Average Ring Size Research Articles

Gamma‐ray irradiation effect on microstructure and physical performances of porous silica

AbstractIn this paper, the gamma irradiation effect on the microstructure and physical performances of porous silica, including mechanical, thermal, and optical performances, are systematically investigated by using molecular dynamics and density‐functional theory‐based methods. The study of bond angle distribution, pair distribution function, coordination number distribution, and average ring size distribution show that, after gamma‐ray irradiation, the microstructure of porous silica is obviously modified. The tight packing of SiO2 tetrahedrons in the porous silica network is broken by gamma‐ray irradiation. Defects of three‐coordinated Si and non‐bridging oxygen are induced by gamma‐ray irradiation. Moreover, we find that the defects concentrations rapidly grow as gamma‐ray dose increases. The mechanical, thermal, and optical performances of porous silica are all seriously degenerated by gamma‐ray irradiation. Our results show that, for mechanical performance, Young's modulus, Bulk modulus, and Shear modulus first decrease and then keep stable as gamma‐ray dose increases, but the change of Poisson's ratio is slight. For thermal performance, the thermal conductivity decreases exponentially as gamma‐ray dose increases. For optical performance, light absorption coefficients increase exponentially and light transmittance drops as gamma‐ray dose increases in the working range (photon energy range around 3.5 eV) of inertial confinement fusion. Present work is expected to be valuable for studying the degradation mechanism of silicate materials under gamma radiation and developing gamma‐ray irradiation protection technology.

Aortic Root Enlargement during Aortic Valve Replacement in Adults

Aortic valve replacement is a gold standard in the treatment of patients with severe aortic stenosis or combined aortic pathology. However, aortic valve pathology is often associated with a narrow aortic orifice, particularly in patients with severe aortic stenosis. In 1978, Rahimtoola first described the term of prosthesis-patient mismatch. He noted that effective orifice area of the prosthesis is smaller than that of the native valve. To minimize this complication, there are several surgical strategies: aortic root enlargement (ARE), implantation of a frameless biological prosthesis in the native position, neocuspidalization procedure, Ross procedure, aortic root replacement with xenograft or homograft. ARE is an excellent option, however, some authors outline additional perioperative risks.
 The aim. To analyze immediate results of ARE during isolated aortic valve replacement and in cases when it is combined with other heart pathologies.
 Materials and methods. Our study included 63 patients who underwent ARE. Isolated aortic valve replacement was performed in the majority of cases, but often aortic root replacement procedure was combined with coronary artery bypass grafting.
 Results and discussion. One of 63 patients died (hospital mortality 1.6%) at an early hospital stage (30 postoperative days). Measurement of the aortic valve ring was performed by two methods, through preoperative echocardiography and perioperative measurement using a valve sizer. However, perioperative dimension was chosen as the basis for the calculations. In 62 patients, the perioperative diameter of the aortic valve ring ranged from 19 to 23 mm, only one patient had a diameter of 24 mm. According to our findings, ARE enabled to achieve an average aortic ring size increase of 2.68 cm2 (from 1.5 to 3.4 cm2) and to prevent prosthesis-patient mismatch in 42 (66.7%) cases.
 Conclusions. Prosthesis-patient mismatch is considered a serious complication in the postoperative period. Narrow aortic root is a common pathology that should be considered during surgery. ARE is a safe procedure and is not associated with an increased risk of mortality and complications.

Relationships between Changes in Guest Ion Properties and in the Host Framework Topology in Ionic Coordination Polymers

In order to identify possible general relationships between changes in the framework topology and changes in the extra framework ion properties, a set consisting of 2202 crystal structures of ionic coordination polymers was extracted from the Cambridge Structural Database. Changes in ion properties served as independent variables for several machine learning models trained to predict the changes in framework dimensionality, topological density, and average ring size of the framework’s tiling. The trained classifiers showed acceptable predictive performance with F1 score in the range 0.4 ÷ 0.6 and were subjected to the validation tests, which confirmed that they fit the data significantly better than by chance. Subsequent feature importance analysis of the classifiers revealed a set of the ion properties being important for prediction of the changes in corresponding framework characteristics in the extracted set of crystal structures. It is shown that in general changes in molecular surface area and molecular flexibility of the guest ions are essential for predicting changes in selected topological characteristics of a framework. Case studies were conducted for several sets of crystal structures with frameworks that are observed to host significant variety of counter ions. The decision tree classifiers allowed us to discover the ion properties determining topological characteristics in particular frameworks.

Predicting the dissolution rate of borosilicate glasses using QSPR analysis based on molecular dynamics simulations

AbstractQuantitative Structure Property Relationship (QSPR) analysis based on molecular dynamics (MD) simulations is a promising approach for establishing the composition‐property relationships of glasses and other materials with complex structures. A series of 20 borosilicate, aluminosilicate, and boroaluminosilicate glasses have been modeled using MD simulations with recently developed effective potentials. Short‐ and medium‐range structures of these glasses were analyzed and, based on the structural information, QSPR analysis of the initial dissolution rates (r0) of these glasses that were measured at 90°C and pH 9 by using various structural descriptors such as percentage of bridging oxygen species, network connectivity, and average ring size. The structural descriptor, Fnet, which contains both energetic information such as single bond strength and structural information such as cation coordination number and Qn distribution, was also used. It was found that while the overall network connectivity, average ring size and Fnet provide reasonable correlations with r0 of studied glasses, Fnet gives the best correlation among the descriptors. For glasses that show incongruent dissolution, it was found that modification of glass compositions to account for preferential release of modifier cations is necessary to achieve best correlations. The findings were discussed with results of recent studies on evaluating the compositional dependence of glass dissolution behavior using the topological‐constraints‐based models.

A classical molecular dynamics simulation method for the formation of “dry” gels from boro-aluminosilicate glass structures

In contact with water, glass transforms into amorphous and porous structures called gels. A simulation method based on classical molecular dynamics is proposed here to mimic “dry” gels forming from initial oxide glass structures. Six glass compositions were investigated. Two behaviours were evidenced depending on the initial glass composition, and in particular on the quantity of elements removed. If a large quantity of soluble elements (B, Na) was removed, it induced an increase in the average pore size within the gels, and the time needed to stabilise the gel structure increased because more local atomic rearrangements occurred. The gel network displayed a higher proportion of Si-Q4 at the expense of Si-Q3 and a lower average ring size compared to the glass network, irrespective of the glass composition. Surface effects were also highlighted in the dry gels, such as the presence of 3-coordinated Al and a decrease in the average angle Si-O-Si and Al-O-Al. These findings will be compared to both wet gels and experimental data in further studies, to help find the best procedure to simulate such structures.

The impact of PbF2 on the ionizing radiation shielding competence and mechanical properties of TeO2–PbF2 glasses and glass-ceramics

The influence of adding PbF2 on the elastic properties and radiation shielding competences of nine different samples labelled as TPb10-TPb90 with the form of (100-X)TeO2-XPbF2: (where X = 10–90 in steps of 10 mol%) glasses and glass-ceramics was investigated via the bond compression model and WinXcom software. The average cross-link density (n‾c) increased from 2.2 to 3.8 and the average atomic ring size (l) decreases from 0.390 to 0.356 nm with the increase of Pb ions content in the TPb10-TPb90 samples. The KBC increased from 84.47 GPa to 111.09 GPa and Young's modulus (EBC) increased from 136.89 GPa to 199.60 GPa for TPb10 to TPb90 samples. Poisson's ratio (σBC) decreased from 0.229 to 0.200, while the hardness (H) increased from 10.02 GPa to 16.59 GPa. The MAC of the glasses follows the order throughout the energy spectrum: TPb10 < TPb20 < TPb30 < TPb40 < TPb50 < TPb60 < TPb70 < TPb80 <TPb90. The maximum value of LAC obtained at 15 keV were: 290, 348, 405, 462, 519, 575, 632, 688, and 744 cm−1 for TPb10, TPb20, TPb30, TPb40, TPb50, TPb60, TPb70, TPb80, and TPb90, respectively. HVT and MFP are inversely proportional to LAC, hence, the highest and the lowest values of HVT and MFP were obtained for TPb10 and TPb90 samples at all energies. The TPb90 had the highest Zeff while TPb10 had the least. The buildup factors of TPb10 was highest at all depths while TPb90 had the least buildup factors. The obtained results showed that the addition of Pb to the proposed samples improves their mechanical and radiation shielding properties. Therefore, TPb90 sample has a better photon, fast neutron, proton, and alpha particle absorber compared to the other eight glass species in this study.

Cyclic Polyester Formation with an [OSSO]-Type Iron(III) Catalyst.

The efficient formation of cyclic polyesters from the ring-opening polymerization of lactide, ϵ-caprolactone, and β-butyrolactone catalyzed by a 1,4-dithiabutanedyl-2,2'-bis(4,6-dicumylphenol) [OSSO]-FeCl complex activated with cyclohexene oxide was achieved. The catalyst was very active (initial turnover frequency up to 2718 h-1 ), robust, and worked with a monomer/Fe ratio up to 10 000. The formation of cyclic polymers was supported by using high-resolution matrix-assisted laser desorption ionization (MALDI) MS, and the average ring size (≈5 kDa for cyclic polylactide) independent of the reaction conditions. A monometallic ring-opening polymerization/cyclization mechanism was proposed from the results of a kinetic investigation.

The influence of germanate anomaly on elastic moduli and Debye temperature behavior of xLi2O-(100 − x)[0.65GeO2·0.35PbO] glass system

Alkali-germanate glass composed of xLi2O-(100 − x)[0.65GeO2·0.35PbO] (x = 0 mol% to 40 mol%) were prepared using the melt-quenching method. The increase in density to a maximum at x = 10 mol% before decreasing indicates presence of the germanate anomaly. Both longitudinal and shear velocities, as well as both independent longitudinal (CL) and shear (μ) moduli, also showed similar non-linear behaviors, with maximum values at x = 10 mol%. The increase in independent moduli at x < 10 mol% is suggested to be related to the germanate anomaly, in which formation of smaller three-membered GeO4 rings connected to [GeO6]2− units leads to an increase in the rigidity and stiffness of the glass network. Meanwhile, the decrease in the elastic moduli for x > 20 mol% is suggested to be due to weakening of the glass network by progressive depolymerization of three-membered GeO4 rings in the germanate network through formation of non-bridging oxygen. In addition, bulk modulus, Ke, Young's modulus, and Debye temperature also showed similar behaviors as CL and μ. Analysis using bulk compression and ring deformation models showed the ratio of theoretical Ke over experimental Kbc and average ring size were also affected by the germanate anomaly, and the closest to ideal isotropic compression was at x = 10 mol%.

BaF2–contained tellurite glasses: Quantitative analysis and prediction of elastic properties and ultrasonic attenuation–Part I

In this article, the semi-empirical formulas of Abd El-Moneim’s model were applied to predict the composition dependence of ultrasonic attenuation coefficient in BaF2-contained tellurite glasses, for the first time. In addition to this, the various experimental elastic moduli and Poisson’s ratio were analyzed quantitatively and predicted on the basis of the bond compression, ring deformation and Makishima-Mackenzie’s models. The results show that:•The molar volume, packing density, fractal bond connectivity, average cross-link density, number of network bonds per unit volume and average atomic ring size play a dominant role in predicting the modifier role of the alkaline earth fluoride BaF2 in binary fluorotellurite BaF2-TeO2 glasses.•The theoretically calculated values of Poisson’s ratio on the basis of the bond compression model are in a better agreement with the corresponding experimentally measured values, when compared with those theoretically calculated on the basis of Makishima-Mackenzie’s model.•The agreement between theoretical and experimental values is excellent for bulk, good for Young’s and longitudinal moduli and unsatisfactory for shear modulus. The divergence between theoretical and experimental values has been attributed to the anomalous behavior between elastic moduli and dissociation energy per unit volume of the glass.•The correlation between ultrasonic attenuation coefficient and the most significant compositional and structural parameters was achieved on the basis of Abd El-Moneim’s model.

Influence of germanate anomaly on elastic, structural, and optical properties of xNa2O-(99–x)[80GeO2:20PbO]-1Er2O3 lead–germanate glasses

Abstract A sodium–lead–germanate glass system, with a composition of xNa2O-((100–y)–x)[80GeO2:20PbO]-yEr2O3 (x = 0–25 mol.%, y = 0, 1 mol.%), was prepared by melt quenching and used to investigate the effect of Na2O and Er2O3 on the germanate anomaly. The structural and optical properties of the glass samples were investigated using X-ray diffraction, Fourier transform infrared, and UV–Vis spectroscopy analyses. Elastic properties of Er2O3-doped glasses (y = 1) were studied by measuring longitudinal and shear velocities through the pulse-echo method at 5 MHz. Based on Fourier transform infrared spectroscopy analysis of Er2O3-doped glasses, the conversion of GeO4 into GeO6 indicates that the glass system possesses the germanate anomaly characteristic but has no density anomaly. Longitudinal, shear, bulk, and Young's moduli (C L, μ, K, and E, respectively) increased to their maximum values at x = 10 mol.% but decreased with increasing amount of Na2O added. This finding reveals the elastic nature of the germanate anomaly. Increase in elastic moduli indicates enhanced network rigidity of the glass system in the germanate anomaly region, where the coordination number increased with the transformation of GeO4 to GeO6. Subsequent decrease in elastic moduli (x &gt; 10 mol.%) denotes weakened network rigidity of the glass system because of enhanced formation of non-bridging oxygen. Furthermore, analysis using bulk compression and ring deformation models reveals the nonlinear trends of K bc /K e ratio and average ring size diameter as a result of the germanate anomaly. The anomaly also influenced optical properties of both Er2O3-doped (y = 1) and Er2O3-free (y = 0) glasses, where the optical energy gap (E opt) decreased with the addition of Na2O up to 10 mol.% and slightly increased with more than 10 mol.% Na2O. By contrast, Urbach energy (E U) and refractive index (n) showed opposite trends to that of E opt. The behavior of E U indicates changes in defect concentration, which affects E opt and n.

Correlation between acoustical and structural properties of glasses: Extension of Abd El-Moneim model for bioactive silica based glasses

Correlation between room temperature ultrasonic attenuation coefficient and the most significant structural parameters has been studied in the bioactive silica based glasses, for the first time. The correlation has been carried out in the quaternary SiO2–Na2O–CaO–P2O5 glass system using the two semi-empirical formulas, which have been presented recently by the author. Changes in the elastic properties, related to the substitution of SiO2 by alkali Na2O and alkaline earth CaO oxides, have also been deduced by evaluating the mean atomic volume, packing density, fractal bond connectivity and density of the analogous crystalline structure. Furthermore, values of the theoretical elastic moduli have been calculated on the basis of Makishima-Mackenzie theory and compared with the corresponding observed values. Results show that the correlation between ultrasonic attenuation coefficient and the oxygen density, average atomic ring size, first-order stretching force constant and experimental bulk modulus was achieved at 5 MHz frequency. Values of the theoretically calculated shear modulus are in excellent correlation (C. R. ≻95%) with the corresponding experimental ones. The divergence between the theoretical and experimental values of bulk modulus has been discussed.

Low temperature structural transitions in dipolar hard spheres: The influence on magnetic properties

We investigate the structural chain-to-ring transition at low temperature in a gas of dipolar hard spheres (DHS). Due to the weakening of entropic contribution, ring formation becomes noticeable when the effective dipole–dipole magnetic interaction increases. It results in the redistribution of particles from usually observed flexible chains into flexible rings. The concentration (ρ) of DHS plays a crucial part in this transition: at a very low ρ only chains and rings are observed, whereas even a slight increase of the volume fraction leads to the formation of branched or defect structures. As a result, the fraction of DHS aggregated in defect-free rings turns out to be a non-monotonic function of ρ. The average ring size is found to be a slower increasing function of ρ when compared to that of chains. Both theory and computer simulations confirm the dramatic influence of the ring formation on the ρ-dependence of the initial magnetic susceptibility (χ) when the temperature decreases. The rings due to their zero total dipole moment are irresponsive to a weak magnetic field and drive to the strong decrease of the initial magnetic susceptibility.

Effects of Increasing Tungsten on Structural, Elastic and Optical Properties of xWO3–(40−x)Ag2O–60Te2O Glass System

Ternary tellurite glasses with composition of xWO3(40−x)Ag2O–60TeO2 (x = 0–40 mol%) have been prepared by melt-quenching method. Elastic and optical properties of the glass system were obtained by ultrasonic velocity measurements and UV–vis spectroscopy, respectively, while structural investigation was carried out by using Raman spectroscopy. The longitudinal and shear velocities, vL and vs showed large increase at x from 0 to 20 mol% before decrease with further addition of WO3. Independent longitudinal modulus CL and shear modulus μ bulk modulus Ke Young's modulus Y and Debye temperature θD showed similar behaviors to both velocities. The large increase of the elastic moduli at x from 0 to 20 mol% is suggested to be due to the increase in WO6 octahedral unit structure indicating the increase of bridging oxygen (BO) and formation of stronger Te–O–W bonds compared to Te–O–Te bonds. On the other hand, for x > 20 mol%, the decrease in the elastic moduli was due to the increase of non-bridging oxygen (NBO) as a result of formation of WO4 tetrahedral via breaking Te–O–W network. Further analysis by using bulk compression and ring deformation models showed a slight decrease in the ratio of ideal bulk modulus Kbc to the experimental bulk modulus Ke and average atomic ring size, l for x 20 mol%. Our analysis also indicates that limited ring deformation took place and the main compression mechanism in this glass system was mainly ideal isotropic compression. On the other hand, optical band gap Eopt showed small variation for x = 0–20 mol% but decreased upon higher WO3 content while refractive index n showed the opposite trend. This optical behavior is suggested to be related to the changes in cross link density and NBO concentration in the glass system.

Correlation between acoustical and structural parameters in some oxide glasses

Correlation between acoustical and structural parameters has been carried out in TiO2–Na2O–CaO–P2O5, ZnO–PbO–B2O3 and BaO–TeO2 glass systems. The correlation is based on the recently presented Abd El-Moneim semi-empirical formulas, which relate the room temperature ultrasonic attenuation coefficient with oxygen density, average atomic ring size, first-order stretching force constant and experimental bulk modulus of the glass. Fractal bond connectivity and mean atomic volume have also been evaluated and interpreted in terms of composition to deduce changes in the structure of the glass. Furthermore, values of the theoretical bulk modulus have been calculated on the basis of Rocherulle et al. model and compared with the corresponding observed values. It has been found that, majority of the studied glass samples have two-dimensional structure and the network strength changes in the order BaO–TeO2 glasses≺TiO2–Na2O–CaO–P2O5 glasses≺ZnO–PbO–B2O3 glasses. Abd El-Moneim semi-empirical formulas can be applied successfully to predict changes of ultrasonic attenuation coefficient in these glasses at room temperature. Also, Rocherulle et al. model is valid for majority of the investigated glass samples.

Ring statistics of silica bilayers

The recent synthesis and characterisation of bilayers of vitreous silica has produced valuable new information on ring sizes and distributions. In this paper, we compare the ring statistics of experimental samples with computer generated samples. The average ring size is fixed at six by topology, but the width, skewness and other moments of the distribution of ring edges are characteristics of particular samples. We examine the Aboav-Weaire law that quantifies the propensity of smaller rings to be adjacent to larger rings, and find similar results for available experimental samples which however differ somewhat from computer-generated bilayers. We introduce a new law for the areas of rings of various sizes.

Study of rigidity of semiconducting vanadate glasses and its importance in use of coatings

The elastic moduli of some multicomponent vanadate based glasses were analysed in terms of the bond compression model by some physical parameters such as, the density, average stretching force constant and average atomic ring size. These parameters were calculated for all the glass series and for all the glass composition to estimate the rigidity of these glasses. The results showed that the average force constant and the elastic moduli of these glasses are sensitive to the decrease in PbO content. This behaviour was attributed to the increase in the molar volume and the role of different modifiers. These parameters along with the coordination number of the glasses affect the glass transition temperature. The correlation between the elastic moduli and thermal properties of these samples showed that 0·25MoO3–0.25PbO–0·5V2O5 glass is the most rigid and has an applicable glass transition temperature for coating.

Hemodynamic outcomes of geometric ring annuloplasty for aortic valve repair: A 4-center pilot trial

A geometric annuloplasty ring could improve efficacy and stability of aortic valve repair. Toward this goal, a 1-piece 3-dimensional titanium annuloplasty ring with Dacron covering was developed and tested successfully in animals. The purpose of this study was to define hemodynamic outcomes with this device used as the annuloplasty component of human aortic valve repair. In a 4-center pilot trial with informed consent, 16 patients underwent aortic valve repair for aortic insufficiency, with the annuloplasty device sutured into the annulus beneath the leaflets. Preoperative annular diameter averaged 26.5 ± 2.0 (mean ± standard deviation) mm, and average ring size was 22.3 ± 1.2 mm. After annuloplasty, leaflet defects were easy to identify, and 14 of 16 patients (88%) required leaflet plication and/or autologous pericardial reconstruction for leaflet defects. Three patients had ascending aortic replacement, and 2 had remodeling root replacement. One had ultrasonic leaflet decalcification and another tricuspid valve annuloplasty. Follow-up data were from site-specific studies at the 6-month postoperative time point. There were no in-hospital mortalities or major complications. Preoperative aortic insufficiency grade (0-4 scale) was 3.6 ± 1.0 and fell to 1.0 ± 0.8 at 6 months (P < .0001). New York Heart Association class fell from 2.5 ± 0.5 to 1.1 ± 0.3 (P < .0001). Postrepair valve area was 2.7 ± 0.2 cm(2), and 6-month mean systolic gradient was 11.3 ± 3.3 mm Hg. Left ventricular end-diastolic diameter and ejection fraction both normalized (both P < .0001). Geometric ring annuloplasty facilitated aortic valve repair, allowing more precise reconstruction of leaflet defects. Aortic insufficiency reduction and systolic gradients were excellent, and expansion of valve reconstruction into broader categories of aortic valve disease seems indicated.

Raman Spectrum of Asphaltene

Asphaltenes extracted from seven different crude oils representing different geological formations from around the globe were analyzed using the Raman spectroscopic technique. Each spectrum is fitted with four main peaks using the Gaussian function. On the basis of D1 and G bands of the Raman spectrum, asphaltene indicated an ordered structure with the presence of boundary defected edges. The average aromatic sheet size of the asphaltene molecules is estimated within the range of 1.52–1.88 nm, which represents approximately seven to eight aromatic fused rings. This estimation is based on the integrated intensity of D1 and G bands, as proposed by Tunistra and Koenig. The results here are in perfect agreement with so many other used techniques and indicate the potential applicability of Raman measurements to determine the average aromatic ring size and its boundary.

Structure and Dynamics of Ring Polymers: Entanglement Effects Because of Solution Density and Ring Topology

The effects of entanglement in solutions and melts of unknotted ring polymers have been addressed by several theoretical and numerical studies. The system properties have been typically profiled as a function of ring contour length at fixed solution density. Here, we use a different approach to investigate numerically the equilibrium and kinetic properties of solutions of model ring polymers. Specifically, the ring contour length is maintained fixed, while the interplay of inter- and intra-chain entanglement is modulated by varying both solution density (from infinite dilution up to \approx 40 % volume occupancy) and ring topology (by considering unknotted and trefoil-knotted chains). The equilibrium metric properties of rings with either topology are found to be only weakly affected by the increase of solution density. Even at the highest density, the average ring size, shape anisotropy and length of the knotted region differ at most by 40% from those of isolated rings. Conversely, kinetics are strongly affected by the degree of inter-chain entanglement: for both unknots and trefoils the characteristic times of ring size relaxation, reorientation and diffusion change by one order of magnitude across the considered range of concentrations. Yet, significant topology-dependent differences in kinetics are observed only for very dilute solutions (much below the ring overlap threshold). For knotted rings, the slowest kinetic process is found to correspond to the diffusion of the knotted region along the ring backbone.

Ultrasonic, FTIR and thermal investigations of SiO2–Na2O–CaO–P2O5 glasses doped with CeO2

The longitudinal and shear ultrasonic wave velocities for different compositions of SiO2–Na2O–CaO–P2O5 glasses were measured at room temperature (305 K) using a pulse-echo method at a frequency of 4 MHz. The elastic moduli, Poisson's ratio, microhardness, Debye temperature and other ultrasonic parameters were obtained from experimental data and analyzed using bond compression theory. By calculating the number of network bonds per unit volume, the average stretching constant, and the average ring size, information about the structure of the glass can be deduced. Structural changes after doping with CeO2 were investigated by FTIR spectroscopy, and by measurements of the thermal expansion coefficient, glass transition and softening temperature to throw more light on the characterization of these glasses.