Unstable Defects Research Articles

Magnetic properties of Co-doped ZnO diluted magnetic semiconductors prepared by low-temperature mechanosynthesis

We report on the magnetic and structural properties of 2% and 4% Co-doped ZnO diluted magnetic semiconductors prepared by mechanosynthesis. We show that this low-temperature preparation technique does not allow obtaining ferromagnetism in such materials, as already observed in samples prepared by high-temperature techniques. Although X-ray diffraction and UV–Vis spectroscopy indicate that Co ions are well inserted in the ZnO würtzite structure and that mechanosynthesis is susceptible to generate thermodynamically unstable defects, ferromagnetism observed at 5 and 295 K is systematically attributed to parasitic phases. The pure ZnO:Co phase presents no ferromagnetism in agreement with the high temperature prepared samples.

Control of Spiral Instabilities in Reaction-Diffusion Systems

Spiral instabilities and their controls are investigated in a reaction-diffusion system using the Belousov-Zhabotinsky reaction. Two spiral instabilities, the long-wavelength in- stability and the Doppler instability, are reported, which can lead to spatiotemporal chaos. The long-wavelength instability occurs in an oscillatory regime, while the Doppler instabil- ity occurs in an excitable regime. To control these two instabilities, two different strategies are proposed according to their defect-generating mechanisms. For the long-wavelength in- stability in an oscillatory system, the control can be achieved by introducing a local pace- maker, which emits stable traveling waves to sweep off the unstable spiral defects. For the Doppler instability, the control can be achieved by trapping the spiral tip with a local area of higher diffusion coefficient than its surroundings.

Iliosacral Resection for Primary Bone Tumors

Iliosacral resection for primary bone tumors creates a large unstable pelvic ring defect, the treatment of which remains controversial. We did this study to determine if skeletal reconstruction of such defects is necessary. Sixteen patients whose data were collected prospectively had iliosacral resection with a minimum followup of 12 months. The surgical and functional results of patients who had skeletal reconstruction (n = 4) were compared with the results of patients who did not have iliosacral repair (n = 12) using a case-control design. Function was evaluated by assessing impairment using the Musculoskeletal Tumor Society 1987 and 1993 rating scales, and disability was measured using the Toronto Extremity Salvage Score. Although all four iliosacral arthrodeses initially healed, one allograft used for reconstruction fractured and another was removed because of progressive lumbosacral spinal instability. Patients treated without pelvic reconstruction had fewer operative complications. Although the Toronto Extremity Salvage Score and the Musculoskeletal Tumor Society 1987 and 1993 scores were similar for both patient groups, those patients who were treated without reconstruction were less likely to require the use of an ambulatory assistive device, less likely to require narcotics or have chronic pain, and more likely to return to work. These results suggest that reconstruction of the skeletal defect to restore pelvic stability after iliosacral resection is not mandatory. Therapeutic study, Level III-1 (case-control study). See the Guidelines for Authors for a complete description of levels of evidence.

Control of spiral instabilities in reaction-diffusion systems

Abstract Spiral instabilities and their controls are investigated in a reaction-diffusion system using the Belousov-Zhabotinsky reaction. Two spiral instabilities, the long-wavelength instability and the Doppler instability, are reported, which can lead to spatiotemporal chaos. The long-wavelength instability occurs in an oscillatory regime, while the Doppler instability occurs in an excitable regime. To control these two instabilities, two different strategies are proposed according to their defect-generating mechanisms. For the long-wavelength instability in an oscillatory system, the control can be achieved by introducing a local pacemaker, which emits stable traveling waves to sweep off the unstable spiral defects. For the Doppler instability, the control can be achieved by trapping the spiral tip with a local area of higher diffusion coefficient than its surroundings.

Dose rate effect on the yield of radiation induced response with thermal fading

A model describing the dependences of the accumulation of thermally unstable radiation induced defects on the dose and dose rate is proposed. The model directly takes into account the track nature of the ionizing radiation represented as accumulation processes of defects in tracks averaged over a crystal volume considering various degrees of overlapping in space and time. The accumulation of the defects in the tracks is phenomenologically described. General expressions are obtained that allows radiation yield simulation of defects involving known creation and transformation processes. The cases considered, of linear accumulation (constant increment of the defects in tracks) and accumulation with saturation (complete saturation of the defects in one track), lead to a set of linear dose dependences with saturation, which are routinely used in luminescence and ESR dating. The accumulation, with increase of sensitivity in regions overlapped by two or more tracks, gave a set of dose dependences, from linear-sublinear-linear-saturation, distinctive of quartz up to linear-supralinear-linear-saturation. It is shown that the effect of the dose rate on dose dependences is determined by a dimensionless parameter a = P τ / D 0 , where P is the dose rate, τ is the defect lifetime and D 0 is the track dose. At a ⪡ 1 the dose rate influences basically the accumulation of thermally unstable defects. In the reverse case the dose dependences did not seems to be influenced by the dose rate.

Saddle point for oxygen reorientation in the vicinity of a silicon vacancy

A piezospectroscopic analysis of the vacancy-oxygen complex in silicon has enabled us to demonstrate that this defect in the unstable configuration of the saddle point on the reconfiguration trajectory has a trigonal symmetry. This unstable defect configuration may be considered as the precursor for an oxygen diffusion process where the migrating oxygen atom is accompanied by a vacancy. The trigonal saddle point configuration results in a strong electrical polarization of the pair which can aid the jumping to a neighboring unit cell. This scenario is very plausible to explain how vacancies can drag oxygen atoms through the crystal to form larger oxygen aggregates.

Effect of the location of Mn sites in ferromagneticGa1−xMnxAson its Curie temperature

We report a strong correlation between the location of Mn sites in ferromagnetic ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ measured by combined channeling Rutherford backscattering and by particle-induced x-ray emission experiments and its Curie temperature. The concentrations of free holes determined by electrochemical capacitance-voltage profiling and of uncompensated ${\mathrm{Mn}}^{2+}$ spins determined from superconducting quantum-interference device magnetization measurements are found to depend on the concentration of unstable defects involving highly mobile Mn interstitials. This leads to large variations in ${T}_{C}$ of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ when it is annealed at different temperatures in a narrow temperature range. The fact that annealing under various conditions has failed to produce Curie temperatures above \ensuremath{\sim}110 K is attributed to the existence of an upper limit on the free hole concentration in low-temperature-grown ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}.$

Mosaicplasty for the treatment of osteochondritis dissecans of the talus: two to seven year results in 36 patients.

An osteochondral defect (OCD) is known as a symptomatic lesion causing pain, recurrent synovitis, and altered joint mechanics most commonly in a weight-bearing joint. Loose bodies may develop, which may then cause joint destruction and/or locking. The damage to the articular surface is most likely a precursor of ankle osteoarthritis. With the recent advances in diagnostic imaging, such as MRI, as well as the development of ankle arthroscopy, the identification and classification of these lesions has become much more precise. This allows more accurate staging and improves treatment recommendations. The assessment of a particular treatment is also improved. A variety of treatment alternatives are now available. These include arthroscopic procedures including debridement, retrograde drilling, and bone grafting. Compared to open treatment, arthroscopic procedures may be particularly advantageous in the treatment of small defects and stable OCD lesions. Until recently, however, favorable results have been less predictable for large or unstable osteochondral defects. We treat these more difficult lesions with a mosaic autogenous osteochondral transplantation. In our hands, this appears to provide an optimal treatment result. The present report evaluates the clinical outcome of 36 patients followed for two to seven years after a mosaicplasty autogenous osteochondral transplantation from a non or less weight bearing portion of the knee to the ipsilateral talus. Ankle function was measured by the Hannover scoring system and showed good to excellent results in 34 cases (94%) with no long term donor site morbidity. The encouraging clinical results are supplemented with radiographs and histology, which support the premise of lasting relief of symptoms and prevention of ankle arthrosis.

Arthroscopically assisted autologous osteochondral transplantation for osteochondral lesions of the talar dome: an MRI and clinical follow-up study.

Osteochondral Lesions of the Talar Dome (OLT) are common problems encountered in orthopedics. Although the etiology remains uncertain, a myriad of treatment options exists. The authors describe arthroscopically assisted autologous osteochondral graft (OCG) transplantation procedures in the treatment of unstable OLTs in nine patients. The patients underwent standard preoperative MRI examination to assess fragment stability (using De Smet criteria for stability). Intraoperative arthroscopy was used to correlate the preoperative MRI assessment (using Cheng/Ferkel grading). After transplantation procedures, MRI (using De Smet criteria for stability) assessed graft incorporation for stability at an average of 9.3 months after the procedure. Preoperative MRI correlated highly with arthroscopic findings of OLT instability (sensitivity = 1.0). This has been demonstrated in the current orthopedic literature. The post transplantation MRI demonstrated stable graft osteointegration by De Smet criteria in all patients. Postoperative visual analogue pain scales showed significant improvement from preoperative assessment. Postoperative AOFAS Ankle-Hindfoot scores averaged 80.2 (S.D. +/- 18.9). Our favorable early results and those of other authors using similar techniques may validate OCG transplantation as a viable alternative for treating unstable osteochondral defects in the talus that are refractive to more commonly used surgical techniques.

MECHANODEGRADATION AND STABILIZATION OF POLYVINYL CHLORIDE DURING VIBROMILLING

The color development of polyvinyl chloride (PVC) during vibromilling and its mechanism, as well as the stabilization method, were studied. Investigations showed that the color development of PVC resulted from its dehydrochlorination reaction, which was caused by the existence of unstable structural defects in the molecular structure of PVC and gross heat generation at the contact interface due to the dissipation of high-frequency impact energy during milling. Three often-used heat stabilizers of PVC—barium-cadmium, lead sulphate, and bis(sulphoisoactyl glycollate)di(n-octyl)tin—were not very effective in preventing color development of PVC during vibromilling. A new stabilizer, designated as VSG96, was developed that was designed to guarantee that PVC sample retain its original color and chlorine content after it had been vibromilled for 120 hr or more.

Mechanisms of creation and destruction of polyhalide V centers in irradiated AHC

The mechanisms of creation and destruction of polyhalide V centers hz AHC were modeled by the MNDO semiempirical quantum-chemical method. It is shown that X ~ molecules dissociate from the ground state into X~ ~ and X- and from the triplet state into X ~ and X o. whereas X ~ and X 7 molecules dissociate into X ~ and X, ~ and X~ and X~ , respectively. At room temperatures. X; or larger aggregates, which consist only of X; centers, form stable polyhalide V centers. It is shown that the thermal stability decreases when going along the series X f, X ~, and X 7 . The radiation-induced instability of stable F centers in AHC at room temperatures is associated with the desmuction of X ~ centers and the thermal instability of X ~ and X 7 centers. The nature of creation of radiation-induced effects in AHC was clarified in detail when experiments on AHC irradiated by electron (or photon) pulses were performed, in which the absorption spectra of radiation-induced defects were registered practically immediately alter their creation [ 1-3]. An analysis of this transient absorption under irradiation at liquid-helium temperatures has unambiguously shown which radiation-induced defects are primary and which are formed by subsequent secondary conversions. Based on the data obtained by the method Of measuring the nonsteady-state absorption, the authors of [4, 5] have shown that 'the creation of unstable defects and the accumulation of stable detects occur according to different laws. By the time the electron pulse ends, absorption bands corresponding to F and H centers are detected in one group of crystals (KCI, RbCI, etc.), whereas bands characteristic of sell-trapped excitons (STE) appear in the other group of crystals (NaC1, KI, etc.). Moreover, it has been shown for many AHC that there are stable and unstable modifications of originally created F, H pairs and STE, and that the unstable component accounts for 85-90% of their number. With increase in temperature, the induced absorption of F, H pairs and STE changes and indicates the decrease of short-lived pairs and excitons and the increase of long-lived ones. Except for vacancy-type detects (co and F centers), interstitial detects having the structure of molecular ions of the X~ family (H, Vk, and V~ centers) and X3 (Vz and V7 centers) have been created by the time the pulse ends. It has been shown that at temperatures 160-250 K tbr RbC1 and 10(O150 K/or KI, the yield of stable F centers significantly increases. With further increase in the temperature up to 500 K, the F-center concentration varies only insignificantly. Under stationary irradiation, the radiation-induced defects are accumuhued as a result of a number of consecutive independent processes: the formation of the primary Frenkel pair, the spatial separation of the components of the pair, and the conversion of separate components of the pair to stable defects. Because dach process depends specifically on the irradiation temperature, the dependence of the total yield of radiation-induced defects on this parameter also appears to be complex. The dependence of the efficiency of F-center accumulation in KC1 on the irradiation temperature, generalized in [6] based on the data of different authors, has the following lorm: a significant decrease at 25-50 K followed by a plateau with the low efficiency of defect survival and a significant increase between 100 and 200 K: above 200 K, the efficiency of Fcenter accumulation decreases. The temperature dependence is due to the change of the tbrm of survival of defects that are halides by their nature and additional to F centers. F centers remain the main radiation-induced electron detects for the entire range of the examined temperatures. Thus, from the experimental data obtained on exposure to pulsed and continuous radiation it can be seen that complex processes, associated with the interaction of radiation-induced defects with each other and with electron excitations that are continuously created iu a crystal during irradiation, proceed in AHC. These processes have opposite effects in the sense that some of them head to the accumulation of defects, whereas others - to mutual annihilation of defects, thereby decreasing the rate of their accumulation.

Leakage currents induced in ultrathin oxides on (100)Si by deep-UV photons

Degradation of ultrathin (dox=3–6 nm) oxides on (100)Si under irradiation with deep-UV (hν=10 eV) photons was studied in order to evaluate the possible oxide damage by deep-UV lithography. The irradiation was found to lead to a significant enhancement in electrical conduction of the ultrathin oxides. The leakage current observed after irradiation correlates with the concentration of radiolytic atomic hydrogen in the oxide and includes two components. A first transient one is related to thermally unstable defects similar to the H-related donor interface states observed after irradiation of thicker oxides. The second, stable component correlates with the enhanced density of Si dangling bond defects in the oxide (E′ center) which points to the H-assisted rupture of SiO bonds. It is also found that the leakage currents observed after electrical stress and after deep-UV irradiation exhibit similar annealing behavior. The two revealed sources of the degradation-induced leakage current may account for the enhanced electron transport across ultrathin Si oxides under various circumstances.

Reversible thermal behavior of the layered double hydroxide of Mg with Al: mechanistic studies

The layered double hydroxide (LDH) of Mg with Al decomposes at 450°C to yield a mixed metal oxide. The oxide reconstructs back to the parent LDH either on cooling in air or soaking in water. This reversible thermal behaviour was attributed to the formation of an unstable defect rocksalt phase (comprising Al3+ ions partially substituting for Mg2+ in MgO) on decomposition of the LDH. However a simple oxide mixture of MgO and Al2O3 taken in the 6∶1 molar ratio is also found to yield a LDH-like phase when soaked in an aqueous solution of a suitable anion, suggesting that the reconstruction occurs through a simple dissolution–reprecipitation mechanism. The formation of the defect rocksalt phase is not a necessary precondition for reconstruction. However, the kinetics of the reconstruction reaction depends upon the nature of the anion and the thermal history of the oxides. The presence of carbonate ions and unsintered oxides hastens the reconstruction reaction.

Hydrogen‐Related Leakage Currents Induced in Ultrathin SiO2 / Si Structures by Vacuum Ultraviolet Radiation

Defects generated by 10 eV photons in ultrathin (3‐6 nm) layers on Si are studied using electron spin resonance and laser photoionization spectroscopy in relation to the observed enhancement of electrical conduction of the oxide. The leakage current is found to include two components: a transient one, related to thermally unstable defect states, and a thermally stable portion. The former current behavior strongly resembles that of the donor interface states observed in structures with thick oxides and is associated with bonding of atomic H in the oxide layer. The stable component correlates with the H‐assisted oxide damage resulting in Si‒O bond rupture. The exposed similarity between leakage currents observed after irradiation and after electrical stress suggests the hydrogen bonding in these two states to be responsible for the dielectric failure of under stress conditions. © 1999 The Electrochemical Society. All rights reserved.

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–104) A/cm2. It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities.

Study of luminescent centers annealing in wide bandgap semiconductors: Color cathodoluminescence-scanning electron microscopy

A color cathodoluminescence-scanning electron microscopy (CCL-SEM) technique was used to study the annealing of luminescent centers in wide bandgap semiconductors (mainly silicon carbide), due to native defects, induced during the growth process or to radiation by high-energy particles. The significant role of dislocations in the crystals was revealed. These dislocations are created by the mechanical failure of crystals and play the role of getters for unstable defects and assist in quenching the luminescence at fairly low annealing temperatures. During the electron beam annealing of the samples, the resolved CL fine structure of luminescence images was measured around failure regions which look like a rosette of strength* at temperatures of 1300–1350°C.

Unstable defects in metals: On some surprising phenomena at high temperatures

The discrepancy between the number of vacancies determined at equilibrium and that determined for quenched samples, as well as anomalies in thermal properties and diffusion in high temperature metals, are discussed qualitatively in terms of a model based on short-living unstable pairs (i.e. a vacancy with an associated interstitial atom).

Kinetic Lattice Monte Carlo Simulations of Cascade Aging in Iron and Dilute Iron-Copper Alloys

AbstractNeutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster- complexes constitute both thermally stable and unstable matrix defect features.

Kinetic Lattice Monte Carlo Simulations of Cascade Aging in Iron and Dilute Iron-Copper Alloys

AbstractNeutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster-complexes constitute both thermally stable and unstable matrix defect features.

Unstable displacement defects and hydrogen trapping in GaAs

Defects produced by implantation of hydrogen into GaAs at 80 K have been investigated. Upon implantation, a predominant absorption band at 2029 cm{sup {minus}1} for As-H centers is observed. A 10-cm{sup {minus}1} full width at half maximum at 80 K for the band, and strong dependence of the frequency and bandwidth on measurement temperature, are indicative of bonding of hydrogen at displacement defects within the lattice. Results from annealing show loss of As-H centers between 180 and 250 K with an activation energy of 0.5{plus_minus}0.5 eV. Characteristics for As-H center annealing are compared to those reported previously for atomic displacement disorder in irradiated GaAs. Release of hydrogen from As-H bonds within thermally unstable damage regions is suggested to explain the annealing loss of As-H centers. An increase in absorption by Ga-H centers upon loss of As-H centers is attributed to retrapping of hydrogen on Ga neighbors of V{sub As} or on V{sub As}-As{sub i} pair defects. {copyright} {ital 1997} {ital The American Physical Society}