Infrared-transparent materials

Care has to be taken in selecting materials for the die and punches. Metals are of little use above 1000 °C because they become ductile, and the die bulges under pressure so that the compact can only be extracted by destroying the die. However, zinc sulphide (an infrared-transparent material) has been hot pressed at 700 °C in stainless steel moulds. Special alloys, mostly based on molybdenum, can be used up to 1000 °C at pressures of about 80 MPa (5 ton in-2). Alumina, silicon carbide and silicon nitride can be used up to about 1400 °C at similar pressures and are widely applied in the production of transparent electro-optical ceramics based on lead lanthanum zirconate as discussed in Section 8.2.1. 

Liquid and gas samples do not need much preparation, but special cells to contain the samples are often necessary. The simplest method to prepare a liquid sample is to make a capillary thin film of the liquid. The capillary thin film is made by placing a drop of liquid on a KBr plate and sandwiching it with another KBr plate. This method, however, is not suitable for volatile liquids. Liquid cells can be used for volatile liquid and toxic liquid samples, particularly for quantitative analysis. The spacing between the bottom and the top of liquid cell is typically from 1 to 100 /u.m. The cell is made of an infrared-transparent material. Typically, KBr is used however, KBr should not be selected as the material for holding samples containing water because water dissolves KBr. Instead, ZeSe or AgCl should be used because they are infrared-transparent but not water soluble. Cells for gas samples are structurally similar to cells for liquid but the dimension is much larger. 

Typical electrochemical cells used for ATR-SEIRAS are shown in Fig. 8.4 [28, 29). The cell can be made of either glass or plastics such as Kel-F. The prism works as the cell window as well as the substrate on which a thin-film electrode is deposited. Infrared-transparent materials with high reflective indices such as... 

In ATR, a beam of infrared light is totally reflected inside a specially cut infrared transparent material that has a high index of refraction. Typical materials used for ATR prisms are Ge, Si, and ZnSe. Because the index of refraction differs between the polymer and the prism, an evanescent wave penetrates the polymer if it intimately contacts the prism. The infrared radiation will interact with molecular vibrations in the same manner as in conventional infrared spectroscopy. The amplitude of the evanescent wave decays exponentially from the surface, so the depth of penetration is arbitrarily taken as the point where the amplitude decays to 1/e (37%) of its initial value. The depth of penetration depends on the ratio of the refractive indices between the polymer and the prism, the angle of incidence, and frequency of radiation in the following manner (Ishida, 1987) ... 

The most common means of recording the spectrum of solid samples, the pressed halide disc method, relies on containing the material to be analysed in a self-supporting matrix of infrared transparent material. KBr is by far the most popular salt to use, although other metal halides, such as Csl or KCl, have their own particular advantages. Csl, for instance, transmits down to 180 cm whereas KBr cuts off at 350 cm ... 

Historically, hard tissues were analyzed by FT-IR as homogenized powders in KBr or other infrared transparent material pellets of known sample concentrations. 

Table 11.1. Optical, Mechanical, and Thermal Properties of Selected Infrared-Transparent Materials...

The relative humidity in the room for infrared spectroscopic measurements should be lower than 50%, because, as described in Section 2.3, some infrared transparent materials used for spectral measurements are hygroscopic. 

To measure an infrared absorption spectrum from a liquid or solution sample, it is necessary to use windows (polished plates of certain crystals) or a cell for containing the sample. The material used for the windows must be transparent to the infrared radiation and appropriate for the purpose of the measurement to be performed. Some characteristics of representative infrared transparent materials are given in Table 2.1. More information is available elsewhere [1 -3]. The windows are commercially available usually in the form of polished plates varying in size and thickness. 

The surfaces of infrared-transparent materials that are available in the form of shaped and polished crystals, such as silicon or germanium, can be studied with good sensitivity by using attenuated total internal reflection (ATR) in conjunction with multiple reflection procedures. 

Another issue with liquid water is that it dissolves some of the materials used in IR sample preparation. Materials such as KBr and NaCl are transparent in the mid-infrared and are used to make windows and cells to hold infrared samples. These materials are highly water soluble, aud any liquid water present in a sample will damage these cells or windows. There are infrared transparent materials that are not water soluble that can be used (see Chapter 4), but they tend to be more expensive than KBr and NaCl. 

Potassium bromide (KBr) is the most commonly used infrared transparent material. It is transparent over a broad spectral range, from 400 cm" up through the visible. KBr windows are commonly used in the beamsplitters found in FTIRs, and their low wavenumber cutoff normally determines the low wavenumber cutoff of the FTIR as well. KBr is also relatively cheap and is easy to machine into windows and cells. A major drawback is that it is highly polar, which leads to two problems. First, KBr is hygroscopic, which means it absorbs water from the atmosphere. Over time a thick layer of water can build up on the surface of KBr, masking sample absorbances. Therefore, KBr in all forms—windows, cells, and powder—should... 

Infrared Transparent Materials Commonly Used in Transmission Analysis... 

A sealed liquid cell consists of two infrared transparent windows, which are typically made from KBr or one of the other infrared transparent materials listed in Table 4.1. Two small holes are drilled in one of the windows, as can be seen in Figure 4.23. 

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