In this article, we show that an effective Hamiltonian fit with first-principles calculations predicts that an order/disorder transition occurs in the high-entropy alloy Mo-Nb-Ta-W. Using the Alloy ...Theoretic Automated Toolkit, we find
T
= 0 K enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in particular, we find the stable structures for binaries at equiatomic concentrations are close in energy to the associated B2 structure, suggesting that at intermediate temperatures, a B2 phase is stabilized in Mo-Nb-Ta-W. Our previously published hybrid Monte Carlo (MC)/molecular dynamics (MD) results for the Mo-Nb-Ta-W system are analyzed to identify certain preferred chemical bonding types. A mean field free energy model incorporating nearest-neighbor bonds is derived, allowing us to predict the mechanism of the order/disorder transition. We find the temperature evolution of the system is driven by strong Mo-Ta bonding. A comparison of the free energy model and our MC/MD results suggests the existence of additional low-temperature phase transitions in the system likely ending with phase segregation into binary phases.
We quantify the thermodynamic equilibrium conditions that govern the formation of crystalline heptazine-based carbon nitride materials, currently of enormous interest for photocatalytic applications ...including solar hydrogen evolution. Key phases studied include the monomeric phase melem, the 1D polymer melon, and the hypothetical hydrogen-free 2D graphitic carbon nitride phase “g-C3N4”. Our study is based on density functional theory including van der Waals dispersion terms with different experimental conditions represented by the chemical potential of NH3. Graphitic carbon nitride is the subject of a vast number of studies, but its existence is still controversial. We show that typical conditions found in experiments pertain to the polymer melon (2D planes of 1D hydrogen-bonded polymer strands). In contrast, equilibrium synthesis of heptazine (h)-based g-h-C3N4 below its experimentally known decomposition temperature requires much less likely conditions, equivalent to low NH3 partial pressures around 1 Pa at 500 °C and around 103 Pa even at 700 °C. A recently reported synthesis of triazine (t)-based g-t-C3N4 in a salt melt is interpreted as a consequence of the altered local chemical environment of the C3N4 nanocrystallites.
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•We have semi-quantitatively modeled the temperature dependent Cu–Pd–S phase diagram.•This work uses only first principles calculations and NIST-JANAF gaseous phase data.•Origin of ...stability of observed phases is explained using simple models.•Excellent agreement with experiment for sulfidization threshholds of Pd–S2 are shown.•Qualitative agreement with CALPHAD for activities in Cu–Pd was obtained.
As an aid to the development of hydrogen separation membranes, we predict the temperature dependent phase diagrams using first principles calculations combined with thermodynamic principles. Our method models the phase diagram without empirical fitting parameters. By applying thermodynamic principles and solid solution models, temperature-dependent features of the Cu–Pd–S system can be explained, specifically solubility ranges for substitutions in select crystalline phases. Electronic densities of states calculations explain the relative favorability of certain chemical substitutions. In addition, we calculate sulfidization thresholds for the Pd–S2 system and activities for the Cu–Pd binary in temperature regimes where the phase diagram contains multiple solid phases.
Motivated by interest in the elastic properties of high-strength amorphous metals, we examine the elastic properties of select crystalline phases. Using first-principles methods, we calculate elastic ...moduli in various chemical systems containing transition metals, specifically early (Ta,W) and late (Co,Ni). Theoretically predicted alloy elastic properties are verified for Ni-Ta by comparison with experimental measurements using resonant ultrasound spectroscopy. Comparison of our computed elastic moduli with effective medium theories shows that alloying leads to enhancement of bulk moduli relative to averages of the pure elements and considerable deviation of predicted and computed shear moduli. Specifically, we find an enhancement of bulk modulus relative to effective medium theory and propose a candidate system for high-strength, ductile amorphous alloys. Trends in the elastic properties of chemical systems are analyzed using force constants, electronic densities of state, and crystal overlap Hamilton populations. We interpret our findings in terms of the electronic structure of the alloys.
In this paper we show that an effective Hamiltonian fit with first principles calculations predicts an order/disorder transition occurs in the high entropy alloy MoNbTaW. Using the Alloy Theoretic ...Automated Toolset, we find T=0K enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in particular we find the stable structures for binaries at equiatomic concentrations are close in energy to the associated B2 structure, suggesting that at intermediate temperatures a B2 phase is stabilized in MoNbTaW. Our previously published hybrid Monte Carlo/molecular dynamics results for the MoNbTaW system are analyzed to identify certain preferred chemical bonding types. A mean field free energy model incorporating nearest neighbor bonds is derived, allowing us to predict the mechanism of the order/disorder transition. We find the temperature evolution of the system is driven by strong Mo-Ta bonding. Comparison of the free energy model and our hybrid Monte Carlo/molecular dynamics results suggest the existence of additional low-temperature phase transitions in the system likely ending with phase segregation into binary phases.
Radiation therapy for intracranial germ cell tumors Haas-Kogan, Daphne A; Missett, Brian T; Wara, William M ...
International journal of radiation oncology, biology, physics,
06/2003, Letnik:
56, Številka:
2
Journal Article
Recenzirano
To review the combined experiences of University of California, San Francisco, and Stanford University Medical Center in the treatment of intracranial germ cell tumors (GCT) and to assess the impact ...of craniospinal radiation (CSI) on patterns of relapse, progression-free survival (PFS), and overall survival (OS).
Ninety-three patients received radiation for newly diagnosed intracranial GCTs, including 49 germinomas, 16 nongerminomatous GCTs (NGGCT), and 28 with no biopsy. Median follow-up for surviving patients was 4.5 years (range 0.25–34). Tests for variables correlating with OS and PFS were conducted using Cox proportional hazards model.
Five-year PFS and OS rates were 60% ± 15% and 68% ± 14% for patients with NGGCT and 88% ± 5% and 93% ± 4% for those with germinoma. Of 6 patients with localized NGGCT who did not receive CSI, 1 experienced an isolated spinal recurrence but was salvaged. Of 41 patients with localized germinoma, 6 who received CSI and 35 who did not, no isolated spinal cord relapses occurred. Twenty-one patients with localized germinoma received neither CSI nor whole brain radiation. Of these, none of 18 with ventricular radiation relapsed. One of 3 patients with primary tumor radiation relapsed intracranially but had only received 11 Gy at initial treatment. On multivariate analysis, germinoma histology but not CSI correlated with improved PFS and OS.
CSI is not indicated in the treatment of localized germinomas. For patients with localized germinomas treated with radiation alone, we recommend ventricular irradiation followed by primary tumor boost to a total of 45–50 Gy.
Previously, we showed that 3% (31/1032)of asymptomatic healthcare workers (HCWs) from a large teaching hospital in Cambridge, UK, tested positive for SARS-CoV-2 in April 2020. About 15% (26/169) HCWs ...with symptoms of coronavirus disease 2019 (COVID-19) also tested positive for SARS-CoV-2 (Rivett et al., 2020). Here, we show that the proportion of both asymptomatic and symptomatic HCWs testing positive for SARS-CoV-2 rapidly declined to near-zero between 25th April and 24th May 2020, corresponding to a decline in patient admissions with COVID-19 during the ongoing UK 'lockdown'. These data demonstrate how infection prevention and control measures including staff testing may help prevent hospitals from becoming independent 'hubs' of SARS-CoV-2 transmission, and illustrate how, with appropriate precautions, organizations in other sectors may be able to resume on-site work safely.
The assessed binary phase diagram of boron-carbon exhibits a single alloy phase designated "B4C" with rhombohedral symmetry occupying a broad composition range that falls just short of the nominal ...carbon content of 20%. As this composition range is nearly temperature independent, the phase diagram suggests a violation of the third law of thermodynamics, which typically requires compounds to achieve a definite stoichiometry at low temperatures. By means of first principles total energy calculations we predict the existence of two stoichiometric phases at T=0K: one of composition B4C with monoclinic symmetry; the other of composition B13C2 with rhombohedral symmetry. Using statistical mechanics to extend to finite temperatures, we demonstrate that the monoclinic phase reverts to the rhombohedral phase above T=600K, along with a slight reduction on carbon content.
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