M. Hautala

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Oxygen detection by non-Rutherford proton backscattering below 2.5 MeV

Abstract Oxygen detection by proton backscattering has been investigated. The oxygen detection sensitivity of 2.5 MeV proton backscattering is shown to exceed that of 4He backscattering by even a factor of about 15 depending on the matrix. The needed proton elastic scattering cross sections of oxygen for θlab = 170° have been measured in the energy range Elab = 770–2480 keV relative to Ti and Sn elastic scattering cross sections using thin TiO2 and SnO2 samples. The angular dependence of the cross section was measured at energies Elab = 1790, 1990, 2191, and 2382 keV for backscattering angles. The experimental cross sections were found to be 1.1–5.7 times the pure Coulomb cross section. Theoretical calculations for the scattering cross sections were performed and their inapplicability to experimental purposes is demonstrated. Fits to experimental data are given.

Thermal- and radiation-stability of hydrogen-implanted silicon standards for ion-beam analysis

The effect of thermal annealing and particle radiation on the depth profile of 25 keV H+ implanted into crystalline and ion-beam amorphised silicon has been studied via the 1H(15N, αγ)12C reaction. Out-diffusion of hydrogen from crystalline silicon was observed after annealing at 100 °C for 45 min. The corresponding temperature for ion-beam (300 keV, 84Kr2+) amorphised silicon was between 300 and 500 °C. Radiation damage produced by 3 × 1015 6.4 MeV 15N2+ ion cm−2 lead to effective trapping of the hydrogen in crystalline silicon whilst 1.1 × 1015 300 keV Kr2+ ions cm−2 gave rise to significant spreading.

Momentum and recoil-flux anisotropies in collision-cascades: Influence on sputtered particle angular distributions

Abstract The anisotropy in the spatial distributions of recoil-flux and recoil-momentum have been studied by Monte Carlo simulation for the specific case of amorphous germanium bombarded with Ar + ions. For all the energies investigated, (1.25 to 320 keV) both the recoil-flux and momentum-flux distributions are strongly backwards directed at the surface, becoming forwards directed at greater depths. Further the angular distributions are (relatively) insensitive to the choice of interatomic potential. The calculations show that the backwards-directedness of sputtered particle angular distributions may be completely explained by the anisotropy in the recoil flux. It is also demonstrated that the adoption of more realistic surface models, than the usual infinite target approximation does not lead to significant changes in the angular distribution of recoil-, momentum- and sputtered particle-fluxes.

Wetting and wicking of fibre plant straw fractions

Abstract Bast fibre plants are attracting increasing attention in Europe, and the plant straw fractions have already been used as a raw material in many new applications. This study focuses on characterising the liquid–fibre interactions of frost-retted and unretted bast fibre plant ( Linum usitatissimum L. and Cannabis sativa L.) straw fractions. A novel approach was used in order to overcome the problems introduced by the surface heterogeneity in contact angle measurements. First, the wicking rates of water, oil and ethanol in plant straw fractions were measured with a capillary rise method. The results were compared with the results calculated from the Lucas–Washburn equation. Second, the wetting properties of a compressed sample were studied by measuring the contact angle of a liquid. Third, the absorption time of a liquid droplet on a compressed sample was measured. Fibre hemp absorbed water better than flax/linseed. After 100 s the difference was about one order of magnitude. For oil the differences were marginal. Wicking and wetting turned out to be very rapid even when the measured (static) contact angle was almost 90°. Big variations in dynamic contact angle calculated from wicking measurements were observed. The wicking rates and droplet absorption times give suitable information when selecting fibre plant straw fractions as raw materials for new industrial products. On the contrary, a droplet based contact angle measurement does not give useful information of dynamic wetting behaviour.

Capillarity of flax/linseed (Linum usitatissimum L.) and fibre hemp (Cannabis sativa L.) straw fractions

Abstract In a study of the wetting properties of the fractions of unretted and frost-retted fibre straws a method to separate fibre, fine shive, and coarse shive from fibre plants is introduced and tested on bast fibre plants ( Linum usitatissimum L. and Cannabis sativa L.) The method consists of optional drying of stalks, cutting of straws, milling the straws with a hammer mill, separating fibres from shives with a drum separator, separating coarse shives from fine shives with a screen and a stream of air and finally cleaning of the fine shive with sieve vibrator. The described method works best for frost-retted plants and for unretted, green flax. The capillarity properties of these fractions were studied by measuring the amount of absorbed water as a function of time. The required time for the best-absorbing fibres of unretted and frost-retted hemp and unretted flax to absorb their own masses of water is less than 10 min compared with 2 h for the poorest-absorbing frost-retted flax. Overretting by frost retting seems to have an opposite effect on flax/linseed and hemp fibre capillarity. Both unretted and frost-retted coarse shive fractions behave quite similarly. If good absorption is needed, (over)retted hemp fibre should be used. If poor fibre absorption is desirable, (over)retted flax/linseed seems to have this feature.

Defect formation in implantation of crystalline Si by MeV Si ions

The distributions of vacancy‐type defects and displaced Si atoms in Si(100) produced by the room‐temperature implantation of 1014–1016 12‐MeV 28Si+ ions/cm2 are measured with low‐energy positron‐ and ion‐beam techniques. The observed damage regions are reproduced by computer simulations. The distribution of displaced Si atoms coincides with the deposited energy distribution in elastic collisions. At the fluence of 1×1016 Si+/cm2, no crystalline structure was found in the peak region of the deposited energy at the depth of z=6 μm. Saturation of the divacancy concentration was observed at the ion fluences 3×1015 Si+/cm2 close to the surface (z 1 μm). In the region z 1 μm. This is also found in the simulated spatial structure of collision cascades.

Calculation of collisional mixing

Collisional mixing of markers is calculated by splitting the calculation into two parts. Relocation cross sections have been calculated using a realistic potential in a Monte Carlo simulation. The cross sections are used in the computation of marker relocation. The cumulative effect of successive relocations is assumed to be an uncorrelated transport process and it is treated as a weighted random walk. Matrix relocation was not included in the calculations. The results from this two-step simulation model are compared with analytical models. A fit to the simulated differential relocation cross sections has been found which makes the numerical integration of the Bothe formula feasible. The influence of primaries has been treated in this way. When all the recoils are included the relocation profiles are nearly Gaussian and the Pearson IV distributions yield acceptable profiles in the studied cases. The approximations and cut-off procedures which cause the major uncertainties in calculations are pointed out. The choice of the cut-off energy is shown to be the source of the largest uncertainty whereas the mathematical approximations can be used with good accuracy. The methods are used to study the broadening of a Pt marker in Si mixed by 300 keV Xe ions, broadening of a Ti marker in Al mixed by 300 keV Xe ions and broadening of a Ti marker in Hf mixed by 750 keV Kr ions. The fluence in each case is 2 × 1016ionscm2. The calculated averages of half widths at half maximum vary between 11–18, 9–12 and 10–15 nm, respectively, depending on the cut-off energy and the mixing efficiencies vary between 11–29, 6–11 and 6–14 A5eV, respectively. The broadenings of Pt in Si and Ti in Al are about two times smaller than the measured values and the broadening of Ti in Hf is in agreement with the measured values.

Measuring Ventilation Rates in Dairy Buildings

Abstract Knowledge of ventilation rates in dairy buildings is essential for determining indoorair quality and for estimating green house gases and particle emissions. Two new methods for estimating ventilation rates are introduced for situations where air velocities atventilation inlets and outlets are tedious or impossible to measure. The first method is applicable to buildings whose ventilation can be stopped or closed totally. The second method is useful in naturally ventilated buildings with large openings and high ventilation rates where spatial gas concentrations are heterogeneously distributed. In addition, traditional heat balance, moisture balance, carbon dioxide balance and direct airflow measurements for ventilation estimation are used. Confirmation experiments were performed to evaluate the different methods. Good agreement was found between heat, moisture and carbon dioxide balances. Direct velocity measurement for ventilation rate estimation was found to be impractical for naturally ventilated buildings. The two introduced methods were found to be simple and adaptable for estimating ventilation rates in dairy buildings.

High energy ion beam mixing in dense collision cascades

Abstract Atomic mixing of tracer atoms in dense collision cascades in metals is calculated. The model used in the calculations describes the atomic transport from the initial collisional phase to the late thermalized stage. The collisional mixing is calculated by Monte Carlo simulation and a thermal spike model is used to describe the late phase of the cascade. The cooling of a thermal spike is described by coupled heat conduction equations for conduction electrons and lattice. Atomic transport takes place above the glass transition temperature of a metallic matrix and the transport is treated as diffusion in liquids. The heat exchange between conduction electrons and lattice allows rapid quenching of thermal spikes in metals with high d-electron density. Mass and heat transport coefficients are calculated on the basis of elementary solid state models, accordingly; the model for ion beam mixing contains no adjustable parameters Calculations for metal markers in Ni, Cu, Ag, Pd, Au and Pt are in agreement with experimental results, when the cohesive energy of marker atoms is sufficiently similar to that of matrix atoms. The model predicts a pronounced temperature dependence for the high-energy ion-beam mixing in Cu, Ag and Au.