CNRT Matériaux

▪ Abstract The synthesis of the two currently used superhard materials, diamond and cubic boron nitride, is briefly described with indications of the factors influencing the quality of the crystals obtained. The physics of hardness is discussed and the importance of covalent bonding and fixed atomic positions in the crystal structure, which determine high hardness values, is outlined. The materials investigated to date are described and new potentially superhard materials are presented. No material that is thermodynamically stable under ambient conditions and composed of light (small) atoms will have a hardness greater than that of diamond. Materials with hardness values similar to that of cubic boron nitride (cBN) can be obtained. However, increasing the capabilities of the high-pressure devices could lead to the production of better quality cBN compacts without binders.

Uncoupling protein-2 and -3 (UCP2 and UCP3) are mitochondrial proteins that show high sequence homology with the brown adipocyte-specific UCP1. UCP1 induces heat production by uncoupling respiration from ATP synthesis. UCP2 is widely expressed in human tissues, whereas UCP3 expression seems restricted to skeletal muscle, an important site of thermogenesis in humans. We have investigated the regulation of UCP2 and UCP3 gene expression in skeletal muscle and adipose tissue from lean and obese humans. UCP2 and -3 mRNA levels were not correlated with body mass index (BMI) in skeletal muscle, but a positive correlation (r = 0.55, P < 0.01, n = 22) was found between UCP2 mRNA level in adipose tissue and BMI. The effect of fasting was investigated in eight lean and six obese subjects maintained on a hypocaloric diet (1,045 kJ/d) for 5 d. Calorie restriction induced a similar 2-2.5-fold increase in UCP2 and -3 mRNA levels in lean and obese subjects. To study the effect of insulin on UCP gene expression, six lean and five obese subjects underwent a 3-h euglycemic hyperinsulinemic clamp. Insulin infusion did not modify UCP2 and -3 mRNA levels. In conclusion, the similar induction of gene expression observed during fasting in lean and obese subjects shows that there is no major alteration of UCP2 and -3 gene regulation in adipose tissue and skeletal muscle of obese subjects. The increase in UCP2 and -3 mRNA levels suggests a role for these proteins in the metabolic adaptation to fasting.

The ontogenesis of the glucose transporters GLUT-1, GLUT-2, and GLUT-4 and the hexokinases HK-I, HK-II, and HK-IV (glucokinase) was studied in rat tissues. In brown adipose tissue, high levels of GLUT-4 and HK-II were observed during fetal life; both decreased at birth and then increased throughout development. At birth, cold exposure increased GLUT-4 and HK-II expression in brown adipose tissue, whereas fasting decreased it. GLUT-1 and HK-I were present in fetal muscle, but GLUT-4 and HK-II were absent. The coordinate appearance of GLUT-4 and HK-II in skeletal muscle was concomitant with the acquisition of insulin sensitivity after weaning. In the heart, the glucose transporter isoform switched from GLUT-1 to GLUT-4 during the suckling period. The coordinate expression of GLUT-4 and HK-II in heart was observed after weaning. GLUT-2, detected in fetal liver, increased throughout development. GLUT-1 and HK-I were detectable in fetal liver, whereas glucokinase appeared after weaning. Consumption of a high-carbohydrate diet after weaning increased GLUT-4 and HK-II in muscle and GLUT-2 in liver, whereas consumption of a high-fat diet prevented these changes. These results showed that 1) GLUT-1 and HK-I are abundant in most fetal rat tissues, 2) GLUT-4 and HK-II expression is associated with the appearance of tissue insulin sensitivity, and 3) GLUT-2 is expressed early in liver, before the appearance of glucokinase.

A decreased mobilization of triglycerides may contribute to fat accumulation in adipocytes, leading to obesity. However, this hypothesis remains to be proven. In this study, epinephrine-induced lipid mobilization was investigated in vivo in nine markedly obese children (160+/-5% ideal body weight) aged 12.1+/-0.1 yr during the dynamic phase of fat deposition, compared with six age-matched nonobese children. As an in vivo index of lipolysis, we measured glycerol flux using a nonradioactive tracer dilution approach, and plasma free fatty acid concentrations. In the basal state, the obese children had a 30% lower rate of glycerol release per unit fat mass than the lean children. To study the regulation of lipolysis, epinephrine was infused stepwise at fixed doses of 0.75 and then 1. 50 microg/min in both groups. In lean children, glycerol flux and plasma free fatty acid increased to an average of 249-246% of basal values, respectively, in response to a mean plasma epinephrine of 396+/-41 pg/ml. The corresponding increase was only 55-72% in the obese children, although their mean plasma epinephrine reached 606+/-68 pg/ml. All obese and nonobese children, except an Arg64Trp heterozygote, were homozygotes for Trp at position 64 of their beta3-adrenoreceptor. The resistance of lipolysis to epinephrine showed no relationship with the Arg64 polymorphism of the beta3-adrenoreceptor gene. In summary, in vivo lipolysis, which mostly reflects the mobilization of lipid stores from subcutaneous adipose tissue, shows a decreased sensitivity to epinephrine in childhood onset obesity. Since our study was carried out in obese children during the dynamic phase of fat accumulation, the observed resistance to catecholamines might possibly be causative rather than the result of obesity.

We study how maximum output power can be obtained from a thermoelectric generator(TEG) with nonideal heat exchangers. We demonstrate with an analytic approach based on a force-flux formalism that the sole improvement of the intrinsic characteristics of thermoelectric modules including the enhancement of the figure of merit is of limited interest: the constraints imposed by the working conditions of the TEG must be considered on the same footing. Introducing an effective thermal conductance we derive the conditions which permit maximization of both efficiency and power production of the TEG dissipatively coupled to heat reservoirs. Thermal impedance matching must be accounted for as well as electrical impedance matching in order to maximize the output power. Our calculations also show that the thermal impedance does not only depend on the thermal conductivity at zero electrical current: it also depends on the TEG figure of merit. Our analysis thus yields both electrical and thermal conditions permitting optimal use of a thermoelectric generator.

Several deuteration experiments on crystalline silicon have been performed for various shallow dopant impurities (B and Al for p-type silicon; P and As for n-type silicon) and for different temperatures and times of plasma exposure. Deuterium diffusion depth profiles obtained by secondary-ion mass spectroscopy (SIMS) were simulated with an improved version of a previously reported model. A careful analysis of the SIMS data has allowed the reduction of the number of fit parameters, by excluding the ${\mathrm{H}}_{2}$-molecule formation and by a rough estimate of the neutral-deuterium diffusion coefficient and of the surface concentration of neutral deuterium. The diffusion coefficients and related activation energies of the hydrogen species ${\mathrm{H}}^{0}$, ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$, and ${\mathrm{H}}^{+}$ were determined, leading to a stated ranking of the mobilities in the order ${\mathrm{H}}^{0}$${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$${\mathrm{H}}^{+}$. The dissociation energies of BH, AlH, and PH complexes were also calculated and have allowed us to deduce the corresponding bonding energies of the complexes, which suggest a scaling of the complex stability in the order PHdepth profiles obtained by high-frequency capacitance-voltage measurements, combined with chemical etching, provided direct evidence of the rate of passivation of the shallow p-type-dopant impurities. The comparison between both couples of depth profiles (deuterium diffusion and carrier concentrations), in the case of p-type silicon, showed good agreement between the deactivation process of dopants and the corresponding depth penetration of deuterium.

The insulin-resistant obese fa/fa rat is a convenient model in which to study a potential effect of metformin, a biguanide used in the treatment of non-insulin-dependent diabetes, on insulin-mediated glucose utilization. Female fa/fa rats were given metformin orally for 8 days. Studies were performed on anaesthetized post-absorptive rats 5 h after the last dose of metformin. Glucose production and utilization were enhanced 1.5-fold in metformin-treated rats. The enhanced glucose production was almost entirely due to increased glucose recycling. The digestive tract was the only tissue responsible for the enhanced glucose utilization.

This volume is the proceedings of the NATO Advanced Study Institute, "Diffusion in Materials", held at "Centre Paul Langevin", Aussois, during March 12-25, 1989. There were 105 participants of whom 24

The localization of the two major placental glucose transporter isoforms, GLUT1 and GLUT3 was studied in 20-d pregnant rats. Immunocytochemical studies revealed that GLUT1 protein is expressed ubiquitously in the junctional zone (maternal side) and the labyrinthine zone (fetal side) of the placenta. In contrast, expression of GLUT3 protein is restricted to the labyrinthine zone, specialized in nutrient transfer. After 19-d maternal insulinopenic diabetes (streptozotocin), placental GLUT3 mRNA and protein levels were increased four-to-fivefold compared to nondiabetic rats, whereas GLUT1 mRNA and protein levels remained unmodified. Placental 2-deoxyglucose uptake and glycogen concentration were also increased fivefold in diabetic rats. These data suggest that GLUT3 plays a major role in placental glucose uptake and metabolism. The role of hyperglycemia in the regulation of GLUT3 expression was assessed by lowering the glycemia of diabetic pregnant rats. After a 5-d phlorizin infusion to pregnant diabetic rats, placental GLUT3 mRNA and protein levels returned to levels similar to those observed in nondiabetic rats. Furthermore, a short-term hyperglycemia (12 h), achieved by performing hyperglycemic clamps induced a fourfold increase in placental GLUT3 mRNA and protein with no concomitant change in GLUT1 expression. This study provides the first evidence that placental GLUT3 mRNA and protein expression can be stimulated in vivo under hyperglycemic conditions. Thus, GLUT3 transporter isoform appears to be highly sensitive to ambient glucose levels and may play a pivotal role in the severe alterations of placental function observed in diabetic pregnancies.

Abstract Chromium self-diffusion coefficients in chromia (Cr2O3) single crystals were determined by both ion implantation and thick-film methods, using the 54Cr and 50Cr isotopes. The concentration profiles were established by secondary-ion mass spectrometry, and the diffusion coefficients were computed using a solution of Fick's law taking into account evaporation and exchange at the surface. Chromium diffusion was studied as a function of temperature T and oxygen pressure Po2. Both methods lead to diffusion coefficients of the same order of magnitude. The diffusion coefficients are lower than those given in the literature and do not depend on the oxygen pressure; they are well described by the relation

${\mathrm{RuO}}_{2}$ was studied up to 40.2 GPa by angle-dispersive x-ray diffraction. Two phase transitions were observed, the first from the rutile-structured ambient phase to an orthorhombic phase below 8.0 GPa with cell parameters a=4.615(17), b=4.230(13), and c=3.112(4) \AA{}, Z=2 at 11.3 GPa. The second transition at just below 13 GPa was to a fluorite-type structure with a=4.727(4) \AA{}, Z=4 at 40.3 GPa. The first transition appeared to be second order producing a structure that is related to the ${\mathrm{CaCl}}_{2}$ type. Several reflexions were found to be common to all three phases, indicating that the rutile-to-fluorite phase transformation occurs via a diffusionless mechanism dominated by oxygen displacement in the xy plane of rutile. Using group theory, we propose possible diffusionless pathways for the transformation from the initial rutile structure, via the observed, orthorhombic, ${\mathrm{CaCl}}_{2}$-like intermediate, to the fluorite structure, based on a series of group-subgroup relationships. Ruthenium dioxide under ambient conditions is isostructural with stishovite and hence provides a model for the high-pressure behavior of ${\mathrm{SiO}}_{2}$.

The effects of fasting on glucose metabolism in the conscious resting rat were studied. Fasting decreased whole-body glucose utilization by 40%. The fast induced a decrease in glucose utilization in muscles which are constantly working even in the resting state, i.e. heart, diaphragm and postural muscles. No modification was observed in other tissues.

Alumina single crystals were compressed perpendicular to the [0001] axis at a constant strain rate between 20° and 950°C. At r&gt;200°C, failure was suppressed by_hydrostatic pressures of 500 to 1500 MPa. Prismatic slip {1120}〈1100〉 was deduced from optical observations of the lateral surfaces and from stress‐optical features in thin sections cut from the specimens. The critical resolved shear stress (CRSS) decreased rapidly with increasing temperature, from a maximum of ∼3000 MPa at 200°C (strain rate 2±10‐ −5 s −1 ). A simple linear law can be fitted with the logarithm of the CRSS as a function of temperature, up to 1800°C. The rate‐controlling mechanism for dislocation glide is likely to be either the Peierls barrier or barriers due to dissociation out of the glide plane.

First-principles plane-wave pseudopotential and full-potential linearized-augmented plane-wave methods have been used to study the elastic and electronic properties of several potential superhard ${\mathrm{RuO}}_{2}$ phases. The structures, relative stabilities, and the elastic constants and bulk moduli of these phases have been calculated within local-density approximation (LDA) and generalized gradient approximation (GGA). In ${\mathrm{RuO}}_{2},$ the LDA and GGA approximations yield smaller and larger lattice constants, respectively, for the $\mathrm{Pa}3\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{-}{\mathrm{RuO}}_{2}$ structure. The internal structural parameter for oxygen atoms in the $\mathrm{Pa}3\ifmmode\bar\else\textasciimacron\fi{}$ structure has a volume dependence that differs from the experimental result and therefore implies a significantly different compression mechanism. The calculated bulk moduli are very similar for the fluorite and $\mathrm{Pa}3\ifmmode\bar\else\textasciimacron\fi{}$ structures and therefore apparently independent of the internal structural parameter. The structure and stability of a hypothetical orthorhombic ${\mathrm{RuO}}_{2}$ phase is investigated.

In most of the mammals, birth and weaning are two periods of nutritional transitions. Whereas the fetus oxidizes mainly glucose, lactate and aminoacids, the newborn is fed with milk, a high fat, low carbohydrate diet. At weaning, milk is replaced progressively by the adult diet which contains less fat and more carbohydrate. In the hours and days following birth, the newborn adapts itself to the new nutritional environment by increasing its capacity to produce glucose de novo (gluconeogenesis) in order to satisfy its high glucose needs. Oxidation of fatty acids is enhanced in the liver and at the peripheral level. Ketone bodies synthetized from fatty acids in the liver in large amounts are utilized by other tissues and specially the brain where they can met energetic and synthetic needs. In the rat, during the suckling period, lipogenesis is decreased in the liver and in white adipose tissue and triglyceride accretion is minimized. At weaning, these adaptations are reversed: decreased gluconeogenic and oxidative capacity of the liver, decrease of the role of ketone bodies, increase of the lipogenic rate in the liver and the adipose tissue, storage of triglycerides. The nutritional and hormonal factors involved in these metabolic adaptations are numerous but insulin and glucagon might play a major role.

Glucose transporter 4 (GLUT4) protein expression was characterized in human and rodent term placentas. A 50-kDa protein was detected, by immunoblotting, in term human placenta at levels averaging 25% of those found in white adipose tissue. It was also present, albeit at lower levels, in mouse and rat placentas. The specificity of the 50-kDa signal was established by using skeletal muscle and placental tissues obtained from GLUT4-null mice as controls. Indirect immunohistochemistry, performed in human placentas, showed that intravillous stromal cells were conspicuously labeled by GLUT4 and revealed colocalization of GLUT4 transporters with insulin receptors. This study provides the first evidence that the insulin-responsive GLUT4 glucose transporter is present in human and rodent hemochorial placentas. Placental GLUT4 gene and protein levels were not modified in human pregnancy complicated by insulin-dependent diabetes mellitus. The significance of the high level of GLUT4 protein in human placenta remains to be elucidated, because, so far, this organ was not considered to be insulin-sensitive, with regard to glucose transport.

Leptin is the product of OB gene. This 16 kDa protein is produced by mature adipocytes and is secreted in plasma. Its plasma levels are strongly correlated with adipose mass in rodents as well as in humans. Leptin inhibits food intake, reduces body weight and stimulates energy expenditure. It has been suggested that leptin could be the link between obesity and diabetes. Recent experiments in rodents have shown that leptin expression in adipocytes is also regulated at short-term by hormones and nutrients. Leptin expression increases after food intake and decreases during fasting and diabetes. Insulin and glucocorticoids increase leptin expression, whereas catecholamines, via beta-adrenergic receptors and cAMP, and long-chain fatty acids (and thiazolidinediones), via PPARy, inhibit leptin expression. Leptin is a cytokine that binds to transmembrane receptors similar to the receptors of cytokine family (type IL-6), and transmit their information inside the cell, after dimerisation. A short-form of leptin receptor (with a cytoplasmic domain of 34 amino residues) has been identified in the choroid plexus. This type of receptor should be used for leptin transport across the blood-brain barrier. Then leptin binds to a long-form of leptin receptor in the hypothalamus (with a cytoplasmic domain of 302 amino residues) and decreases the production of neuropeptide Y, a neuromediator of food intake. The long-form of leptin receptor, transmits its information via the Janus Kinases (JAK) who subsequently phosphorylate transcription factors of the STAT family. Intermediary forms of leptin receptor have been identified in other tissues: liver, heart, skeletal muscles, endocrine pancreas. The role of leptin receptors in these tissues remains obscure, but is of considerable interest. Recent studies have shown that leptin inhibits insulin secretion and have anti-insulin effects on liver and adipose tissue. If these effects are confirmed, leptin could play a role similar to TNF alpha and could participate in the insulin-resistance of obesity and type II diabetes.

Abstract The thermal conductivity of carbon dioxide and steam has been measured as a function of temperature and density using a concentric cylinder method. Earlier measurements of the thermal conductivity of CO 2 , obtained with a parallel plate method, covered a range of temperatures from 25 °C to 75 °C and revealed the existence of an anomalous thermal conductivity in the critical region. In this paper the experimental temperature range for the thermal conductivity of CO 2 is extended to 700°C. The high temperature data enable us to determine a “background” thermal conductivity needed for a quantitative analysis of the anomalous thermal conductivity in the critical region. In addition, we provide experimental evidence that the thermal conductivity of steam exhibits an anomalous increase in the critical region similar to the behavior observed for the thermal conductivity of CO 2.

Specific-heat, magnetic-susceptibility, and resistivity measurements on annealed polycrystalline samples of Ce${\mathit{M}}_{2}$${\mathrm{Sn}}_{2}$, with M=Ni, Ir, Cu, Rh, Pd, and Pt, indicate that each of these compounds orders antiferromagnetically at N\'eel temperatures ${\mathit{T}}_{\mathit{N}}$ between \ensuremath{\sim}0.5 and 4.2 K. Just above ${\mathit{T}}_{\mathit{N}}$, all these materials have a significant enhancement of their electronic specific heat, which can be as large as 3--4 J/${\mathrm{molK}}^{2}$. We discuss the role of critical fluctuations on the specific heat and argue that the enhancement is associated with a large effective mass and not fluctuations. The anomalously low ordering temperatures and very large electronic specific heat suggest that ${\mathit{T}}_{\mathit{N}}$ and the Kondo temperature ${\mathit{T}}_{\mathit{K}}$ are comparable, making these materials particularly attractive for studying the competition between Kondo and Ruderman-Kittel-Kasuya-Yosida interactions. Measurements of the pressure dependence of ${\mathit{T}}_{\mathit{N}}$ in ${\mathrm{CeIr}}_{2}$${\mathrm{Sn}}_{2}$ were performed to explore this competition, which is discussed in terms of Doniach's Kondo-necklace model.

We study the efficiency at maximum power of two coupled heat engines, using thermoelectric generators (TEGs) as engines. Assuming that the heat and electric charge fluxes in the TEGs are strongly coupled, we simulate numerically the dependence of the behavior of the global system on the electrical load resistance of each generator in order to obtain the working condition that permits maximization of the output power. It turns out that this condition is not unique. We derive a simple analytic expression giving the relation between the electrical load resistance of each generator permitting output power maximization. We then focus on the efficiency at maximum power (EMP) of the whole system to demonstrate that the Curzon-Ahlborn efficiency may not always be recovered: The EMP varies with the specific working conditions of each generator but remains in the range predicted by irreversible thermodynamics theory. We discuss our results in light of nonideal Carnot engine behavior.