The thermionic emission properties of the borides of the alkaline-earth and rare-earth metals and thorium have been investigated. These compounds all have the same formula MB6 and the same crystal structure consisting of a three-dimensional boron framework in whose interlattice spaces the metal atoms are embedded. The valence electrons of the metal atoms are not accepted by the B6 complex, thus giving rise to the presence of free electrons which impart a metallic character to these compounds. This, together with the strong bonds between the boron atoms in the framework, produces a series of compounds which have high electrical conductivities and high thermal and chemical stabilities—ideal properties for a cathode material. When this structure is heated to a sufficiently high temperature, the metal atoms at the surface evaporate away. They are, however, immediately replaced by diffusion of metal atoms from the underlying cells. The boron frame work does not evaporate but remains intact. This process gives a mechanism for constantly maintaining an active cathode surface. Thermionic emission measurements made on these materials show the rare-earth metal borides to be superior to the others. The highest emission was obtained from lanthanum boride. Its emission constants for the Dushman equation were φ=2.66 volts and A=29 amps/cm2/degK2. This is higher than the emission normally obtained from thoria. Lanthanum boride has a relatively low evaporation rate corresponding to a latent heat of evaporation of 169 kilocalories per mole. If the hexaborides are operated at high temperature in contact with the refractory metals, boron diffuses into their metal lattices forming interstitial boron alloys with them. When this occurs, the boron framework which holds the alkaline-earth or rare-earth metal atoms collapses, permitting the latter to evaporate. However, the hexaboride cathodes may be operated at high temperatures in contact with tantalum carbide or graphite. Lanthanum boride cathodes are especially useful in applications where high current densities are required. They are also suitable for high voltage applications because they stand up well under positive ion bombardment. Since they are atmospherically stable and activate easily, they have found wide use in experimental demountable systems.
* Examines all types of pressure measurement techniques, including the latest quadrupole mass spectrometer techniques for partial pressure analysis * Explores the state of the art in calibration and standards.
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In order to extend the low-pressure limit of the conventional hot-cathode ionization gauge it is necessary, at a given emission current, to increase the ratio of the ion current to the x-ray photo-current. This ratio may be increased by increasing the sensitivity of the ionization gauge. The gauge may be modified to permit the electrons to travel in long paths before they are collected by the positive grid or anode. Under these conditions the probability of the electrons' colliding with an ionizing a gas molecule will be enhanced, and the sensitivity of the gauge will be improved with no increase in x-ray photoemission. To accomplish this a magnetron with a cylindrical anode was selected for the ionization gauge and operated in a magnetic field with an intensity of 2.5 times the cutoff value. Two end plates maintained at a negative potential relative to the cathode collect the ion current generated in the magnetron and prevent the escape of electrons. Positive ion emission from the cathode is suppressed by mounting a hairpin filament on the axis of the cylindrical anode and well removed from the region of the negative ion collector. A very low level of electron emission is used to prevent instabilities in operation and to give a maximum ratio of ion current to x-ray photocurrent. Mearurements of sensitivity and x-ray photocurrent indicate that the magnetron gauge is linear down to a pressure of 4×10−14 mm Hg. Ability to measure low pressures with the gauge appears to be limited by the sensitivity of the circuit measuring the low ion currents. The ion pumping speed was found to be 0.003 liter/sec under normal operating conditions. Magnetron gauges have been built with ceramic metal envelopes.
It was found that a considerable portion of the instability in the FP-54 is caused by variations in emission from the thoriated tungsten filament. Operation of the filament at a current which neither activates nor deactivates was found to be a good criterion for adjusting the circuits employing these tubes. Large fluctuations were observed in emission immediately after activation of the filament. Greater stability was obtained by increasing the activation time from 8 to 40 minutes. Filament and shields improved the stability, showing that some rapid fluctuations in emission occur at the poorly activated end portions of the filaments. Long-time drifts were not improved by end shields. Tubes with oxide-coated filaments gave greater sensitivity and less grid current than tubes with thoriated tungsten filaments. These tubes, however, had a tendency to drift. A split type of FP-54 was constructed with a common filament and space-charge grid, but with twin control grids and plates. Both oxide-coated and thoriated tungsten filaments were used. Operation of these tubes in a bridge type circuit eliminated long-time drift and decreased the amplitude of rapid fluctuations.
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