the use of diamond as a window material in high power Gyrotron tubes can be listed as follows. 

Compression cell Allows two sample windows with flat surfaces to be squeezed together for the flattening of samples to make them transparent or to flatten them for easier transmittance measurement. KBr and diamond are common window materials. 

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. 

X-ray transmission of diamond is excellent by virtue of its low atomic number and, in thin sections, it even allows the transmission of characteristic x-rays generated by low-energy elements such as boron, carbon, and oxygen. In this respect, it compares favorably with the standard x-ray window material beryllium. The x-ray transmission of a 0.5 mm-thick diamond of the characteristic radiation of a series of elements is shown in Fig.11.14.120]... 

Diamond for X-ray Windows. CVD diamond is an excellent material for windows of x-ray spectrometers. In thin cross section, it is transparent to the x-rays generated by low-energy elements such as boron, carbon, and oxygen (see Ch. 11, Sec. 7.0, and Fig. 11.14) and is superior to beryllium which is the present standard material. Such a window is now commercially available and is shown schematically in Fig. 13.11 It is leak-tight to helium and capable of withstanding at least one atmosphere differential pressure. 

Figure 5 Raman spectra obtained from three of the four common window materials (a) fused silica, (b) quartz, and (c) diamond. Note that the spectrum of the fourth popular window, sapphire, is shown in Fig. 7.

A few years ago, diamond was considered as an exotic, rather expensive material that was only used as a window material in high-pressure cells (see Section 14.3). Now synthetic diamonds are becoming more available, and diamond can also be used as a coating for less chemically resistant materials with and far lower cost and superior infrared transparency. Diamond has a characteristic doublet absorbing between about 1900 and 2300 cm , but fortunately, few functional groups absorb in this region. 

The window materials that can be used in the far-infrared region are limited (see Table 2.1). A practically useful material not listed in Table 2.1 is spectroscopic-grade polyethylene high-density polyethylene plates of a few millimeters thickness can be used as cell windows in the far-infrared region. Polyethylene has two weak bands in the vicinity of 100 cm but these bands do not seriously disturb measurements of far-infrared spectra. Quartz (z-cut) may be used as windows in the low-wavenumber region (below about 250 cm ). Synthetic diamond and sapphire crystal windows may also be used. 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

Infrared optics is a fast growing area in which CVD plays a maj or role, particularly in the manufacture of optical IR windows. 1 The earths atmosphere absorbs much of the infrared radiation but possesses three important bandpasses (wavelengths where the transmission is high) at 1-3 im, 3-5 im and 8-17 pm. As shown in Table 16.2, only three materials can transmit in all these three bandpasses single crystal diamond, germanium, and zinc selenide. 

Single-crystal diamond is the ideal material with remarkable optical properties, high heat resistance, extreme hardness, and excellent chemical resistance. But, because of its high cost and size limitation, it is only used in exceptional cases, such as the window... 

Materials such as cubic zirconia, diamond and sapphire may be used for transmission windows for special applications. 

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