Search results for "defect"

Variation of lattice constant and cluster formation in GaAsBi

2013

We investigate the structural properties of GaAsBi layers grown by molecular beam epitaxy on GaAs at substrate temperatures between 220–315 C. Irrespective of the growth temperature, the structures exhibited similar Bi compositions, and good overall crystal quality as deduced from X-Ray diffraction measurements. After thermal annealing at temperatures as low as 500 C, the GaAsBi layers grown at the lowest temperatures exhibited a significant reduction of the lattice constant. The lattice variation was significantly larger for Bi-containing samples than for Bi-free low-temperature GaAs samples grown as a reference. Rutherford backscattering spectrometry gave no evidence of Bi diffusing out o…

The kinetics of F-center aggregation under irradiation: many-particle effects in ionic solids

1994

The accumulation kinetics of primary Frenkel defects created in solids under permanent irradiation is calculated using the microscopic formalism of many-particle densities. It is based on the Kirkwood superposition approximation for three-particle densities as described in our previous paper p. N. Kuzovkov and E. A. Kotomin, Physica Scripta 47, 585 (1993)l. This formalism is generalized in this paper by incorporating the elastic attraction between similar defects (called in ionic solids F-centers) which causes their efficient aggregation. It is shown that the aggregation process starts only if the dose rate and elastic attraction energy exceed certain critical values; it also happpens in th…

A mesoscopic approach to point-defect clustering in solids during irradiation

1993

Accumulation of point defects in solids during irradiation is often accompanied by self-organization processes which lead to point-defect clustering and thus to the formation of a spatially inhomogeneous defect structure. Within the framework of a mesoscopic phenomenological approach, the conditions for clustering of mobile point defects caused by their elastic interactions are studied. It is shown that differences between the elastic interaction of similar and that of dissimilar defects may lead to such clustering. Further, it is shown that the presence of impurities acting as traps for interstitials may promote the clustering process. The conditions for spatial clustering are studied for …

The Deformation of Single, Nanometer-Sized Metal Crystals in Graphitic Shells

2005

Discrete-lattice theory for Frenkel-defect aggregation in irradiated ionic solids

1998

Institut fu ¨r Physikalische und Theoretische Chemie, Technische Universitat Braunschweig, D-38106 Braunschweig, Germany~Received 11 September 1997; revised manuscript received 6 April 1998!A microscopic theory of diffusion-controlled aggregation of radiation Frenkel defects—called in ionic solidsH and F centers—is presented. This is based on a discrete-lattice formalism for the single defect densities~concentrations! and the coupled joint densities of similar and dissimilar defects treated in terms of a modifiedKirkwood superposition approximation. The kinetics of defect aggregation is studied in detail; the cooperativecharacter of this process for both types of complementary defects is sho…

The methylenetetrahydrofolate reductase C677T polymorphism and the risk of congenital heart diseases: a literature review

2014

Congenital Heart Diseases (CHDs) are the most commonand serious developmental anomaly and the leading non-infectious cause of mortality in the first year of life. Despite the advances in diagnosis and treatment, understanding of the developmental causes and aetiologies of CHDs has been limited. The hyperhomocysteinemia is one of the proved risk factors related to the occurrence of CHDs. The connection between cardiac defects, folate and hyperhomocysteinemia could be explained by a mutation in the methylenetetrahydrofolate reductase (MTHFR) gene. Indeed, the C677T MTHFR mutation produces a thermolabile variant of MTHFR with reduced enzymatic action resulting in higher plasma levels of homocy…

Self-assembly of semiflexible polymers confined to thin spherical shells

2018

Confinement effects are critical for stiff macromolecules in biological cells, vesicles, and other systems in soft matter. For these molecules, the competition between the packing entropy and the enthalpic cost of bending is further shaped by strong confinement effects. Through coarse-grained molecular dynamics simulations, we explore the self-assembly of semiflexible polymers confined in thin spherical shells for various chain lengths, chain stiffnesses, and shell thicknesses. Here, we focus on the case where the contour and persistence length of the polymers are comparable to the radius of the confining cavity. The range of ordered structures is analyzed using several order parameters to …

Calculation of bone graft volume using 3D reconstruction system.

2010

Objectives: To obtain bone ?bioreplicas? for determining precisely the amount of biomaterial required for bone regeneration procedure. Study design: A case-control comparison with a total sample size of 20 cases, 10 control and 10 test samples. ?Bioreplicas? were generated from helical CAT scans with 0.5 mm slices, without 3D reconstruction or image filters. Bone defects in premolar and molar areas were treated with titanium mesh and xenograft (MP3) combined with resorbable carriers. Time taken to carry out procedures with and without the use of ?bioreplicas? (in minutes), discrepancies between the grafts performed with and without ?bioreplicas? and bone defects (mm), and postoperative comp…

Periodontal disease affecting tooth furcations. A review of the treatments available

2007

The molars are the teeth that suffer the greatest periodontal destruction in untreated patients. When periodontal disease affects the furcation of a tooth, the chance that it will be lost increases considerably. An increase in the exposed root surface, anatomical peculiarities and irregularities of the furcation surface all favor the growth of bacteria. These problems make it harder for the patient to maintain hygiene, and impede adequate treatment. The treatment of furcations affected by periodontal disease is one of the most difficult problems for the general dentist and periodontist. The motivation of both the attending professional and of the patient are therefore of great importance. N…

Evaluation of a porous hydroxyapatite alloplast in the management of grade II furcation defects in molars: a case series

2012

Introduction: The present case series evaluates the efficacy of a hydroxyapatite graft material in the management of grade II furcation involvement in first molars. Material and methods: Eight patients presenting with a total of 9 grade II furcation defects in relation to the facial aspect of either maxillary or mandibular first molars were treated with a porous hydroxyapatite alloplast. The clinical outcomes were measured in terms of change in probing pocket depth and clinical attachment level (vertical and horizontal) at 6 months post-operatively. Radiographs were used as supplements. Results: At 6 months, there was a mean pocket depth reduction of 3.12±1.25 mm, a mean vertical attachment…