Titanium vapor pressure, high temperature

Many elements evaporate, but many such as chromium (Cr), cadmium (Cd), magnesium (Mg), arsenic (As), and carbon (C) sublime, and many others such as antimony (Sb), selenium (Se), and titanium (Ti), are on the borderline between evaporation and sublimation. For example, chromium, which has a vapor pressure of 10 Torr 600°C below its melting point, is generally vaporized by sublimation. Carbon cannot be melted except under high hydrostatic pressure. Materials such as aluminum, tin, gallium, and lead have very low vapor pressures at temperatures above the points at which they are just-molten. For example, tin has a vapor pressure of lO Torr 1000°C above its melting point. Aluminum and lead have vapor pressures of 10 Torr at about 500°C above their melting points. 

Another candidate material for high temperature fiber is titanium diboride. It has a melting point of around 3000°C. Diefendorf and Mazlout (1994) used a gas mixture of titanium tetrachloride boron trichloride, hydrogen, and hydrochloride to make titanium diboride fibers by chemical vapor deposition (CVD) in a cold wall reactor at atmospheric pressure. 

Let us consider a somewhat unusual application of this type of instrument. In the manufacture of titanium it is important to keep liquid TiC at a particular high temperature and pressure in a vessel. TiC has a triple point (i.e. the pressure and temperature conditions at which all three phases - solid, liquid, and vapor - of a substance are at equilibrium) in the neighborhood of the particular technical conditions. By using a y-density gauge it is possible to detect when the triple point is exceeded because of the disappearance of the vapor-liquid interface. This allows a simple method of control of the process conditions in the vessel. 

At 10 torr, a vapor pressure of 10 torr is required to produce an acceptable evaporation rate. For most metals, this temperature is in excess of 1000°C. The refractory metals, or metals with a high melting point such as tungsten, titanium, or molybdenum, are frequently used as carriers, or boats, to hold other metals during the evaporation process. To prevent reactions with the metals being evaporated, the boats may be coated with alumina or other ceramic materials. 

Chemical and physical vapor deposition technique has been widely applied for the preparation of such photocatalytic thin films. Since these vapor methods need an instrumental setup which enables control of temperature and pressure, their initial and running costs are generally high and the size of substrate is limited. Spray method, in which titanium alkoxide and water is sprayed on a substrate heated at a desired temperature, affords Ti02 thin films.69) However, like the sol-gel route, the physical properties and photocatalytic activity of Ti02 strongly depend on many factors such as temperature of substrate, flow rate of carrier gas, and partial pressure of starting material in the system. 

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