Diffusion of particles in an expanding sphere with an absorbing boundary

We study the problem of particles undergoing Brownian motion in an expanding sphere whose surface is an absorbing boundary for the particles. The problem is akin to that of the diffusion of impurities in a grain of polycrystalline material undergoing grain growth. We solve the time dependent diffusion equation for particles in a d-dimensional expanding sphere to obtain the particle density function (function of space and time). The survival rate or the total number of particles per unit volume as a function of time is evaluated. We have obtained particular solutions exactly for the case where d=3 and a parabolic growth of the sphere. Asymptotic solutions for the particle density when the sphere growth rate is small relative to particle diffusivity and vice versa are derived.Comment: 12 pages. To appear in J. Phys. A: Math. Theor. 41 (2008

Hydrogen Futures and Technologies

Concerns about the security of oil supplies and the environmental consequences of burning fossil fuels have transformed the idea of a hydrogen (H2) economy from science fiction into a political bipartisan vision of our energy future. The challenge is now one of economics and technology. In one context, we already have a rapidly growing H2 economy, driven by the need for increased supplies of H2 to convert more abundant lower-grade crude oils into clean liquid fuels. This development is creating the infrastructure for a global H2 economy and provides powerful incentives to develop better methods of H2 production. The H2 content of liquid fuels is a variable; thus, there is also the option to add additional H2 to conventional liquid fuels to create H2-enhanced fuels. This option increases the liquid fuel yield per barrel of oil, creates a greatly expanded H2 production infrastructure, and may provide the easiest transition to a full H2 economy. It is primarily the characteristics of H2 as a fuel, rather than the type of device in which it is used (fuel cell or internal combustion engine), that creates the environmental benefits of a H2-fueled economy at the point at which the device is used. Water is the only waste product of H2 fuel. The other potential benefits of a H2 economy require methods of production that do not depend upon foreign energy resources and greatly reduce emission of greenhouse gases to the environment. Consequently, the most important challenges are the development of better methods to produce H2 and to store (deliver) H2 onboard vehicles. While fuel cells are not required for a revolution in transportation (internal combustion engines can burn H2), they add another dimension to the H2 economy by their potential impacts on electricity production and distribution. Hydrogen fuel cells may provide a storable form of electricity to meet peak electric demands. This benefits high-capital-cost low-production-cost energy sources such as nuclear and renewables by providing a demand for their energy output that is not tied to the daily cycle of electricity demand. The methods to produce and store H2 define the technical challenges. These challenges, in turn, define the challenges in heat transfer—the subject of this Rohsenow Symposium. The likely characteristics of our transition to a H2 economy and some of the accompanying technical challenges in heat transfer are described herein

Combined congenic mapping and nuclease-based gene targeting for studying allele-specific effects of Tnfrsf9 within the Idd9.3 autoimmune diabetes locus.

Rodent complex trait genetic studies involving a cross between two inbred strains are usually followed by congenic mapping to refine the loci responsible for the phenotype. However, progressing from a chromosomal region to the actual causal gene remains challenging because multiple polymorphic genes are often closely linked. The goal of this study was to develop a strategy that allows candidate gene testing by allele-specific expression without prior knowledge of the credible causal variant. Tnfrsf9 (encoding CD137) is a candidate gene for the Idd9.3 type 1 diabetes (T1D) susceptibility locus in the nonobese diabetic (NOD) mouse model. A C57BL/10Sn (B10)-derived diabetes resistance Idd9.3 congenic region has been shown to enhance accumulation of CD137+ regulatory T cells and serum soluble CD137 in NOD mice. By combining the power of congenic mapping and nuclease-based gene targeting, we established a system where a pair of F1 hybrids expressed either the B10 or NOD Tnfrsf9 allele mimicking coisogenic strains. Using this approach, we demonstrated that the allelic difference in B10 and NOD Tnfrsf9 alone was sufficient to cause differential accumulation of CD137+ regulatory T cells and serum soluble CD137 levels. This strategy can be broadly applied to other rodent genetic mapping studies

Ion concentrations in cerebrospinal fluid in wakefulness, sleep and sleep deprivation in healthy humans

Sleep is controlled by a circadian rhythmicity, via a reduction of arousal-promoting neuromodulatory activity, and by accumulation of somnogenic factors in the interstitial fluid of the brain. Recent experiments in mice suggest that a reduced neuronal excitability caused by a reduced concentration of potassium in the brain, concomitant with an increased concentration of calcium and magnesium, constitutes an important mediator of sleep. In the present study, we examined whether such changes in ion concentrations could be detected in the cerebrospinal fluid of healthy humans. Each subject underwent cerebrospinal fluid collection at three occasions in a randomized order: at 15:00 hours–17:00 hours during waking, at 06:00 hours–07:00 hours immediately following 1 night of sleep, and at 06:00 hours–07:00 hours following 1 night of sleep deprivation. When compared with wakefulness, both sleep and sleep deprivation produced the same effect of a small (0.1 mm, about 3%), but robust and highly significant, reduction in potassium concentration. Calcium and magnesium concentrations were unchanged. Our results support a circadian modulation of neuronal excitability in the brain mediated via changes of the interstitial potassium concentration

Vibronic structure in triatomic molecules : The hydrocarbon flame bands of the formyl radical (HCO). A theoretical study

A theoretical study of the vibrational structure of the math 2A′ ground and math 2A′ excited states of the formyl radical, HCO, and its deuterated form, DCO, has been performed. The potential energy surfaces have been computed by means of a multiconfigurational perturbative method, CASPT2. The computed geometries and the harmonic and anharmonic frequencies are successfully compared to the available experimental information. The vibrational intensities of the transition math 2A′↔math 2A′ have been computed both for absorption and emission. The results lead to accurate determinations of several structural parameters and some reassignments of the vibrational transitions of the so-called hydrocarbon flame bands of the formyl [email protected]

Estimating Survival in Patients with Operable Skeletal Metastases: An Application of a Bayesian Belief Network

BACKGROUND: Accurate estimations of life expectancy are important in the management of patients with metastatic cancer affecting the extremities, and help set patient, family, and physician expectations. Clinically, the decision whether to operate on patients with skeletal metastases, as well as the choice of surgical procedure, are predicated on an individual patient's estimated survival. Currently, there are no reliable methods for estimating survival in this patient population. Bayesian classification, which includes bayesian belief network (BBN) modeling, is a statistical method that explores conditional, probabilistic relationships between variables to estimate the likelihood of an outcome using observed data. Thus, BBN models are being used with increasing frequency in a variety of diagnoses to codify complex clinical data into prognostic models. The purpose of this study was to determine the feasibility of developing bayesian classifiers to estimate survival in patients undergoing surgery for metastases of the axial and appendicular skeleton. METHODS: We searched an institution-owned patient management database for all patients who underwent surgery for skeletal metastases between 1999 and 2003. We then developed and trained a machine-learned BBN model to estimate survival in months using candidate features based on historical data. Ten-fold cross-validation and receiver operating characteristic (ROC) curve analysis were performed to evaluate the BNN model's accuracy and robustness. RESULTS: A total of 189 consecutive patients were included. First-degree predictors of survival differed between the 3-month and 12-month models. Following cross validation, the area under the ROC curve was 0.85 (95% CI: 0.80-0.93) for 3-month probability of survival and 0.83 (95% CI: 0.77-0.90) for 12-month probability of survival. CONCLUSIONS: A robust, accurate, probabilistic naïve BBN model was successfully developed using observed clinical data to estimate individualized survival in patients with operable skeletal metastases. This method warrants further development and must be externally validated in other patient populations

Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 \ub0C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 \ub0C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3 710-8 m2 K/W, measured at an elevated temperature of 160 \ub0C, in a GaN-on-SiC transistor structure was achieved