V. N. Shrednik

Heat-field treatment of tips made of tungsten-hafnium alloy

The combined effect of a high electric field and a high temperature on the morphology of W-Hf field-emission tips is studied. It is shown that, while the tip reconfiguration stages for this alloy are basically the same as for pure tungsten, they have a number of specific features due to the surface segregation of hafnium. The heat-field treatment of these emitters narrows the solid angle of emission significantly, improves the emissivity of the tips, and is accompanied by the high-temperature evaporation primarily of Hf atoms.

Field evaporation of a Hf-Mo alloy

The field evaporation of a Hf-Mo alloy (15 wt. % Hf) is investigated using a time-of-flight atom probe. A moderately heated tip detects an impurity of Hf and Mo oxides on the surface. Thermofield microprotrusions grown at T=1440–1850 K in an electric field (retarding to electrons) of intensity E=(3.2–5)×107 V/cm are analyzed at room temperature and above.

Field evaporation of carbidized tungsten

The field evaporation of carbidized tungsten (ribbed crystals) was studied by the atom probe method at various temperatures of the sample in the regions of cryogenic, room, and elevated (1300–1600 K) temperatures. For a high pulsed (not less than 40%) component in the evaporating field, the room-temperature field evaporation mass spectra correctly reflect the surface composition of the evaporated sample. The mass spectra reveal a large number of molecular and polymerized (cluster) ions. Data from field ion desorption microscopy indicate that high-temperature field evaporation of a ribbed crystal proceeds via ion emission from walking and merging thermal-field microroughnesses.

High-temperature field evaporation of rhenium

High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/Å and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed.

Cluster ions in the field evaporation spectra of lanthanum hexaboride measured at room and elevated temperatures

The field evaporation of LaB6 single crystal was studied by the atom probe method at room temperature and at elevated (1145–1260 K) sample point temperatures. The field evaporation spectra of LaB6 showed a surprisingly large variety of cluster ions: ion species of the LaBn+m type (n=1–9; m=1–4) and La2Bn+m type (n=1–15; m=2, 3) were observed at room temperature, and doubly charged ions of the LaBn+2 type (n=1–5) were observed in the high-temperature spectra. The physical aspects of this phenomenon are discussed.

Formation and stability of free charged lanthanum hexaboride clusters at field evaporation

Field evaporation of LaB6 single crystals resulting in a large variety of ionized clusters of LaBnm+ type, where n=1,2,…,6 and m=1, 2, 3, 4, is investigated in a collaborative experimental and theoretical study. The aim of the present work was to compare the measured appearance probability of various clusters in a flux with the calculated energetic stability of corresponding free clusters. The experiments were carried out with a time-of-flight atom probe. The appearance probability of cluster ions in the atom probe spectra was estimated by calculating its rating in many selected spectra. The methods for determining such ratings have been developed and discussed. The theoretical calculations are based on ab initio quantum chemical Hartree–Fock self-consistent field and configuration interaction methods. A double ζ basis set was used to determine the cluster geometries, energies, and relative stability. For the inner-shell electrons of lanthanum we have used relativistic compact effective potentials. The ge...

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, αT, for the islands was estimated at (4–6)×10−4 eV grad−1 or more, whereas for pure tungsten αT was found to be 2×10−5 eV grad−1.

Field electron emitter for air ionization in a supersonic flow

It is suggested to use a field electron emitter based on liquid gallium for the ionization of molecules in a supersonic air flow.

The properties of the nonequilibrium LaB6 surface resulting from field evaporation

The nonequilibrium surface of single-crystal lanthanum hexaboride needles and its modifications are studied with a time-of-flight atomic probe. The surface is obtained by room-temperature field evaporation. The mass spectra of field evaporation shed light on the surface composition at the needle tip immediately after tip etching, corrosion in residual gases, intense cleaning by field evaporation, and the relaxation of the nonequilibrium surface by heating to 1250 K. Conditions for the breakdown of an oxide film on the tip surface and for obtaining the mass spectra of field evaporation for stoichiometric or lanthanum-enriched pure LaB6 single crystals are discussed.

Field evaporation of lanthanum hexaboride and estimate of the stability of forming clusters

In addition to boron and lanthanum ions, mass spectra of the field evaporation of lanthanum hexaboride (LaB6) reveal various ionized clusters of the type LaBn+m, where n=1,2...6, m=1,2,3,4. The probability of observing specific clusters in an ion flux is compared with the energy stability of the free clusters calculated theoretically. Clusters possessing low stability in the free state or completely unstable clusters tending to dissociate leave the surface more readily.