Melting temperature sapphire

The Verneuil, or flame-fusion, method is illustrated in Figure 29.1. It is a well-established technique for growing single crystals of oxides that have high melting temperatures. The largest application of the Verneuil method is for the growth of sapphire and ruby. 

The elements of an infrared melt temperature sensor are a sapphire window, an optical fiber, and a radiation sensor with associated signal-conditioning electronics as shown in Fig. 4.17. IR melt temperature probes are commercially available [85, 86] and fit in standard pressure transducer mounting holes. Because the sapphire window is flush with the barrel or die, the sensor does not protrude into the polymer melt. As a result, the sensor is less susceptible to damage, there is no chance of dead spots behind the sensor, and the melt velocities are not altered around the sensor. When melt velocities are changed, the melt temperatures will change as well. Therefore, the melt temperatures measured with an IR sensor are less affected by the actual measurement than with an immersion sensor. 

Besides applications in optical fiber beam delivery, single crystal fibers also find potential uses in fiber-based sensors. In applications where sensor must operate in harsh environment, the optical property of the fiber materials is not the only consideration. High melting temperature, chemical inertness, and mechanical strength often dictate the choice of fiber materials. Sapphire is one example of a single crystal that posses an unusual combination of these properties. 

Sapphire fibers have good physical properties. Sapphire is a strong material. It is chemically stable, not soluble in water, has high melting temperature and it is biocompatible. Sapphire fibers are transparent up to 3.5pm [21,22]. 

The melting transition of ultra-pure metals is usually used for calibration of DSC instruments. Metals such as indium, lead, and zinc are useful and cover the usual temperature range of interest. Calibration of DSC instruments can be extended to temperatures other than the melting points of the standard materials applied through the recording of specific heat capacity of a standard material (e.g., sapphire) over the temperature range of interest. Several procedures for the performance of a DSC experiment and the calibration of the equipment are available [84-86]. A typical sensitivity of DSC apparatus is approximately 1 to 20 W/kg [15, 87]. 

Alumina is a widespread component of siliceous minerals. It occurs as single crystals in the form of sapphire, and with chromium impurity as ruby, and in large deposits as the hydrated oxide bauxite (A1203-2H20). The dehydration of this and other hydrated oxides at temperatures below 1000°C leads to the formation of y-Al203 which is converted to a-Al203 above 1000 °C. The transformation is irreversible and the a-polymorph is stable from absolute zero to its melting point at 2050 °C. 

Figure 1. Schematic diagram of furnace used to pull lithium niobate crystals 1, alumina pull rod 2, after heater 3, temperature control signal to auxiliary controller 4, Pt seed holder 5, LiNbOj crystal 6, LiNbOs melt 7, Pt crucible 8, alumina plate 9, rf coil 10, alumina tubes 11, firebricks 12, sapphire rod-radiation pyrometer 13, temperature signal to main controller 14, Fiberfrax insulation. (Courtesy W. L. Kway.)...

MgO is has cubic crystal structure, as shown in Fig. 2.10. MgO ceramics have excellent thermal and mechanical properties with a high melting point of 2850 °C and low density of 3.58 g cm. It has an isotropic cubic crystal strucmre, which meets one of the requirements to be transparent. PolycrystaUine infrared-transparent MgO is a potential substitute for sapphire IR windows and protectors for sensors. Due to the high sintering temperature required for full densification of MgO, fabrication of transparent MgO ceramics is stiU a challenge. Almost aU techniques discussed above have been used to prepare transparent MgO ceramics [3]. 

We investigated the PVC compounds with an NIR in-line diffuse reflectance probe on the end of a twin screw extruder (Viskosystem, Reifenhaeuser). The probe was connected via optical fiber cables to the NIR spectrometer. On the end of the extruder we adapted a melt-at-die interface for different probes. We used the diffuse reflectance fiber optic probe FDR 650 (Axiom Analytical Inc., USA) with a sapphire-window. The pressure in the melt-at-die interface varied in the 100 to 270 bar (1500 to 4000 psi) range and the processing temperature was 190 °C. 

The next calibration concerns the area of the DSC trace or the amplitude at any one temperature. The peak area below the baseline in Fig. 4.62 can be compared with the melting peaks of standard materials such as the benzoic acid, urea, indium, or anthracene, listed at the bottom of the figure. The amplitudes measured from the baseline established in the heat-capacity mode of measurement are usually compared with the heat capacity of standard aluminum oxide in the form of sapphire. The heat capacity of sapphire is free of transitions over a wide temperature range and has been... 

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