Sapphire windows

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Halogen lamp, (2) elliptical reflector, (3) sapphire window welded at the center of the bottom of the high-pressure reactor, (4) the assembled sample, (5) high-pressure reactor. Turquoise dashed line shows the alignment of light beam and sample center. 

Hegedus (23), for the study of adsorbed CO over a limited range of frequencies, was able to use sapphire windows which could be operated at 450°C. His reactor cell thus requires no cooling it operates in transmission. The cell volume is about 22 mL. A flow rate of 250 mL/s was used so that the residence time was only 0.09s. 

The reactor assembly was heated by electric heaters. The maximum operating temperature Is determined by the window construction. Sapphire windows (from EIMAC), brazed into Kovar sleeves, were used the sleeves were then welded directly into the stainless steel reactor housing. We found that the cell so constructed was capable of trouble-free, continuous operation at 450°C operations at somewhat higher temperatures are probably still possible but were not explored. Sapphire was chosen as a window material because it is insensitive to water vapor and is transparent in tljie wave number range of our interest (about 2400 cm to 2000 cm in these experiments). Moreover, the thermal expansion characteristics of the reactor were found to match well with those of the window fixture. 

Figure 1.38 shows the layout of the system. By the use of a very good heat-conducting sapphire window and a cooling system with LN2 the authors reach cooling rates of several hundred degrees per minute down to -60 °C, and temperature gradients in the sample of 0.1 °C at a temperature of approx. 0 °C. 

Figure 6.7 A typical batch reactor used for small-scale laboratory reactions. The view through the cell is shown (a) together with the dismantled cell (b) Here, the screw thread which holds the window in place can be seen, along with the sapphire window. The holes on top of the cell allow it to be connected to the high-pressure system with the relevant adaptors. (Photograph by Dr A. P. Abbott)...

Fig. 7.18 (continued) (c) Light scattering apparatus used to detect polymer-polymer demixing 1 - HeNe laser, 2 - sapphire window, 3 - polymer film, 4 - photodiode array, 5 - copper block, 6 - resistance thermometer, 7 - temperature controlled jacket, (Reproduced with permission from Zywocinski, A., et al. J. Polymer Sci. Polym. Phys. 33, 595 (1995))... 

As discussed in previous chapters, the phase behavior with changing temperature and pressure may be strongly influenced by small concentration gradients in multi-component systems already. Therefore, experimental control should take this into account. It is a common practice to use reactors with glass or sapphire windows. The transition of an inhomogeneous multiphase system to a homogeneous one can be observed visually as cloud point (Sect. 2.2, with the pressure and temperature values being monitored. 

The extmders were monitored using a pair of custom-made transmission probes inserted into the extruder die just downstream from the screws. Each probe consisted of a sapphire window brazed into a metal body a quartz rod behind the window piped the light to the end of the probe low-OH silica fiber-optic bundles connected the probe to the NIR analyzer, an LT Quantum 12001. Optical path lengths were typically between 0.3 and 2.0 cm. 

Reaction cell. 1 Cylindrical body with 80 mm o.d. and 30 mm i.d. 2 Sealing cones. 3 Threaded screws. 4 Sapphire windows. 5 Opening for the introduction of a burner. 6 Openings for a sheathed thermocouple and two capillaries... 

For visual observation of the cell interior through the sapphire windows a lamp mounted behind one end is used. A mirror and stereo microscope at the other end facilitate the observation. The microscope is equipped with a normal camera or a video camera. Normally the phenomena within the cell are continuously observed and controlled with video camera and colour monitor. A video recorder serves for documentation, for inspection of short time processes and for the production of standing flame pictures for size and shape determination. Instead of the microscope a Jarrell-Ash diode array rapid scan spectrometer can be attached to the cell to obtain flame spectra in the visible and UV-regions. 

It can be used under very high pressure and temperatures. Oil reservoirs are found typically at 100°C and 300 atm pressure. The surface tension of such systems can be conveniently studied by using high pressure and temperature cells with optical clear windows (sapphire windows 1 cm thick up to 2000 atm). For example, yof inorganic salts at high temperatures (ca. 1000°C) can be measured using this method. The variation in surface tension can be studied as a function of various parameters (temperature and pressure additives [gas, etc.]). 

The view-cell reactor is made of titanium and has two sapphire windows, a gas inlet valve and an outlet valve, as shown in Figure 3. The view cell is interfaced with a pressure transducer, a thermocouple, and a pressure relief valve. The pressure and temperature are computer-monitored during the reaction. 0.6 ml of 50 wt% H2O2/H2O (10.41 mmoles), 0.20ml of pyridine (2.47 mmoles), or some other base, was dissolved in 5 ml of acetonitrile or methanol, and was added to the reactor. 2.2 ml of supercritical CO2 was charged after lOOmg of propylene (2.38 mmoles) had been added to the reactor. The reactor was heated with a band heater at 40 - 70°C for 3, 6, 12, and 24 hr reaction periods. Following a batch conversion experiment, the amounts of products formed were determined by GC and GC/MS. 

The last ATR cell described here in detail was designed for the study of catalytic reactions at high pressures and in particular in supercritical fluids. A schematic representation of the design is shown in Fig. 17 (76). An important issue in this type of reaction is the phase behavior of the system, which can have a large influence on the catalytic reaction 77,IS). The cell consists of a horizontal stainless-steel cylinder. It is designed to allow monitoring of the phase behavior via a video camera. For this purpose, one end of the cylinder is sealed with a sapphire window, behind... 

First, a simple cell for 200 MPa and 250 °C, with two sapphire windows, is shown. The volume of the cell is approx. 110 cm3, the free diameter of the windows is 16 mm. This cell is suitable for spectroscopic measurements, as well as for phase observation (Fig. 4.3-28) [41],... 

As has been indicated, the sensor can be adapted to almost any process application. The sensor is designed to withstand pressures to 300 psi and can be designed to withstand process temperatures as high as 450° C using sapphire window materials. Accuracies between 0.01 an 0.5 % are typical. Sensitivities to 0.01% have been obtained and are primarily limited by the sophistication of the network analyzer utilized. 

Raman spectroscopy with high pressure windowed cell (Sum et al., 1997 Thieu et al., 2000) P, T and hydrate phase Yes P, P, hydrate phase vs. time (mins) Typically for sapphire window < 10,000 psi (for capillary tubes <60,000 psi diamond anvil cell GPa s) Guest occupancy ratios, structure, structural transitions... 

Experimental. All photodimerizations were carried out in a stainless steel fixed volume cell (1.75 cm ID with a 1.0 cm path length) with sapphire windows under the irradiation of a Hanovia medium pressure mercury lamp filtered through water and Pyrex for a 13.5 hour exposure. The cell and lamp assembly have been described previously (31). For selected runs a custom built 0.9 mL variable-volume pump was connected to the cell and the pressure was varied to determine the exact location of the phase boundary, based on light scattering measured in a Cary 2290 UV-Vis spectrophotometer (Varian Inst.). The spectrophotometer was also used to measure the concentrations of the monomeric cyclohexenone before and after reaction. 

The gray values in Fig. 10 and in Fig. 11 are 2D projections into the. rv-plane. Because of the phase contrast technique, they are approximately linear functions of the integral over the refractive index along the z-direction. The temperature and concentration distribution and, hence, also the refractive index are fully 3D objects. The high thermal conductivity of the sapphire windows enforces a constant temperature boundary condition at the top and bottom windows. 

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