Experimental evidence of auxeticity in ion implanted single crystal calcite.

We report initial experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite with Ar+ at room temperature using an ion energy of 400 keV and a dose of 1 × 1014 cm-2. Lattice compression normal to the substrate surface was observed, which is an atypical result for ion implanted materials. The auxetic behavior is consistent with predictions that indicate auxeticity had been predicted along two crystallographic directions including [1010]. Materials with a positive Poisson's ratio experience lattice expansion normal to the substrate surface when ion implanted, whereas lattice contraction normal to the surface is evidence of auxetic behavior. Triple-axis X-ray diffraction measurements confirmed the auxetic strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate, as is typical with implanted single crystals. A symmetric (3030) ω:2θ line scan was used with X-ray dynamical diffraction simulations to model the strain profile and extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were performed to obtain a displacement-per-atom profile and implanted Ar+ concentration profile. It was found that the strain profile matches the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxeticity.

Kinetic Model of Incipient Hydride Formation in Zr Clad under Dynamic Oxide Growth Conditions.

The formation of elongated zirconium hydride platelets during corrosion of nuclear fuel clad is linked to its premature failure due to embrittlement and delayed hydride cracking. Despite their importance, however, most existing models of hydride nucleation and growth in Zr alloys are phenomenological and lack sufficient physical detail to become predictive under the variety of conditions found in nuclear reactors during operation. Moreover, most models ignore the dynamic nature of clad oxidation, which requires that hydrogen transport and precipitation be considered in a scenario where the oxide layer is continuously growing at the expense of the metal substrate. In this paper, we perform simulations of hydride formation in Zr clads with a moving oxide/metal boundary using a stochastic kinetic diffusion/reaction model parameterized with state-of-the-art defect and solute energetics. Our model uses the solutions of the hydrogen diffusion problem across an increasingly-coarse oxide layer to define boundary conditions for the kinetic simulations of hydrogen penetration, precipitation, and dissolution in the metal clad. Our method captures the spatial dependence of the problem by discretizing all spatial derivatives using a stochastic finite difference scheme. Our results include hydride number densities and size distributions along the radial coordinate of the clad for the first 1.6 h of evolution, providing a quantitative picture of hydride incipient nucleation and growth under clad service conditions.

Parenting style and oral health practices in early childhood caries: a case-control study.

BACKGROUND: There is a need to carry out controlled investigations regarding risk factors for early childhood caries (ECC). AIM: To study the type of parenting style and oral health practices as risk factors among children with ECC in an Indian preschool population. METHODS: Two hundred and eleven children with ECC and equal number of controls participated in this case-control study. A questionnaire was answered by parents regarding oral health practices such as oral hygiene methods, feeding habits, daily sugar intake, and dental attendance pattern along with socioeconomic and demographic status. The parenting style was determined using Parenting Styles Dimension Questionnaire (PSDQ) index. Data were statistically analyzed using chi-square test and multiple logistic regression analysis. RESULTS: Risk factors associated with ECC were higher birth order, lower socioeconomic status, non-use of fluoridated toothpaste, breast/bottle feeding for more than one year, presence of formula milk or milk with sugar in the feeding bottle while falling asleep, higher sweet scores in the diet chart, and visiting dentist only when a problem was perceived. Majority of parents of children with and without ECC had authoritative parenting style. CONCLUSION: Improper oral health practices are the risk factors for ECC. The association of parenting style with ECC could not be confirmed.

Periodontal regeneration using platelet-derived growth factor in infrabony defects: a series of three cases.

The periodontal researchers and clinicians, in an effort to develop effective regenerative therapies, have sought to understand key events involved in the regeneration of periodontium. Polypeptide growth factors are a class of natural biological mediators which regulate key cellular events in tissue repair. Platelet-derived growth factor (PDGF) is the most thoroughly studied growth factor in periodontal regeneration. The present case series evaluate the effectiveness of platelet-derived growth factor (rh-PDGF-BB) in combination with beta-tricalcium phosphate ( ß -TCP) to achieve periodontal regeneration in 3 infrabony defects.