Hot-filament

Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament. 

Positive ions are obtained from a sample by placing it in contact with the filament, which can be done by directing a gas or vapor over the hot filament but usually the sample is placed directly onto a cold filament, which is then inserted into the instrument and heated. The positive ions are accelerated from the filament by a negative electrode and then passed into a mass analyzer, where their m/z values are measured (Figure 7.1). The use of a suppressor grid in the ion source assembly reduces background ion effects to a very low level. Many types of mass analyzer could be used, but since very high resolutions are normally not needed and the masses involved are quite low, the mass analyzer can be a simple quadrupole. 

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

Dispersion-strengthened copper is made by dispersing a thoria or alumina phase through copper powder. The resulting P/M product retains its strength at elevated temperatures. It is used, for example, as the conductor or lead wine that supports the hot filament inside incandescent lamps. 

Rhenium exhibits a greater resistance than tungsten to the water cycle effect, in which lamps and electron tubes become blackened by deposition of metal. This phenomenon involves catalysis by small quantities of water that react with the metal in a hot filament to produce a volatile metal oxide and hydrogen. The oxide condenses on the surface of the bulb and is reduced back to the metal by hydrogen. 

In the microwave-assisted or hot-filament-assisted CVD of diamond, methane and hydrogen gases (CH ca 1—5% and 95—99%) are used. In... 

In practice, it is often necessary to take readings from hot-filament ionization gauges or other devices. Figure 5 gives pump-down curves for six different types of pumping equipment on the same vacuum chamber (23). The shape of curve 1 indicates that a real leak could be responsible for the zero slope demonstrated by the Bayard-Alpert gauge (BAG). The shape of the other curves could result from a combination of real and virtual leaks. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

A large number of CVD diamond deposition technologies have emerged these can be broadly classified as thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 pm-h to 1 mm-h depending upon the method used. The following are essential features of all methods. 

With the rotary and diffusion pumps in tandem, aided by a liquid-nitrogen trap, a vacuum of 10 Torr became readily attainable between the wars by degrees, as oils and vacuum greases improved, this was inched up towards 10 Torr (a hundred-billionth of atmospheric pressure), but there it stuck. These low pressures were beyond the range of the McLeod gauge and even beyond the Pirani gauge based on heat conduction from a hot filament (limit Torr), and it was necessary to... 

Heiz-dampf, m. steam for heating, -draht, m. heating wire hot filament. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

Our data give a rate constant of 4 X 10-13 cc. molecule-1 sec.,-1 assuming the production of N02- solely by this reaction. However, one must consider the probability that N02, present as a minor impurity or produced by pyrolysis or N20 of or near the hot filament, would react by Reaction 20. 

Figure 7.7. Schematic of hot-filament apparatus for the deposition of diamond.

Schafer, L., Saltier, M., and Klages, C. P., Upscaling of the Hot-Filament CVD Process for Deposition of Diamond Films on Large-Area Substrates, m Applications of Diamond Films and Related Materials, (Y. Tzeng, et al., eds.), Elsevier Science Publishers, pp. 453-460(1991)... 

Like synthetic diamond, C-BN is normally obtained by high-pressure processing. Efforts to synthesize it by CVD at low pressure are promising. It is deposited in an electron-cyclotron-resonance (ECR) plasma from a mixture of BF3 and either ammonia or nitrogen at 675°C on an experimental basis.F l Like CVD diamond, it is also deposited by the hot-filament method using diborane and ammonia diluted with hydrogen at 800°C.P 1... 

Poly crystalline boron nitride films, with a structure similar to rhombohedral boron carbide and a ratio of boron to nitrogen of 3 1, were produced by hot-filament CVD. This work indicates the possible existence of other boron-nitride structures. 

Hot Filament CVD (see Figure 5.2(a)) is relatively cheap and easy to operate and produces reasonable quality polycrystalline diamond films at... 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

Microwave Plasma CVD reactors use very similar conditions to hot filament reactors, and despite being significantly more expensive, are now among the most widely used techniques for diamond growth. In these... 

A number of other deposition methods have been used for growing diamond, with varying degrees of success. These include oxyacetylene welding torches, arc jets and plasma torches, laser ablation and hquid phase crystallisation, but none of these yet reahstically compete with the hot filament or microwave systems for reliability and reproducibility. 

It is now believed that the most critical component in the gas phase mixture is atomic hydrogen, and indeed, this reactive atom drives the whole chemical system. Two hydrogen atoms are made when a hydrogen molecule (H ) splits apart. In a hot filament system, the thermal energy... 

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