Loop Punching Research Articles

Effect of viscous flow on ion damage near solid surfaces.

Molecular dynamics computer simulations of 10 and 20 keV Au bombardment of Au substrates were performed to elucidate the influence of surfaces on defect production. Nearly all of the damage was due to local melting and viscous flow of hot liquid onto the surface. For the 10 keV event, 554 atoms flowed to the surface, and this led to the formation of a dislocation network that extended \ensuremath{\approxeq}6 nm below the surface. Only three interstitial atoms, and no isolated vacancies or small vacancy clusters were produced. Loop punching and the nature of nonlinear sputtering are also discussed.

Self-organization of cavities under irradiation

In the present paper two examples of self-organization in solids under irradiation are considered on the basis of original mechanisms, namely, the ordering of voids in void lattices under high temperature irradiation and the alignment of gas bubbles in bubble lattices under low-temperature gas atom implantation. The ordering of cavities (i.e. voids or gas bubbles) is proposed to arise due to a dissipative interaction between cavities induced by the interstitial dislocation loop absorption and punching, respectively, which represent anisotropic mechanisms of atomic transport.

New insight on bubble-void transition effects in irradiated materials

An account of elastic interaction between cavities and point defects is shown to result in new critical quantities for bubble-void transition effects in irradiated cubic crystals. In contrast to previous theories, the present one gives not only critical quantities which determine the onset of bias-driven void swelling but the maximum stationary number density and the corresponding mean radius of voids as well as the duration of the bimodal regime. The void density and swelling rate are shown to be independent from the gas level. In the region of low temperatures/high dose rates, the void density appears to be independent from irradiation parameters as well. The relationships among material constants are found at which the stabilization of gas bubbles occurs via the dislocation loop punching mechanism resulting in a drastic change in the cavity behaviour under irradiation such as the saturation (or even suppression) of void swelling and void lattice formation. The theoretical results are compared with experimental data and further experimental tests are proposed.

Theory of te precipitation and related effects in CdTe Crystals

A thermodynamic calculation is presented which explains the origin of often reported large stress fields in and around Te precipitates and associated punching of dislocation loops in star like patterns. The calculation is based on the consideration that the stoichiometric deviation in Te saturated crystals are accommodated by Te interstitials, Cd multivacancy complexes and Te antisites; the Te interstitials are most mobile of them, and dominate the precipitation process; and the precipitates when they appear first are in droplet form. The droplet state is modelled on the basis of the Lennard-Jones 6-12 interaction potential. The droplet growth is envisaged to occur via the following two processes operating in tandem: first, capture of excess Te interstitials to cause droplet overpressurization, and then, punching of interstitial dislocation loops whenever the pressure exceeds a threshold value. The flow of Te interstitials into the droplets is driven by the difference between their formation energies in the lattice and the free energy change of the droplets per Te atom added. It is shown that the achievable droplet pressures far exceed that required for the loop punching and sustaining the growth cycle. The occurrence of varying precipitate morphologies are explained, and the possibility of nucleating high presssure Te phases is examined. Some experimental evidence is also presented which corroborate the theoretical arguments involved.

SiO2 precipitate strain relief in Czochralski Si: Self-interstitial emission versus prismatic dislocation loop punching

Formation of SiO2 precipitates in Czochralski Si is associated with a volume expansion of more than 100%. The needed extra volume for precipitate growth to occur is primarily supplied by emission of Si self-interstitials (I) into the Si matrix, in balance with a compressive growth residual strain. During cooling after the anneal, an additional compressive cooling strain component also develops because of the different thermal expansion coefficients of SiO2 and Si. For precipitates grown to a sufficiently large size, the growth residual strain and/or the cooling strain can be further relieved by punching interstitial type prismatic dislocation loops into the Si matrix. Otherwise, only I emission can occur. Up to now, there have been no quantitatively determined strain values, which constitute in a given experiment a measure of the I emission efficiency on the one hand, and a basis for determining whether prismatic punching can also occur on the other. In this study, we have calculated the strain values and obtained a quantitative criterion for prismatic punching to occur. In the order of ∼10−3–10−2, the growth residual strain component values indicate that I emission has attained an efficiency of relieving the precipitate growth strain by ∼90%–99%. Available experimental data on the precipitate size dependence of prismatic dislocation loop punching have been satisfactorily fitted using the obtained strain values and the punching criterion, indicating that these calculated values are in acceptable accuracy ranges.

Quenching of dilute Be [sbnd]H alloys: A TEM study

Be is a relevant material in nuclear and reactor technology. It has recently gained additional prominence as a coating material for Tokamak fusion devices. In this latter context, the reactions of Be with the hydrogen isotopes are very important. Recently, rather precise hydrogen solubility measurements both in {alpha}-Be and in {beta}-Be became available. This paper presents an adapted form the solubility of H in Be in the {alpha}-phase. The solubility in terms of the atom ratio H/M can be expressed by: S = 1.723 {times} 10{sup {minus}4} sq. root p exp({minus}16700/RT) (H){sub (M)} where p is the pressure in MPa, T the temperature and R the gas constant. Thus, {alpha}-Be is an endothermic occluder of hydrogen. Another endothermic occluder of H is, for instance, Cu where TEM techniques showed the quenching of dilute Cu-H alloys from high temperatures leads to the formation of hydrogen bubbles around room temperature. Matter is ejected from the growing bubble by prismatic dislocation loop punching. In other TEM studies, it was established that quenching of the exothermic occluder of hydrogen, Mg, does not lead to bubble precipitation but to the formation of small hydride particles.

The expansion of the tritide TaT0.37 at 10 K due to single interstitial 3He atoms

The expansion of the tritide TaT0.37 at 10 K was measured using strain gauge dilatometry. The volume increase as expressed by ΔvHe/Ω = 0.26 may be attributed to a random distribution of single interstitial 3He atoms. To the knowledge of the authors this is the first direct measurement of the volume requirement of interstitial 3He in a metal. For purposes of comparison swelling of the same sample was also measured at room temperature. Here, however, 3He atoms cluster and form bubbles leading to ΔvHe/Ω ≊ 0.38, which represents the volume requirement of 3He in a bubble. Conversion of this value to a pressure normalized to the shear modulus μ yields in p/μ=0.22 which is in excellent agreement with other recent experimental and theoretical work on the pressure required for dislocation loop punching.

On the relaxation of a mismatching spheroid by prismatic loop punching

Keywords: Glass - Fiber Reinforcement ; Silver Compounds ; Titanium Metallography - Inclusions ; Silver Chloride ; Titanium and Alloys Note: Massachusetts Inst of Tech, Cambridge, United States0956716X (ISSN) Reference LMM-ARTICLE-1991-009View record in Scopus Record created on 2006-10-09, modified on 2017-05-10

Helium bubbles in molybdenum investigated by positron annihilation spectroscopy

Abstract Positron lifetime and Doppler broadening measurements have been performed in homogeneously helium implanted (715 appm) molybdenum. A new stage beyond the temperature range of stability of voids has been observed. The shorter lifetime componet τ1 = 158 ± 2 ps of 60% intensity is assigned to the positron state in helium-decorated dislocation loops while the longer lifetime component τ2 = 408 ± 5 ps of 40% intensity is explained in terms of positron trapping at voids with a small percentage of helium associated with them. Dissociation of helium from loops is observed in the range 300–800 K followed by loop annealing above 900 K. A sharp reduction of τ2 around 1300 K is assigned to multiple helium occupancy of voids, resulting in their transformation into stable bubbles. Helium retention in bubbles is found stable up to the highest annealing temperature of 1700 K. The bubble parameters deduced from the positron lifetime results reveal that the pressure in the bubbles is maintained at the near-equilibrium value and that the vacancy mode of bubble growth is operative, in contrast to the reported growth of athermal bubbles in molybdenum following very low energy helium implantation where loop punching mechanism seems to control the excess pressure in bubbles. A brief comparison is also made between the observed behaviour of helium with that of hydrogen in molybdenum and the differences are discussed.

A model of melting of rare gases within bubbles of crystalline metal matrix

Abstract On the basis of experimental results obtained in the present and some other works a model of melting of rare gas solids within bubbles formed in a crystalline metal matrix as a result of ion implantation is proposed. Rare gas solid is supposed to melt on heating at the expense of the bubble volume expansion by emission of a dislocation loop. On this basis the melting temperature can be estimated as one which is enough to provide for a pressure inside a bubble sufficient for the initiation of the dislocation loop punching. Values of melting temperatures obtained in this way are in good agreement with available experimental data.

Room-temperature creep of tantalum tritides

We report on long-term creep experiments on dilute tantalum tritides at room temperature. Significant deviations of the recorded strain rates from isotropic swelling are found above approximately 30 MPa. We attribute this room-temperature creep to a stress-induced preferential dislocation loop punching by bubbles in crystallographic directions close the stress axis. Quantitative estimates show that this mechanism can indeed account for the observed creep rates.

Characterization of macroscopic defects in silicon after processing for CMOS and bipolar circuits

We have been engaged in a systematic and careful study of the early stages of oxygen precipitation at temperatures up to 850°C which involved transmission electron microscopy correlated with diffraction, optical and IR spectroscopy. We have simultaneously made a detailed study of macroscopic defects induced in Czochralski silicon wafers after repeated heat cycling during the fabrication of circuits. Even though there have been a number of such studies there remain many questions about the effects which are observed, such as why do stacking faults sometimes occur while on other occasions prismatic loop punching predominates? Variable precipitate morphologies are also observed such as {100} plates or octahedra without any general agreement on the reasons for such differences. The question of how precipitates nucleated at lower temperatures grow and develop after repeated high temperature cycles has also not often been addressed. Further questions exist over the heterogeneous or homogeneous nucleation of precipitates, the involvement of transition metals, the unfaulting of stacking fault loops. Answers to many of these questions arise out of our work which seeks to provide an over-view of the variety of oxygen precipitation routes in silicon.

Simulation of the growth of inert gas bubbles near the surface of an amorphous alloy

Using an interatomic potential technique, an overpressurized gas bubble in an amorphous metal was simulated for various pressures. Above an excess pressure P exc of nearly 75 kbar, the material around the bubble undergoes a continuous irreversible deformation, as found in experiments. On the basis of this value of P exc, a good description can be given of several experimental facts concerning gas bubble growth in amorphous alloys (e.g. the dimension of the bubbles as a function of dose). This P exc value is considerably higher than bubble pressures usually observed in crystalline material, which reveals a different growth mechanism for bubbles. Instead of loop punching as observed in crystals, local rearrangements of small groups of atoms are a more probable mechanism. The excess gas pressure found in the simulation approximates ( 1 2π ) times the material shear modulus, which is approximately the theoretical strength of a crystalline material. The value of P exc is confirmed by a recent measurement using EELS techniques. The high P exc permits in principle the crystallization of some noble gases in the precipitates at room temperature.

Dislocation loop punching in bubble arrays

Abstract High densities of inert gas bubbles can be produced in metals by ion implantation or nuclear transmutations. At low temperatures, these bubbles grow by repeated emission of prismatic dislocation loops. An analysis is presented for loop punching considering the effects of the stress fields between bubbles and the presence of retained loops. It is shown that the retained loops increase the pressure required for further loop growth dramatically as the interbubble spacing decreases. At a critical helium content, further bubble growth ceases by one of two mechanisms: equilibration of the chemical potentials for gas atoms in the bubble and in interstitial solution, or interbubble fracture when the average ligament stress exceeds the theoretical tensile strength of the material. The critical concentrations predicted for gas release or blistering are shown to agree with experimental findings. Furthermore, the influence of retained loops on bubble growth also leads to a natural explanation for bubble supe...

NMR Studies of Aging Effects in Palladium Tritide

This paper presents Nuclear Magnetic Resonance (NMR) studies of aging phenomena in palladium tritide. /sup 3/He NMR relaxation parameters have been measured as a function of temperature for 6-, 13-, and 22-month-old beta phase palladium tritide. The most significant result of this study is the observation of a solid/fluid phase transition near 250 K of /sup 3/He that has accumulated in the PdT/sub x/ substrate via triton decay. Although the existence of solid helium at relatively high temperatures had been predicted for helium in metals, it had not previously been confirmed in any metal/helium system. The observed melting temperatures, together with the known equation of state for /sup 4/He, allow a determination of the helium density as a function of age. The atomic density obtained in this way is approximately 2.0 times that of palladium metal, agreeing with densities inferred from dilatometric measurements of other metal tritides and also with predictions based on the concept of dislocation loop punching by highly overpressurized He bubbles. The /sup 3/He signal in the 22-month-old sample was sufficiently strong to allow a detailed study of melting as a function of temperature, and provides information on the distribution of densities over the ensemble of bubbles.

The pressure for dislocation loop punching by a single bubble

Abstract A detailed analysis is given on the energetics of bubble growth in a solid by prismatic loop punching. Interactions between the bubble and the dislocation loop are included as well as the effect of the loop proximity on the actual bubble volume. It is shown that there exists an attraction between bubble and loop and that the bubble volume increases gradually as the loop recedes from the bubble. As a result, there exists an energy barrier for loop punching which requires a significantly higher pressure than previously assumed. Furthermore, the critical pressure for loop punching reaches a nearly constant value with increasing bubble radius. The calculations are carried out for the case of nickel at room temperature, and it is predicted that helium within the bubbles is in the solid state.

Microstructural evolution in high energy helium implanted nickel—I. Room temperature ( t − 100° c) implantation

Nickel foils of about 12 μm thickness have been homogeneously implanted with 0 to 6 MeV He ions at room temperature. For helium concentrations ranging between 0.1 and 5 at.% the implantation induced changes of the microstructure have been investigated by measurements of the electrical resistivity, the length and the lattice parameter and by transmission electron microscopy (TEM). Beginning from the lowest implantation dose clustering of irradiation induced vacancies and self-interstitial atoms leads to the formation of stacking fault tetrahedra (SFT), dislocation loops and of a dislocation network. In addition a large number of small He-vacancy complexes is present that are invisible in the TEM at low He concentrations, c He , and can be identified as small cavities for c He ≳ 0.7 at.%. The He-vacancy complexes are characterized by a He-density that is rather low compared to the density necessary to activate the loop punching mechanism for bubble growth: i.e. c He is about one half of the vacancy concentration c V for c He ≲ 0.8 at.%. At higher implantation doses the ratio c He / c V increases slowly up to a value of 1.4. Hence, the limiting pressure for loop punching may be obtained only for c He ≈ 8 at.%. A “model” is proposed that describes this microstructural evolution by the continuous nucleation of stable He-vacancy complexes as a result of high energy collision cascades. This model directly explains the low average helium density observed during the early stages as well as the coexistence of small cavities and SFT's. A saturation of the bubble density at high doses and the slow increase of the He density in bubbles might be understood by cascade overlap.

A TEM study of the aging of Zr tritides

Zr-tritides were investigated by TEM for aging times up to almost 3 y. After ∼ 3 weeks 3He bubbles of ∼1 nm in diameter were detected. The bubble density soon reached a value of ∼ 5 × 1023 m−3 and then apparently remained constant. Using strong diffraction contrast the interstitial loop and dislocation damage accompanying bubble formation could be imaged. Loops, network dislocations and internal boundaries were found to be heavily decorated with tiny bubbles. After ∼ 2 y an interconnected system of He filled channels evolved from these structures. The observed microstructural evolution is discussed in the light of bubble nucleation and growth models. From the bubble densities, energies of He diffusion or He dissociation from He clusters, respectively, of ∼ 1 eV are derived. Under the present conditions, bubbles can grow by loop punching or by the diffusional transfer of self-interstitials (SIA's) from the bubble surface to dislocations attached to them. Under high pressure, bubbles at dislocations could be unstable against elongation along the dislocations. The evolving system of channels presumably is responsible for 3He loss in the “state of accelerated 3 He release”.

Conditions for dislocation loop punching by helium bubbles

Under continuous helium but insufficient vacancy supply, bubbles in metals grow by pressure-driven athermal processes such as metal interstitial emission and dislocaton loop punching. To discuss the energetic conditions for these processes, the formation and interaction energies of bubbles and interstitial type metal defects are analyzed. The only condition which has been considered to date is that the decrease in the free energy of a bubble associated with the emission of an interstitial type metal defect must be equal to or larger than the formation free energy of the latter defect. Consideration of the elastic interaction energies result in additional conditions controlling loop punching for bubbles with radii larger than about 10 Burgers vectors. Possible modes for the loop punching process are discussed.

Comparison of mechanisms for cavity growth by athermal and thermal processes

The energy expended per metal atom removed from an overpressurized bubble is evaluated for three bubble growth mechanisms under constant helium generation or implantation rates. These mechanisms are self-interstitial expulsion, loop punching, and thermal vacancy absorption. It is found that the predominance of each one of the three mechanisms depends strongly on the bubble size, the temperature and the helium generation rate.