Formation Of Defect Clusters Research Articles

Spatially dependent cluster dynamics model of He plasma surface interaction in tungsten for fusion relevant conditions

In fusion reactors, plasma facing components (PFC) and, in particular, the divertor will be irradiated with high fluxes of low-energy (∼100 eV) helium and hydrogen ions. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behaviour of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The PFC response and performance changes are intimately related to microstructural changes, such as the formation of point defect clusters, helium and hydrogen bubbles or dislocation loops. Computational materials' modelling results are described here that investigate the mechanisms controlling microstructural evolution in tungsten. The aim of this study is to understand and predict sub-surface helium bubble growth under high flux helium ion implantation (∼1022 m−2 s−1) at high temperatures (>1000 K). We report results from a spatially dependent cluster dynamics model based on reaction–diffusion rate theory to describe the evolution of the microstructure under these conditions. The key input parameters to the model (diffusion coefficients, migration and binding energies, initial defect production) are determined from a combination of atomistic modelling and available experimental data. The results are in good agreement with results of an analytical model that is presented in a separate paper. In particular, it is found that the sub-surface evolution with respect to bubble size and concentration of the helium bubbles strongly depends on the flux and temperature.

Radiation damage in cubic ZrO2 and yttria-stabilized zirconia from molecular dynamics simulations

We perform molecular dynamics simulation on cubic ZrO2 and yttria-stabilized zirconia (YSZ) to elucidate defect cluster formation resulting from radiation damage and evaluate the impact of Y dopants. Interstitial clusters composed of split-interstitial building blocks, i.e. Zr–Zr or Y–Zr were formed. Oxygen vacancies control cation defect migration; in their presence, Zr interstitials aggregate to form split interstitials, whereas in their absence Zr interstitials remain immobile, as isolated single-interstitials. Y-doping prevents interstitial cluster formation due to sequestration of oxygen vacancies.

The effect of annealing ambient on carrier recombination in boron implanted silicon

The selection of either an oxidising or inert ambient during high temperature annealing is shown to affect dopant activation and electron–hole recombination in boron implanted silicon samples. Samples implanted with B at fluence between 3 × 1014 cm–2 to 3 × 1015 cm–2 are shown to have lower dopant activation after oxidation at 1000 °C compared to an equivalent anneal in an inert ambient. In addition, emitter recombination is shown to be up to 15 times higher after oxidation compared with an inert anneal for samples with equivalent passivation from deposited Al2O3 films. The observed increase in recombination for oxidised samples is attributed to the enhanced formation of boron-interstitial defect clusters and dislocation loops under oxidising conditions. It is also shown that an inert anneal for 10 minutes at 1000 °C prior to oxidation has no significant impact on sheet resistance or recombination compared with a standard oxidation process. J0p+ as a function of sheet resistance for Al2O3 passivated samples after 1000 °C anneal for 70 minutes. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Spatially dependent cluster dynamics modeling of microstructure evolution in low energy helium irradiated tungsten

In fusion reactors, plasma facing components (PFC) and in particular the divertor will be irradiated with high fluxes of low energy (∼100 eV) helium and hydrogen ions. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behavior of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The PFC response and performance changes are intimately related to microstructural changes, such as the formation of point defect clusters, helium and hydrogen bubbles or dislocation loops. Computational materials modeling has been used to investigate the mechanisms controlling microstructural evolution in tungsten following high dose, high temperature helium exposure. The aim of this study is to understand and predict helium implantation, primary defect production and defect diffusion, helium-defect clustering and interactions below a tungsten surface exposed to low energy helium irradiation. The important defects include interstitial clusters, vacancy clusters, helium interstitials and helium-vacancy clusters. We report results from a one-dimensional, spatially dependent cluster dynamics model based on the continuum reaction–diffusion rate theory to describe the evolution in space and time of all these defects. The key parameter inputs to the model (diffusion coefficients, migration and binding energies, initial defect production) are determined from a combination of atomistic materials modeling and available experimental data.

Temperature Dependent Young's Modulus of Si and Ge

1. Introduction. Mechanical and thermal stresses play an important role in many of the processes used to produce electronic devices. For example, bulk Si and Ge single crystals are grown from a melt. Near the melt/solid interface high thermal stresses develop that can lead to dislocation formation and the loss of the crystal. As recently demonstrated, also thermal stress levels below the yield stress have an important impact on intrinsic point defect cluster formation in the crystal [1,2]. It is therefore surprising that few reports can be found on Young's modulus Eof Si and Ge at high temperatures [3].In the present paper, the impulse excitation technique (IET) [4] is used to determine E <100> , E <110> and E <111> between room and melt temperature of Si and Ge. IET measures the resonant frequency of a specimen by exciting it mechanically with an impulse tool. The resonance frequencies are characteristic for the sample material and are related to its stiffness, mass and geometry. The amplitude decay of a free vibration can be related to the internal friction, i.e. dissipation of the vibration energy or damping. Measurement of the resonance frequency allows the calculation of E. 2. Experimental. Czochralski-grown Si and Ge specimens are prepared with typical dimensions of about 45×7×2 mm3. To remove the mechanical damage, the samples are mirror etched. Before and after the IET measurement, the geometry and weight of each sample is measured accurately.The measurements are performed using a IMCE HTVP 1750 IET set-up equipped with automated impulse excitation and vibration detection devices. The samples are suspended in flexural mode inside the furnace using carbon wires at the bending nodes, which have minimum motions at these points. Mechanical excitation is performed by periodically hitting the sample with a ceramic rod. The acoustic wave generated in the sample is guided to a microphone outside the furnace using a alumina tube The samples are heated in argon atmosphere from room temperature up to melt temperature using heating rates between 3 and 7 ºC/min. 3. Results and discussion. The Young's moduli obtained in the present study are slightly above those of Burenkov et al as illustrated in the Figure showing the normalised E <111> versus the normalized temperature T /T m for Si and Ge obtained in this study [5.6], compared with the results of Burenkov et al [3]. The figure illustrates also that the temperature dependence of the elastic constants of both materials is very similar when expressed as function of temperature normalized with respect to T m as is also the case for some of the other material properties [7]. The experimental data can be reproduced well using a second order polynome E <ijk> = A 0 + A 1 T + A 2 T 2, (1)and the results of the best fits are listed in Table 1. The listed coefficients allow calculating the Si and Ge temperature dependent mechanical properties to be used in device and process simulations.Table 1: Coefficients of Eq. (1), with T in ºC and E <ijk> in GPa, obtained from best fits to experimental data [5,6]. A 0 A 1 (x10-3) A 2 (x10-6) Si <100>127.5- 6.3- 4.4<110>161.26- 7.00- 5.60<111>181.8- 7.9- 5.9 Ge <100>101.86- 8.78- 5.57<110>136.33- 10.85- 8.28<111>152.91- 11.76- 9.54 References [1] J. Vanhellemont, E. Kamiyama and K. Sueoka, ECS J. Solid State Sci. Technol. 2, P166 (2013).[2] K. Sueoka, E. Kamiyama, J. Vanhellemont and K. Nakamura, submitted for publication in ECS Solid State Letters. [3] Yu. A. Burenkov, S. P. Nikanorov and A. V. Stepanov, Soviet Phys. - Solid State 12, 1940 (1971).[4] G. Roebben, B. Bollen, A. Brebels, J. Van Humbeeck and O. Van der Biest, Rev. Sci. Instrum. 68, 4511 (1997).[5] J. Vanhellemont and E. Simoen, J. Electrochem. Soc. 154, H572 (2007). [6] A. K. Swarnakar, O. Van der Biest and J. Vanhellemont, Phys. Status Solidi C 11 , 150 (2014).[7] A. K. Swarnakar, O. Van der Biest and J. Vanhellemont, to be presented at Symposium X “Materials research for group IV semiconductors: growth, characterization and technological developments” of the 2014 EMRS Spring Meeting, to be published in Phys. Status Solidi C .

In-situ TEM observation of the response of ultrafine- and nanocrystalline-grained tungsten to extreme irradiation environments.

The accumulation of defects, and in particular He bubbles, can have significant implications for the performance of materials exposed to the plasma in magnetic-confinement nuclear fusion reactors. Some of the most promising candidates for deployment into such environments are nanocrystalline materials as the engineering of grain boundary density offers the possibility of tailoring their radiation resistance properties. In order to investigate the microstructural evolution of ultrafine- and nanocrystalline-grained tungsten under conditions similar to those in a reactor, a transmission electron microscopy study with in situ 2 keV He+ ion irradiation at 950°C has been completed. A dynamic and complex evolution in the microstructure was observed including the formation of defect clusters, dislocations and bubbles. Nanocrystalline grains with dimensions less than around 60 nm demonstrated lower bubble density and greater bubble size than larger nanocrystalline (60–100 nm) and ultrafine (100–500 nm) grains. In grains over 100 nm, uniform distributions of bubbles and defects were formed. At higher fluences, large faceted bubbles were observed on the grain boundaries, especially on those of nanocrystalline grains, indicating the important role grain boundaries can play in trapping He and thus in giving rise to the enhanced radiation tolerance of nanocrystalline materials.

Tensile properties of K-doped W–3%Re

In this work, the tensile properties of K-doped W–3%Re were investigated. This material was fabricated by powder metallurgy and hot rolling on an industrial scale. It is expected that there would be improvement of the high-temperature strength, an increase of the recrystallization temperature, and a decrease in the ductile–brittle transition temperature (DBTT) of pure tungsten due to the dispersion of K bubbles and the addition of 3% Re. In addition, suppression of the formation of irradiation-induced defect clusters is also expected. Tensile tests in the temperature range from room temperature to 1800°C were conducted. After the tensile tests, fracture surface observations were carried out using a scanning electron microscope (SEM). The tensile strength decreased with increasing test temperature. Elongation of K-doped W–3%Re was observed above 500°C. The results of fracture surface observation showed that delamination of the layered structure occurred at 500, 700, and 900°C and cracking along the grain boundaries occurred at 1500 and 1800°C.

Microstructural Evolution of an Ion Irradiated Ni&amp;ndash;Mo&amp;ndash;Cr&amp;ndash;Fe Alloy at Elevated Temperatures

The irradiation behavior of a Ni­Mo­Cr­Fe alloy, of the type currently being considered for use in future molten salt cooled reactors, has been investigated in situ using 1MeV Kr ions at temperatures of 723 and 973K. When irradiated to 5 dpa, experimental observations reveal the instantaneous formation and annihilation of point defect clusters, with such processes attributed to the long range elastic interactions that occur between defects through multiple intra-cascade overlap. Corresponding differences in the defect cluster density and size distribution suggest that changes to the microstructure were dependent upon temperature and dose, affecting the growth, accumulation and mobility of irradiationinduced defect clusters under these conditions. [doi:10.2320/matertrans.MD201311]

Lithium diffusion in congruent LiNbO3 single crystals at low temperatures probed by neutron reflectometry

The self-diffusion of lithium in congruent LiNbO3 single crystals was investigated at low temperatures between 379 and 523 K by neutron reflectometry. From measurements on (6)LiNbO3 (amorphous film)/(nat)LiNbO3 (single crystal) samples, Li self-diffusivities were determined in single crystals down to extremely low values of 1 × 10(-25) m(2) s(-1) on small length scales of 1-10 nm. The measured diffusivities are in excellent agreement with (extrapolated) literature data obtained by experiments based on Secondary Ion Mass Spectrometry and Impedance Spectroscopy. The tracer diffusivities can be described by a single Arrhenius line over ten orders of magnitude with an activation enthalpy of 1.33 eV, which corresponds to the migration energy of a single Li vacancy. A deviation from the Arrhenius behaviour at low temperatures, e.g., due to defect cluster formation is not observed.

Order and temperature dependence of surface blistering in H and He co-implanted Ge

The effect of implantation order and post-annealing temperature on surface blistering of H and He co-implanted germanium was investigated in the samples implanted with H and He in different orders (H first then He or He first then H). It was found that blisters were easily formed in H-alone sample after annealing at 350°C. However, with the increasing annealing temperature, the blistering phenomena were inclined to occur in the co-implanted samples, especially in He-first sample. The different behaviors of blister formation were closely correlated with the thermal stability of He implantation-induced damage. For low temperature annealing, most of the implanted He atoms were localized at the original site as implanted and the diffusion of He was efficiently suppressed, thus the blistering mainly originated from the platelets which were pressurized by implanted H. In consequence, blisters were more easily formed in H-alone sample in which the implantation dose of H was relatively higher than the co-implanted samples. However, for higher temperature annealing, the enhanced blistering behavior of co-implanted samples was attributed to an increasing contribution of He to the internal pressure of H-platelets. Compared to the He-first sample, the formation of large defect clusters with high vacancy/hydrogen ratio in the H-first sample may retard the blistering phenomenon.

On the origin and formation of large defect clusters in multicrystalline silicon solar cells

Large defect clusters can represent a serious reduction of the material quality of multicrystalline silicon and the efficiency of the resulting solar cells. It is useful to find the origin of these defect-rich regions in order to understand their formations. For this work, multicrystalline silicon wafers from different positions of a compensated p-type silicon brick were processed to solar cells with a homogeneous emitter and screen-printed metallization. The solar cells from the main part of the brick showed efficiencies between 15.6% and 16.1%. The characterization for this work focuses on the positions of the three largest defect clusters by means of detailed optical, crystal orientation and electrical loss measurements. This allows the localization of the defect clusters' origins during crystallization. The locations where the observed defect clusters started to grow, show similar crystal configurations. This implies that the large clusters formed preferentially at grain boundaries between specific grain orientations. A smaller cluster disappeared at a grain boundary. The characterization showed the same crystal configuration as for the three large growing clusters.

Properties of Point Defects in Silicon: New Results after a Long-Time Debate

The contributions of vacancies and self-interstitials to silicon (Si) self-diffusion are a matter of debate since many years. These native defects are involved in dopant diffusion and the formation of defect clusters and thus influence many processes that take place during Si single crystal growth and the fabrication of silicon based electronic devices. Considering their relevance it is remarkable that present data about the properties of native point defects in Si are still limited and controversy. This work reports recent results on the properties of native point defects in silicon deduced from self-diffusion experiments below 850°C. The temperature dependence of silicon self-diffusion is accurately described by contributions due to vacancies and self-interstitials assuming temperature dependent vacancy properties. The concept of vacancies whose thermodynamic properties change with temperature solves the inconsistency between self-and dopant diffusion in Si but further experiments are required to verify this concept and to prove its relevance for other material systems.

Crystallographic comparison of zirconium and molybdenum dioxide-based pyrochlores

For AO1.5–MoO2 and -ZrO2 equimolar systems (A = La, Nd, Sm, Gd, Dy, Y, Er, Lu), oxide compounds samples were prepared by solid-state sintering at 1673 K–1873 K in argon and air, respectively. In addition, oxide samples of yNdO1.5 − (1 − y)ZrO2 with y = 0.3–0.7 were prepared to investigate the pyrochlore–fluorite (P–F) phase transition. X-ray diffraction measurements clarified that molybdenum oxides formed cubic-pyrochlore phase except for A = La, and zirconate pyrochlores formed for A = La, Nd, Sm, Gd, whereas other zirconium oxides exhibited the defect-fluorite structure. With decreasing ionic radius of A3+, the broadening of Raman bands were observed for both pyrochlore systems. This might be due to the decrease of energy for defect cluster formation, i.e. cation antisite reaction and oxygen Frenkel defect formation. Especially for molybdate pyrochlores, it might be also attributed to the fact that the ionic charge of molybdenum was likely to be changeable. The P–F phase transition of the yNdO1.5 − (1 − y)ZrO2 system was clearly confirmed by Raman spectroscopy, and the changes in band shape were influenced mainly by the oxygen environment around Zr, which was elucidated by molecular dynamics calculations.

High-energy collision cascades in tungsten: Dislocation loops structure and clustering scaling laws

Recent experiments on in situ high-energy self-ion irradiation of tungsten (W) show the occurrence of unusual cascade damage effects resulting from single-ion impacts, shedding light on the nature of radiation damage expected in the tungsten components of a fusion reactor. In this paper, we investigate the dynamics of defect production in 150 keV collision cascades in W at atomic resolution, using molecular-dynamics simulations and comparing predictions with experimental observations. We show that cascades in W exhibit no subcascade break-up even at high energies, producing a massive, unbroken molten area, which facilitates the formation of large defect clusters. Simulations show evidence of the formation of both and interstitial-type dislocation loops, as well as the occurrence of cascade collapse resulting in vacancy-type dislocation loops, in excellent agreement with experimental observations. The fractal nature of the cascades gives rise to a scale-less power-law–type size distribution of defect clusters.

The Effects of Annealing on the Inter-diffusion of Cations Across the LaCrO3/SrTiO3(100) Interface, and the Formation of La Anti-site Defect Clusters

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

First principle-based AKMC modelling of the formation and medium-term evolution of point defect and solute-rich clusters in a neutron irradiated complex Fe–CuMnNiSiP alloy representative of reactor pressure vessel steels

The formation and medium-term evolution of point defect and solute-rich clusters under neutron irradiation have been modelled in a complex Fe–CuMnNiSiP alloy representative of RPV steels, by means of first principle-based atomistic kinetic Monte Carlo simulations. The results obtained reproduce most features observed in available experimental studies, highlighting the very good agreement between both series. According to simulation, solute-rich clusters form and develop via an induced segregation mechanism on either the vacancy or interstitial clusters, and these point defect clusters are efficiently generated only in cascade debris and not Frenkel pair flux. The results have revealed the existence of two distinct populations of clusters with different characteristic features. Solute-rich clusters in the first group are bound essentially to interstitial clusters and they are enriched in Mn mostly, but also Ni to a lesser extent. Over the low dose regime, their density increases in the alloy as a result of the accumulation of highly stable interstitial clusters. In the second group, the solute-rich clusters are merged with vacancy clusters, and they contain mostly Cu and Si, but also substantial amount of Mn and Ni. The formation of a sub-population of pure solute clusters has been observed, which results from annihilation of the low stable vacancy clusters on sinks. The results indicate finally that the Mn content in clusters is up to 50%, Cu, Si, and Ni sharing the other half in more or less equivalent amounts. This composition has not demonstrated any noticeable modification with increasing dose over irradiation.

Irradiation behavior evaluation of oxide dispersion strengthened ferritic steel cladding tubes irradiated in JOYO

Irradiation behavior of ODS steel cladding tubes was evaluated for the further progress in understanding of the neutron-irradiation effects on ODS steel.Two types of ODS (9Cr–ODS_F/M, 12Cr–ODS_F) steel cladding tubes with differences in basic compositions and matrix phases were irradiated in JOYO. Post-irradiation examination data concerning hardness, ring tensile property, and microstructure were obtained.Hardness measurement after irradiation showed that there was an apparent irradiation temperature dependence on hardness for 9Cr–ODS_F/M steel whereas no distinct temperature dependence for 12Cr–ODS_F steel. Also, there was no significant change in tensile strengths after irradiation below 923K, but those above 1023K up to 6.6×1026n/m2 (E>0.1MeV) were decreased by about 20%. TEM observations showed that the radiation-induced defect cluster formation during irradiation was suppressed because of high density sink site for defect such as initially-existed dislocation, and precipitate interfaces. In addition, oxide particles were stable up to the maximum doses of this irradiation test.

Displacement damage rate dependence of defect cluster formation in α-Fe during irradiation

Formation kinetics of defect clusters in pure iron during irradiation has been numerically investigated by reaction rate theory, with focusing on nucleation process of vacancy clusters (voids) and self-interstitial-atoms (SIA) clusters under a wide range of atomic displacement damage rate (dpa rate) and temperature conditions. In the rate theory model, the size dependence of thermal stability of a defect cluster is treated for a wide range of cluster size. The numerical analysis shows that the nucleation processes of voids and SIA-clusters are quite different from each other. As to the voids, the nucleation rate of voids depends much on temperature and dpa rate, and has the individual peak temperature for each dpa rate, during which the peak temperature increases with increasing dpa rate. This tendency for void nucleation is similar to that for void swelling observed in experiments. As to the SIA-clusters, the nucleation rate of SIA-clusters does not depend much on temperature and has no peak temperatures because of the relatively high thermal stability of an SIA-cluster, indicating that the conventional model (di-interstitial model) is applicable to describe the nucleation of SIA-clusters in a wide range of temperature.

Effect of crystal structure on the luminescence properties of CaF2-PrF3 solid solutions

The structure and luminescent properties of lightly doped CaF2:Pr crystals and Ca0.65Pr0.35F2.35 solid solution have been studied. The results demonstrate that the evolution of the emission spectra of the crystals with increasing PrF3 concentration can be understood in terms of the structural changes in the fluorite solid solutions, namely, the formation of defect clusters.

Defects clustering and ordering in di- and trivalently doped ceria

The formation and growth of defect clusters in CeO2M2O3 (M=La3+, Pr3+, Sm3+, Gd3+, Dy3+, Y3+, Yb3+) and CeO2DO (N=Cd2+, Ca2+, Sr2+, Ba2+) binary solid solutions have been comparatively studied by atomistic simulations based on energy minimization of atomic interactions. The calculation ensemble, including both divalent and trivalent dopants, shows a similar energetic tendency for defect species (dopants and associated oxygen vacancies) to aggregate and grow. The dumbbell structure has been verified as a universal vacancy structure in oxygen deficient fluorite lattice. Nevertheless, it also demonstrates different tendencies of dopant–vacancy associations that depend on dopant valence and radius. The dopant is site-selective in trivalent defect clusters correlated with dopant size. While in divalent solid solutions, clusters adopt similar dopant–vacancy locations. Furthermore, all clusters in divalently doped ceria have stronger dopant–vacancy associations/interactions compared to those in trivalent ones. As a consequence, the correlation of the dopant size as well as valence effects on oxygen-ion conductivity has been illustrated based on an ordered defect cluster model. This study thereby offers insight into the physical picture of ionic conductivity behavior experimentally obtained in aliovalently doped ceria.