We have measured the energy distribution of excess charge trapped by an electron irradiated mica-glass ceramic, Macor™. Photoelectron emission as a function of incident wavelength shows two distinctly different contributions to the measured density of electron trap states. First, there is a broad continuum of binding states which extends well above the Fermi level. This results in spontaneous electron emission which depletes the excess charge in these states rapidly at room temperature and more slowly at 170K. The free decay of charge emission can be described semiclassically. Second, localized trap states appear as peaks on the continuum distribution which occur at 2.7 and 3.1 eV. These correspond to two thermionic emission peaks which occur during heating at 550K and 595K respectively. Photoemission at high intensities using the 458, 488, and 514 nm wavelengths of an argon ion laser and the 632 nm wavelength of a He-Ne laser greatly enhances the depletion of excess charge from the sample. The laser-induced emission intensity is initially high and drops rapidly, followed by a more gradual decrease. The initial intensity measures the population of electrons with binding energies less than the photon energy. The later emission is related to the repopulation of these states by the dynamic redistribution of electrons from deeper states. This behavior may lead to a model for the mechanism of electron redistribution in charged insulating materials.

Surface-initiated radiation damage processes in a number of the maximum valence transition metal oxides - TiO2, V2O5, WO3, MoO3, Ta2O5, Al2O3, and Nb2O5 - have been investigated by in-situ high resolution electron microscopy under UHV conditions and at incident energies ranging from 3 keV to 300 keV. The relative contributions of ballistic, electronic, and thermal effects are evaluated by a comparison of the observed structural changes and damage rates - made under various experimental conditions of incident electron energy and flux - to theoretical predictions based on thermodynamic and available energy criteria.

Many properties of implanted fused silica (e.g., surface stress, hardness) exhibit maximum implantation-induced changes for collisional energy deposition values of ∼1020 keV/cm3. We have observed a second critical energy deposition threshold value of about 1022 keV/cm3 in stress and hardness measurements as well as in many other experiments on silicate glasses (leaching, alkali depletion, etching rate, gaseous implant redistribution). The latter show evidence for damage depths exceeding TRIM ranges by about a factor of 2. For crystalline quartz, a similar threshold value value has been found for extended damage depths (greater than TRIM) for 250 kev ions (H-Au) as measured by RBS and interference fringes. This phenomenon at high damage deposition energy may involve the large stress gradients between damaged and undamaged regions and the much increased diffusion coefficient for defect transport.

We present here experimental results using transmission electron microscopy (TEM) and real time video recordings, of the many dynamic structural instabilities in amorphous silica structures during intense electron beam irradiation. It is observed that while the silica layers on oxidized silicon particles sinter when irradiated uniformly, they undergo necking and extensive plastic deformation when the beam is focused in the neck regions. Extremely thin silica fibers (less than five nanometers in diameter and microns in length) can be produced by the deformation of such necks between particles. The thin fibers and the sharp tips developed from the failure of such structures are seen to spheroidize and blunt when exposed to the electron beam. The instabilities are thought to arise from irradiation induced charging of the insulator surface which expands to reduce the density of accumulated surface charges.

We report new measurements of the regrowth behaviour of Pb-implanted SrTiO3 crystals in the presence of water vapour. Doubly labelled water vapour, D218O, at greater than 95% enrichment in each isotope has been added to the annealing ambient and depth profiles of D and 18O have been obtained from the regrown crystals using secondary ion mass spectrometry (SIMS). The D and 18O content has also been measured by nuclear reaction analysis (NRA) using the reactions D(3He,p)4 He and 18O(p,α)15 N. The crystals were regrown in a conventional furnace under a controlled gas ambient and time-resolved optical reflectivity (TRR) was used to dynamically monitor the regrowth rate during the anneal. An enhancement of the solid-phase epitaxial regrowth rate is observed when water vapour is added to the annealing ambient. This rate increase is accompanied by incorporation of D throughout the regrown layer. 18O is incorporated into the lattice but does not appear to penetrate deep enough to influence the regrowth rate.

The optical emission spectra resulting from ion beam sputtering a SrTiO3 target were observed as a function of position. A collimated optical fiber bundle parallel to the plane of die sputter target was translated vertically and horizontally to spectroscopically profile the sputter process. Three different angles of incidence of the beam to die target were examined for evidence of bombardment of candidate substrate positions by undesirable species. When the ion beam was at normal incidence to the target or 22.5* from normal incidence the Ar and O contours were of similar shape. However, when die target was inclined at an angle of 45* to die incident ion beam die O contours deviated markedly. Under these conditions it appears that there is a substantial oxygen flux in a direction normal to die target surface. Under these conditions die placement of a substrate facing die sputter target would probably result in undesirable bombardment of die growing film by energetic oxygen.

Damaged layers, buried underneath the surface of type IIa diamond slabs by 2.8 MeV C+ ion implantation have been subjected to irradiation with 14 ns pulses from a focused Nd-Glass laser. At high powers, ablation of the implanted surface was observed. However, the correct choice of laser power and wavelength results in annealing of the implanted layer without any disruption to the surface morphology. Annealing was confirmed by optical measurements showing a decrease in the GR-1 absorption band, and by channelled Rutherford Backscattering Spectroscopy on the small 10μm diameter laser irradiated spots. The results suggest that an undamaged diamond cap can be utilized to promote damage annealing of diamond by pulsed laser beams, for which the annealing in other materials has been shown to proceed via the melt. The surface cap is believed to aid in the production of extremely high internal pressures in the damaged layer during the laser pulse which prevents graphitization and promotes diamond regrowth.

A computer model is developed to simulate the behavior of self-interstitials with particular attention to clustering. Owing to the layer structure of graphite, atomistic simulations can be performed using a large parallelepipedic supercell containing a few layers. In particular, interstitial clustering is studied here using a supercell that contains two basal planes only. Frenkel pairs are randomly produced. Interstitials are placed at sites between the crystal planes while vacancies are distributed in the two crystal planes. The size of the computational cell is 20000 atoms and periodic boundary conditions are used in two dimensions. Vacancies are assumed immobile whereas interstitials are given a certain mobility.

Two point defect sinks are considered, direct recombination of Frenkel pairs and interstitial clusters. The clusters are assumed to be mobile up to a certain size where they are presumed to become loop nuclei. Clusters can shrink by emission of singly bonded interstitials or by recombination of a peripheral interstitial with a neighboring vacancy. The conditions under which interstitial clustering occurs are reported. It is shown that when clustering occurs the cluster size population gradually shifts towards the largest size cluster. The implications of the present results for irradiation growth and irradiation-induced amorphization are discussed.

Pressureless-sintered AlN was neutron irradiated and the hardness change was examined by Vickers indentation. The hardness was increased by irradiation. When the samples were annealed at high temperature, the hardness gradually decreased. Length was also found to increase and to change in the same way as the hardness. A considerable density of dislocation loops still remained, even after the hardness completely recovered to the value of the unirradiated sample. Thus, it is concluded that the hardening in AlN is caused by isolated point defects and small clusters of point defects, rather than by dislocation loops.

Hardness was found to increase in proportion to the length change. If the length change is assumed to be proportional to the point defect density, then the curve could be fitted qualitatively to that predicted by models of solution hardening in metals. Furthermore, the curves for three samples irradiated at different temperatures and fluences are identical. There should be different kinds of defect clusters in samples irradiated at different conditions, e.g, the fraction of single point defects is the highest in the sample irradiated at the lowest temperature. Thus, hardening is insensitive to the kind of defects remaining in the sample and is influenced only by those which contribute to length change.

Motivated at least initially by materials needs for nuclear reactor development, extensive irradiation effects studies employing transmission electron microscopes (TEM) have been performed for several decades, involving irradiation-induced and irradiation-enhanced microstructural changes, including phase transformations such as precipitation, dissolution, crystallization, amorphization, and order-disorder phenomena. From the introduction of commercial high voltage electron microscopes (HVEM) in the mid-1960s, studies of electron irradiation effects have constituted a major aspect of HVEM application in materials science. For irradiation effects studies two additional developments have had particularly significant impact; (1) the development of TEM specimen holders in which specimen temperature can be controlled in the range 10–2200 K and (2) the interfacing of ion accelerators which allows in situ TEM studies of irradiation effects and the ion beam modification of materials within this broad temperature range. This paper treats several aspects of in situ studies of electron and ion beam-induced and enhanced phase changes and presents two case studies involving in situ experiments performed in an HVEM to illustrate the strategies of such an approach of the materials research of irradiation effects.

Ion bombardments, transmission-electron-microscopy (TEM) and analytical-electron-microscopy (AEM) techniques were used to obtain information on the effect of irradiation on the phase stability of Zr3 Fe, which has an orthorhombic crystal structure. Irradiations were performed at 35–773 K using 0.5–1.5 MeV 40Ar ions and at 40 K using 15–120 keV 209Bi ions.

At low ion fluences (1011–1013 ions cm−2), most of the visible damaged regions (∼1.0–5.0 nm diameter) exhibited strong structure-factor contrast features and it is believed that they are probably amorphous. With increasing ion energy, multiple damaged regions were observed to form within individual collision cascades, and the fraction of the theoretical cascade volume occupied by these heavily-damaged regions decreased rapidly. Irradiations at higher ion fluences showed that an irradiation-induced phase change from the crystalline-to-amorphous state occurred at 35–573 K, but not above 623 K. The effect of varying the ion flux was also investigated. The results showed that the fluence required to amorphize a given fraction of the irradiated volume was quite dependent upon the amount of annealing that occurred within the collision cascade at the temperature of the bombardment. The crystalline-to-amorphous transformation appeared to be the result of direct amorphization within the collision cascades and a critical defect concentration being reached in the cascade overlap regions, thus producing additional amorphous regions.

Changes induced by 1.5 MeV Kr ion irradiation of both U3Si and U3Si2 have been followed by in situ transmission electron microcopy. When irradiated at sufficiently low temperatures, both alloys transform from the crystalline to the amorphous state. When irradiated at temperatures above the temperature limit for ion-beam amorphization, both compounds disorder, with the Martensite twin structure in U3Si disappearing from view in TEM. Prolonged irradiation of the disordered crystalline phases results in nucleation of small crystallites within the initially large crystal grains. The new crystallites increase in number during continued irradiation until a fine grain structure is formed. Electron diffraction yields a powder-like diffraction pattern that indicates a random alignment of the small crystallites. During a second irradiation at lower temperatures, the small crystallizes retard amorphization. After 2 dpa at high temperatures, the amorphization dose is increased by over twenty times compared to that of initially unirradiated material.

In order to help establish the role of Cr in high-dose, high-dose-rate, elevated temperature N implantation of austenitic (fcc) stainless steels, similar implantations into fee Ni80Fe20 and Ni80Cr20 alloys have been made and characterized by Auger depth profiling and X-ray diffraction. For the Ni-Fe alloy a shallow layer fcc(∼ 0.2 μm) containing an ordered fee γ'-(Ni0.8Fe0.4)4N phase is induced. In contrast, for the Ni-Cr alloy a much thicker N-containing layer (∼ 0.2 μm) is produced consisting primarily of a high-N solid solution fee phase. The fractions of the implanted N retained in Ni-Fe and Ni-Cr were approximately 10 and 100%, respectively. The mechanisms by which Cr is promoting the deep migration and high retention of N in solid solution are proposed.

Phase transformations by yttrium implantation into pure aluminum using 60kV ions and 3×1017 ions/cm2 dose have been studied by transmission electron microscopy (TEM). YAl3(12R) particles with an average size of 0.3μm were formed in the modified aluminum surface, which transformed into YFe3·5Alx (predominantly) and YAl3(8H) phase particles with 2 μm average diameter after being annealed at 600°C for one hour. The Fe ions were introduced by ion sputtering from the iron screws of the specimen holder, a previously unnoticed effect during heavy metal ion implantation for surface modification of materials. Electron diffraction experiments indicated that the YFe3·5Alx was a new phase and belongs to the orthorhombic system with a C-face-centered lattice (a=c=0.887nm, b=1.024nm). The reciprocal lattice of the YFe3·5Alx phase is related to that of the YFe4Al8 magnetic phase. The microstructure of the modified aluminum surface was also investigated by high-resolution TEM.

Ion implantation of lead or indium into aluminium results in spontaneous phase separation and formation of lead or indium precipitates. The precipitates grow in topotactical alignment with the matrix, giving TEM images characterized by moiré fringes. The size and density of the precipitates increase with increasing fluence until coalescence begins to occur. Implantations at elevated temperatures lead to formation of larger precipitates with well developed facets. This is particularly significant for implantations above the bulk melting point of the implanted species. Melting and solidification have been followed by in-situ TEM heating and cooling experiments. Superheating up to ∼ 50 K above the bulk melting point has been observed, and the largest inclusions melt first. Melting is associated with only partial loss of facetting of the largest inclusions. Initial growth of the inclusions occurs by trapping of atoms retained in supersaturated solution. Further growth occurs by coalescence of neighbouring inclusions in the liquid phase. Solidification is accompanied by a strong undercooling ∼ 30 K below the bulk melting point, where the smallest inclusions solidify first. Solidification is characterized by spontaneous restoration of the facets and the topotactical alignment.

Ti/Al, Ni/Ti, and Ni/Al bilayers were produced by evaporation on a Si substrate. The thicknesses of the composing layers were in the 50–200 nm range. The as-evaporated bilayers were implanted with doses of 0.4–2.5 · 1017 of 15N ions/cm2. The 15N energy was chosen so that the calculated projected range was either in the middle of the top Ni layer, or coincided with the interface between the metal layers. The implantation temperature was varied in the range from 25 °C to 245 °C. The 15N depth profiles as measured with nuclear reaction analysis (NRA) were found to have the expected Gaussian shape for the Ti/Al bilayers. However, in the Ni/Ti and Ni/Al layers very asymmetric 15N profiles were observed, with a major fraction of N atoms in the Ti or Al layer, and a minor fraction in the Ni layer. The N concentrations in the Al or Ti layers were found to be almost constant. A massive redistribution of N atoms must have taken place in all three metals during the implantations, to form the observed profiles. We speculate that this remarkable phenomenon, which occurs even below 80°C, can be explained by interstitial diffusion of N atoms and subsequent trapping in Ti or Al, which have a high chemical affinity for nitrogen. The N atoms remaining in the Ni layer after implantation were found to migrate out of this layer during annealing at temperatures ≥250°C.

RBS/channeling analyses of high energy Au implantation into Mg are presented. The diffusion behaviour of Au was studied and the occurrence of essentially two distinct regimes were observed: the segregation regime at lower temperatures correlated with the damage introduced by the high energy implantation and the higher temperature regime as a normal thermally activated process of back-diffusion.

We have continued to investigate ion-beam mixing in bilay-er targets irradiated by 2-MeV He+ microbeams at room temperature. Although we have previously reported a linear dependence of interface width on dose for Cu/Al targets 1, more extensive results have not supported this conclusion, within statistical uncertainty, it appears that the interface width in Cu/Al (1) is proportional to the square root of dose, at constant dose rate, (2) is larger in Al than in Cu, for the same dose, (3) is proportional to the 1/4 power of dose rate, and (4) is absent at liquid nitrogen temperature. Calculations of the expected interface growth rate from a radiation-enhanced diffusion model have provided order-of-magnitude agreement with observed rates. Additionally, intermixing of Cu and Al outside the damaged area may indicate significant transverse diffusion of vacancies.

The role of alloying elements such as Cr and Al in the formation and stability of the nitride phases formed due to N ion implantation into metallic iron was studied by conversion electron Mössbauer spectroscopy (CEMS). The thermal stability of nitride phases upon 1 h annealing was greatly increased as a result of co-implanting either Cr or Al with N as compared to pure α-Fe implanted only with N.

Amorphization behavior was studied for the Y-Ti system, which has rather positive heat of formation being around + 22 kJ/mol, by room temperature 360 keV xenon ion mixing of YxTi100−xmultilayered films to various doses, ranging from 7×1014 to 1×1016 xe/cm2 Single and uniform amorphous phase was obtained in a narrow composition region, i.e. x=65 to 75, after ion mixing to the relevant doses. Moreover, a metastable fee crystalline Y-Ti phase was observed, for the first time, in this system. The crystalline lattice constant of the metastable phase was determined to be 4.012 Å. The re-crystallization temperature of the formed amorphous alloy was found out to be 600°C by in situ transmission electron microscope annealing as well as by vacuum furnace experiments. Possible interpretation is also discussed by comparing the experimental results with those proposed models for predicting glass forming ability.