Sapphire melting point

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]. 

The laser system consisted of a home-built Ti sapphire fs laser oscillator and regenerative amplifier (RGA). The pulse duration was 50 fs at 800 nm and 1 kHz repetition rate. The output of the RGA was split into two parts. One part was used as pump pulse. The other part served as a source for the generation of probe pulses with the help of a non-collinear optical parametric amplifier (NOPA, Clark). The sample preparation was explained elsewhere [7]. Briefly, sodium (Alfa Aesar) was used as received and sodium bromide (Alfa Aesar) was dried and re-crystallized under vacuum. The preparation of the samples was carried out in a glovebox under argon atmosphere. Localized electrons were generated by heating the metal-salt mixture to 800 °C, i.e. well above the melting point of the salt. 

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. 

The solid is placed in a hopper, heated with an oxy-hydrogen torch and allowed to drip on to a seed crystal. This method is often used to grow synthetic gemstones, such as sapphires, because the other methods are not possible owing to the very high melting point of the material. 

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]. 

In the flame fusion method (Verneuil), the powder falls onto an O2-H2 flame and the melt drops on a seed crystal, which is slowly lowered as the crystal grows. The method has been applied to grow high-melting-point oxides (ruby and sapphire). The starting powder is usually placed on a gently hopped mesh to get a continuous flow of solid particles of the same size. 

Single crystal High melting point (sapphire). Low theoretical attenuation. Some of the materials are sensitive to UV radiation. Some of the materials are poisonous. Hard to manufacture. Brittle Soluble in water. Impurities cause scattering... 

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