Grain boundary diffusion in ionic crystals

The diffusion of ¹³¹I in single crystals and along the tilt grain boundaries of pure and calcium chloride doped (142±5 ppm) bicrystals of sodium chloride, grown by Czochralski technique, was investigated using a microtome sectioning technique. Results on bulk diffusion in pure NaCl agree well with the results of earlier investigators and are represented by the D=(504 cm²/sec)exp (-2.27 ev/kT). Enhanced diffusion of ¹³¹I is observed along the grain boundaries and the presence of the dopant lowers the grain boundary diffusivity over the entire temperature range of 430ºC - 570ºC studied in the present investigation. Thermal activation and electrical polarization effects, acting simultaneously on the diffusing ion, determine the magnitude of the grain boundary diffusivity in these ionic bicrystals. In the low temperature range of 430ºC - 490ºC, the diffusion of ¹³¹I along the grain boundaries is described by the relationship where (D\u27δ) = (D\u27δ)ooexp(-Qb/kT), where (D\u27δ)o has values of 8.45 x 10⁻⁵ and 0.39 cm³/sec for the pure and the doped bicrystals, respectively. Qb is observed to be 1.36 and 1.93 eV for the same two cases. In the high temperature range of 490ºC - 570ºC, grain boundary charge and ionic polarization cause distinct but similar diffusion anomalies in the grain boundary diffusivity plots of both the pure and the doped bicrystals, depending upon the proximity of the measuring temperature to the isoelectric temperature. Grain boundary diffusion seems to occur by movement of ions in the grain boundary core by an interstitial motion of iodine ions --Abstract, pages ii-iii

Thermal fatigue testing of die casting die steels

Thermal fatigue is a form of surface failure produced by repeated thermal stresses and is the main cause of die retirement in the die casting industry. A general review of the fundamental and practical aspects of thermal fatigue as encountered in the dies, based on earlier published work, is given and preliminary experiments in an effort to set up a suitable method of testing to study this phenomenon are then described. In the present technique developed to create the thermal conditions experienced by a die steel in aluminum die casting, a small portion of one face of a cylindrical test-piece is intermittantly [sic] heated by H.F. induction whilst the main body of the test-piece is kept at a lower temperature by means of a copper jig. Air under pressure is used as a cooling medium during the cooling part of the cycle. Thermal fatigue resistance of various die steels and prospective die materials for aluminum die casting is determined on the basis of number of cycles for crack initiation, propagation, frequency and severity of cracking supported by graphs and series of photographs. Photographic evidence presented gives a detailed look into the mechanism of thermal fatigue. Present investigations reveal that 18% nickel maraging steel is the most suitable steel for aluminum die casting dies, followed by H-21, H-13 and H-11 respectively --Abstract, page ii

Parameter Estimation for Log-Pearson Type III Distribution by Pome

The principle of maximum entropy (POME) is employed to derive an alternative method of parameter estimation for the log-Pearson type (LPT) III distribution. Historical flood data are used to evaluate this method and compare it with the methods of moments (MOM) and maximum likelihood estimation (MLE). The parameter estimates yielded by POME are comparable to those by MOM and MLE

Cation induced differential effect on structural and functional properties of Mycobacterium tuberculosis α-Isopropylmalate synthase

<p>Abstract</p> <p>Background</p> <p>α-isopropylmalate synthase (MtαIPMS), an enzyme that catalyzes the first committed step of the leucine biosynthetic pathway of <it>Mycobacterium tuberculosis </it>is a potential drug target for the anti-tuberculosis drugs. Cations induce differential effect of activation and inhibition of MtαIPMS. To date no concrete mechanism for such an opposite effect of similarly charged cations on the functional activity of enzyme has been presented.</p> <p>Results</p> <p>Effect of cations on the structure and function of the MtαIPMS has been studied in detail. The studies for the first time demonstrate that different cations interact specifically at different sites in the enzyme and modulate the enzyme structure differentially. The inhibitors Zn²⁺and Cd²⁺ions interact directly with the catalytic domain of the enzyme and induce unfolding/denaturation of the domain. The activator K⁺also interacts with the catalytic TIM barrel domain however, it does not induce any significant effect on the enzyme structure. Studies with isolated catalytic TIM barrel domain showed that it can carry out the catalytic function on its own but probably requires the non-catalytic C-terminal domain for optimum functioning. An important observation was that divalent cations induce significant interaction between the regulatory and the catalytic domain of MtαIPMS thus inducing structural cooperativity in the enzyme. This divalent cation induced structural cooperativity might result in modulation of activity of the catalytic domain by regulatory domain.</p> <p>Conclusion</p> <p>The studies for the first time demonstrate that different cations bind at different sites in the enzyme leading to their differential effects on the structure and functional activity of the enzyme.</p

Guanidine hydrochloride- and urea-induced unfolding of Toxoplasma gondii ferredoxin-NADP<SUP>+</SUP> reductase: stabilization of a functionally inactive holo-intermediate

Usually during the folding/unfolding of flavoproteins, an apo-intermediate is stabilized before global unfolding of the enzymes occurs. However, stabilization of a holo-intermediate has also been reported for a few flavoproteins. We have studied the unfolding of Toxoplasma gondii ferredoxin-NADP+ reductase (TgFNR) using GdnHCl and urea. A functionally inactive holo-intermediate of the enzyme was found to be stabilized during this unfolding process. The intermediate species had cofactor FAD bound to it, but it showed free movement due to which the stabilized intermediates were functionally inactive. The native TgFNR behaves cooperatively with the two structural domains interacting strongly with each other. The denaturants GdnHCl and urea, at low concentrations, were found to interact selectively with the NADP+-binding domain of TgFNR and to induce structural modifications in it. These selective modifications in the protein molecule lead to loss of interactions between two domains and the enzyme behaved non-cooperatively resulting in stabilization of an intermediate species. Significant differences in the structural properties of the GdnHCl- and urea-stabilized holo-intermediates of TgFNR were observed. Comparison of the unfolding pathway of TgFNR (a plant-type FNR) with that of FprA (a GR-type FNR) demonstrates that they follow very different pathways of unfolding

Analysis of 226Ra, 232Th and 40K in soil samples for the assessment of the average effective dose

The activity concentrations of the natural radionuclides namely 238Ra, 232Th and 40K are measured for soil samples collected from different locations of Faridkot and Mansa districts of Punjab. HPGe detector, based on high-resolution gamma spectrometry system is used for the measurement of activity concentration. The range of activity concentrations of 226Ra, 232Th and 40K in the soil from the studied areas varies from 21.42 Bq kg–1 to 40.23 Bq kg–1 , 61.01 Bq kg–1 to 142.34 Bq kg–1 and 227.11 Bq kg–1 to 357.13 Bq kg–1 with overall mean values of 27.17 Bq kg–1, 95.22 Bq kg–1 and 312.76 Bq kg–1, respectively. Radium equivalent activities are calculated for the analyzed samples to assess the radiation hazards arising due to the use of these soil samples in the construction of dwellings. The absorbed dose rate calculated from activity concentration of 226Ra, 232Th and 40K ranges between 9.87 and 18.55, 38.01 and 88.68 and 9.40 and 14.79 nGy h–1, respectively. The total absorbed dose in the study area ranges from 61.10 nGy h–1 to 112.86 nGy h–1 with an average value of 84.80 nGy h–1. The calculated values of external hazard index (Hex) for the soil samples of the study area range from 0.36 to 0.68. Since these values are lower than unity, according to the Radiation Protection 112 (European Commission, 1999) report, soil from these regions is safe and can be used as construction material without posing any significant radiological threat to population. The corresponding average annual effective dose for indoor and outdoor measured in the study area are 0.42 mSv and 0.10 mSv respectively.Author Affiliation: Rohit Mehra, Surinder Singh and Kulwant Singh 1.Department of Applied Sciences, Malout Institute of Management and Information Technology, Malout-152 107, Punjab, India 2.Department of Physics, Guru Nanak Dev University, Amritsar-143 005, Punjab, India E-mail : [email protected] of Applied Sciences, Malout Institute of Management and Information Technology, Malout-152 107, Punjab, India Department of Physics, Guru Nanak Dev University, Amritsar-143 005, Punjab, Indi

Redirecting Traffic in the Nucleus

Nuclear spatial organization of genes has emerged as an important determinant of their transcriptional activity. In this issue, Wang et al. (2011) show that the Msx1 homeoprotein induces a dramatic redistribution of Ezh2 and H3K27me3 to the nuclear periphery of muscle progenitor cells to repress transcription of developmentally regulated genes

What primary school teachers say about their beliefs on teaching and learning of English as a Second Language (ESL)

The belief system upheld by a teacher largely controls and shapes his or her views and practices of the subject matter and pedagogical content knowledge. Whether or not this applies to an English as a Second Language (ESL) context underpinned the purpose of the study presented and discussed in this paper. This Case Study aimed to explore three purposively selected rural primary school teachers’ beliefs and practices about teaching and learning of English. They each had fifteen, six and one year of experience; respectively. Data was gathered through individual interviews and triangulated with observations and document analysis. Themes were identified through constant comparative and thematic analysis and assisted with the use of CAQDAS, Atlas Ti version 7; a software for analyzing qualitative data. Findings indicate that despite the different number of years of experience, the teachers’ beliefs had an impact on how they felt English should be taught and therefore employed pedagogical strategies they believed would work for their students. This implies there is a need to for sharing good practice which will enable even novice teachers to get the pedagogical content knowledge which are tested through real teaching experiences

Simulation Based Analysis of Temperature Effect on Breakdown Voltage of Ion Implanted Co/n-Si Schottky Diode

In semiconductor devices, breakdown voltage variation with temperature is a very significant study, since the reliability and performance of semiconductor devices especially depends upon the temperature. In this paper, the influence of temperature on breakdown characteristic of Ion Implanted edge terminated Co/n-Si Schottky Diode formed on n-Si epitaxial layer has been investigated by using SILVACO TCAD. It is also reported that not only resistive area present in close proximity to the edges of boron ion implanted Schottky diode are responsible for improvement in breakdown voltage but also the formation of PN junction near the edges, affect the breakdown voltage to a significant amount. The dopant concentration of epitaxial layer is 1 × 1015/cm3. The variation in reverse breakdown characteristics as a junction of temperature in the range of 300-1000 K is presented in this paper. A comparative study of breakdown voltages of Ion Implanted and as-prepared Schottky diode is also presented. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3026