Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal ...conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κ
(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf
Zr
NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κ
, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κ
of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κ
of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.
Predictions of the anisotropic coefficients of thermal expansion are needed to not only compare to experimental measurement, but also as input for macroscopic modeling of devices which operate over a ...large temperature range. While most current methods are limited to isotropic systems within the quasiharmonic approximation, our method uses first-principles calculations and includes anharmonic effects to determine the temperature-dependent properties of materials. These include the lattice parameters, anisotropic coefficients of thermal expansion, isotherm bulk modulus, and specific heat at constant pressure. Our method has been tested on two compounds (Cu and AlN) and predicts thermal properties which compare favorably to experimental measurement over a wide temperature range.
Background and objectives
Metformin is used to treat gestational diabetes. It is also used to treat women with polycystic ovary syndrome and has been shown to prevent late miscarriage and preterm ...birth. However, increased renal clearance during pregnancy causes a decline in serum concentrations of metformin. The aim of this study was to explore the time course of the pregnancy‐related changes in metformin pharmacokinetics and the return to the non‐pregnant state.
Method
A subgroup of women in the PregMet2 study (n = 73) agreed to provide serum samples at three time‐points in pregnancy (gestational weeks 19, 28 and 32) and once in post partum, (either 2, 4 or 8 weeks after delivery). Serum metformin concentrations were compared using a four‐parameter logistic model.
Findings
The mean steady‐state serum concentration of metformin during pregnancy was 9.39 μmoL/L, whereas the post partum concentration was 12.36 μmoL/L, an increase of 32% (p = 0,019). This change took place already during the first 2 weeks post partum.
Conclusion
Clinicians who treat pregnant women with metformin should be aware of the significant decrease in metformin concentration mediated by pregnancy, and the rapid increase after delivery, as it may impact both the therapeutic efficacy and the risk of adverse drug reactions.
► Electronic factors predominately control S interactions with Pd alloys. ► Adsorption enthalpy of 0.25 monolayer S on Pd alloys inversely correlates with density of states d-band center – S p ...bonding peak. ► Sulfur incorporation and Pd segregation at high S coverages leads to Pd
4S formation.
Atomic modeling was conducted to investigate the origin of S interactions with Pd alloy H selective membrane candidates selected from the Pd–Cu, Pd–Ag, and Pd–Au binary systems, as well as their constitutive metals. The electronic characteristics of these alloy/metal systems played a more predominant role in controlling S bonding behavior than surface site geometries. The electronic coupling of S p orbitals bonding with alloy/metal d-bands in the adsorbate/slab density of states split the lower energy p bonding state and the d-band center further apart with increasing S bonding strength. A universal linear correlation was established for increasing adsorption strength (decreasing adsorption enthalpy) of 0.25 monolayer S with the increasing density of states energy difference: d-band center – S p bonding peak. The S interactions predicted at higher coverage provided indications of alloy susceptibility to irreversible S corrosion. The reversible adsorption of 1.0 monolayer S was only the most stable configuration on the more open Pd
0.5Cu
0.5
I
m
3
¯
m
and
P
4
mmm (1
1
0) surfaces. The most competitive configuration for the interaction of a full S monolayer with the Pd
0.75Cu
0.25
P
m
3
¯
m
and Pd
0.875Au
0.125
F
m
3
¯
m
surfaces was the partial desorption and coupling of S. Partial incorporation of S to form a mixed absorbed/adsorbed S monolayer was more favorable for the Pd
F
m
3
¯
m
(1
1
1) surface, and also on the Pd
0.5Cu
0.5
P
4
mmm (1
0
1) and Pd
0.75Ag
0.25
P
m
3
¯
m
(1
1
1) surfaces when accompanied by Pd segregation. The combination of S incorporation and Pd segregation was interpreted to be the first step towards nucleation of irreversible Pd
4S formation.
Boron (B) has the potential for generating an intermediate band in
cubic silicon carbide (3C-SiC), turning this material into a highly
efficient absorber for single-junction solar cells. The ...formation of a
delocalized band demands high concentration of the foreign element, but
the precipitation behavior of B in the 3C polymorph of SiC is not well
known. Here, probe-corrected scanning transmission electron microscopy
and secondary-ion mass spectrometry are used to investigate
precipitation mechanisms in B-implanted 3C-SiC as a function of
temperature. Point-defect clustering was detected after annealing at
1273 K while stacking faults, B-rich precipitates and dislocation
networks developed in the 1573 - 1773 K range. The precipitates adopted
the rhombohedral B
13
C
2
structure and trapped B up
to 1773 K. Above this temperature, higher solubility reduced
precipitation and free B diffused out of the implantation layer. Dopant
concentrations of
\mathbf{10^{19}\:\mathrm{\mathbf{at.cm}}^{-3}}
10
19
.
−
3
were achieved at 1873 K.
Thermodynamic assessments were made to optimize thermodynamic models and parameter fits to selected experimental and first principles hypothetical predicted phase data within the Na–Al–Ti–H system. ...This enabled thermodynamic modeling of Ti solubility within the sodium alanates: NaAlH
4 and Na
3AlH
6, and the relative stability of Ti-bearing phases. The modeling provides insights into the role of Ti originating from Ti-based activating agents commonly referred to as ‘catalysts’ in promoting reversibility of the Na–Al–H dehydrogenation and rehydrogenation reactions under moderate temperature and pressure conditions relevant to H storage applications. Preliminary assessments were made to evaluate H solubility in bcc-Ti and hcp-Ti, and stability of the hydride δ-TiH
2. To model possible Ti dissolution in NaAlH
4 and α-Na
3AlH
6, sub-lattice models were applied. A repulsive interaction is predicted by first principles calculations when Ti is dissolved in NaAlH
4 or α-Na
3AlH
6, which becomes stronger with increasing temperature. Although Ti is virtually insoluble in NaAlH
4 or α-Na
3AlH
6, a small addition of TiCl
3 will induce a thermodynamic driving force for formation of TiH
2 and/or TiAl
3. The addition of pure Ti shows a weaker effect than TiCl
3 and leads to formation of TiH
2 only. Based on a combined interpretation of present thermodynamic modeling and prior experimental observations, the TiAl
3 and TiH
2 phases are ascribed to have a catalytic effect, not a thermodynamic destabilization effect, on the reversibility of the dehydrogenation/rehydrogenation reactions in the Na–Al–H system.
LiMgAlH6 is the intermediate phase when LiMg(AlH4)3 is heated. It contains 9.4 wt.% hydrogen, of which 4.8 wt.% is released during the decomposition step to MgH2 and LiH. Deuterated LiMgAlD6 was ...prepared by heat-treating LiMg(AlD4)3 at 130 deg C. Powder neutron and synchrotron X-ray diffraction patterns were measured and the structure was refined using the Rietveld technique on both patterns simultaneously. LiMgAlD6 crystallizes in the trigonal space group P321 with a = 7.9856(4) A and c = 4.3789(3) A. The structure consists of isolated AlD6 octahedra connected through octahedrally coordinated Mg- and Li-atoms.
Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×10 17 atoms/cm 2 in order to form ...intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 oC for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ~3×10 21 cm -3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ~1.7-1.8 eV, which may be associated with B-precipitates having a sufficiently high density.
PDF
