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‣ Síntese e auto-organização de nanopartículas ferromagnéticas metálicas visando aplicações em gravação magnética de ultra-alta densidade e imãs permanentes de elevado desempenho; Synthesis and self-assembling of metallic ferromagnetic nanoparticles for ultrahigh density magnetic recording and high-performance permanent magnets applications

Silva, Tiago Luis da
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 15/05/2015 Português
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Nanomateriais de fct-FePt, SmCo e Carbeto de Cobalto têm sido bastante estudados para a aplicação em gravação magnética e imãs de elevado desempenho, devido as suas energias magnetocristalinas e coercividades elevadas. Nanopartículas de FePt unidimensionais foram propostas na tentativa de obter melhora no alinhamento magnético das estruturas auto-organizadas. Neste trabalho, a formação de nanobastão e nanofios de FePt foi estudada através da presença de oleilamina e pequena quantidade de monóxido de carbono liberado pelo pentacarbonilferro(0). Estes dois parâmetros foram estudados a fim de analisar a influência no alongamento das nanopartículas e verificou-se que ambos atuam sinergicamente. Foram obtidos tanto nanofios de FePt ramificados de comprimento de 20-100 nm quanto nanobastões de FePt de 20-60 nm de comprimento, ambos com diâmetro de 2-3 nm. Todas as nanopartículas sintetizadas foram obtidas na fase cúbica de face centrada e o processo de tratamento térmico nas temperaturas de 450 oC e 560 oC levou a conversão para a fase tetragonal de face centrada, com custo da sinterização das nanopartículas. Os nanobastões, entretanto, apresentaram maior estabilidade térmica se comparado com o nanofio ramificado...

‣ Ferroelectric and structural instability of (Pb,Ca)TiO3 thin films prepared in an oxygen atmosphere and deposited on LSCO thin films which act as a buffer layer

Pontes, D. S L; Pontes, F. M.; Pereira-da-Silva, Marcelo A.; Zampieri, M.; Chiquito, A. J.; Pizani, P. S.; Longo, Elson
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica
Português
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Structural, microstructural and ferroelectric properties of Pb0.90Ca0.10TiO3 (PCT10) thin films deposited using La0.50Sr0.50CoO3 (LSCO) thin films which serve only as a buffer layer were compared with properties of the thin films grown using a platinum-coated silicon substrate. LSCO and PCT10 thin films were grown using the chemical solution deposition method and heat-treated in an oxygen atmosphere at 700 °C and 650 °C in a tube oven, respectively. X-ray diffraction (XRD) and Raman spectroscopy results showed that PCT10 thin films deposited directly on a platinum-coated silicon substrate exhibit a strong tetragonal character while thin films with the LSCO buffer layer displayed a smaller tetragonal character. Surface morphology observations by atomic force microscopy (AFM) revealed that PCT10 thin films with a LSCO buffer layer had a smoother surface and smaller grain size compared with thin films grown on a platinum-coated silicon substrate. Additionally, the capacitance versus voltage curves and hysteresis loop measurement indicated that the degree of polarization decreased for PCT10 thin films on a LSCO buffer layer compared with PCT10 thin films deposited directly on a platinum-coated silicon substrate. This phenomenon can be described as the smaller shift off-center of Ti atoms along the c-direction 〈001〉 inside the TiO6 octahedron unit due to the reduction of lattice parameters. Remnant polarization (P r ) values are about 30 μC/cm2 and 12 μC/cm2 for PCT10/Pt and PCT10/LSCO thin films...

‣ Platinum instability in PEM fuel cells MEA’s subjected to chloride contamination

Rangel, C. M.; Paiva, T. I.; Hashimoto, T.; Thompson, G. E.
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em 10/11/2011 Português
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In this work a low power fuel cell, intended for passive management of water, was operated integrating a range of relative humidity (RH) from ~30 to 80% and temperatures from 5 to 55 ºC. The stack was fed with pure hydrogen. An open air cathode was designed for easy water removal and stack cooling. The stack uses own design flow field drawn on graphite plates from Schunk and a commercial MEA with carbon supported catalyst containing 0.3 mgcm-2 Pt. Polarization curves were registered for a full stack characterization using a purpose-built test station and a climatic chamber with temperature and RH control. Results indicated that 60% RH is associated to maximum fuel cell performance over the studied temperature range. While water management is done in a passive fashion, heat management is done on the basis of the injection of air at the cathode with the fuel cell showing good performances at relatively low currents where back diffusion towards the anode is favored. The loss of performance with temperature increase was related to an increase in the membrane resistance which may correspond to loss of water on the anode side. Performances at temperatures lower that room temperature showed only slight decrease in power. An examination of the fuel cell components after 100 h of operation revealed that chloride contamination has produced cathode failure associated to catalyst migration anomalies favored by operation conditions that allowed platinum particles to break free from their carbon backing and migrate toward the polymer electrolyte. Migration resulted in precipitation with larger mean particle size distribution within the solid electrolyte when compared to the original catalyst layer...

‣ Oscillatory instabilities in the electrooxidation of borohydride on platinum

Machado,Eduardo G.; Varela,Hamilton
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2014 Português
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The borohydride ion has been pointed as a promising alternative fuel. Most of the investigation on its electrochemistry is devoted to the electrocatalytic aspects of its electrooxidation on platinum and gold surfaces. Besides the known kinetic limitations and intricate mechanism, our Group has recently found the occurrence of two regions of bi-stability and autocatalysis in the electrode potential during the open circuit interaction of borohydride and oxidized platinum surfaces. Following this previous contribution, the occurrence of more complicated phenomena is here presented: namely the presence of electrochemical oscillations during the electrooxidation of borohydride on platinum in alkaline media. Current oscillations were found to be associated to two distinct instability windows and characterized in the resistance-potential parameter plane. The dynamic features of such oscillations suggest the existence of distinct mechanisms according to the potential region. Previously published results obtained under non-oscillatory regime were used to give some hints on the surface chemistry behind the observed dynamics.

‣ Instability of the monofunctional adducts in cis-[Pt(NH3)2(N7-N-methyl-2-diazapyrenium)Cl](2+)-modified DNA: rates of cross-linking reactions in cis-platinum-modified DNA.

Payet, D; Gaucheron, F; Sip, M; Leng, M
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 25/12/1993 Português
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Single- and double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)(N7-N-methyl-2-diazapyrenium)]3+ adduct have been studied at two NaCl concentrations. In 50 mM and 1 M NaCl, the adducts within the single-stranded oligonucleotides are stable. In contrast, they are unstable within the corresponding double-stranded oligonucleotides. In 50 mM NaCl, the bonds between platinum and guanine or N-methyl-2,7-diazapyrenium residues are cleaved and subsequently, intra- or interstrand cross-links are formed as in the reaction between DNA and cis-DDP. In 1 M NaCl, the main reaction is the replacement of N-methyl-2,7-diazapyrenium residues by chloride which generates double-stranded oligonucleotides containing a single monofunctional cis-[Pt(NH3)2(dG)Cl]+ adduct. The rates of closure of these monofunctional adducts to bifunctional cross-links have been studied in 60 mM NaClO4. Within d(TG.CT/AGCA), d(CG.CT/AGCG) and d(AG.CT/AGCT) (the symbol.indicates the location of the adducts in the central sequences of oligonucleotides), the half-lifes (t1/2) of the cis-[Pt(NH3)2(dG)Cl]+ adducts are respectively 12, 6 and 2.8 hr and the cross-linking reactions occur between guanine residues on the opposite strands. Within d(AG.TC/GACT)...

‣ Nucleotide excision repair deficiency is intrinsic in sporadic stage I breast cancer

Latimer, Jean J.; Johnson, Jennifer M.; Kelly, Crystal M.; Miles, Tiffany D.; Beaudry-Rodgers, Kelly A.; Lalanne, Nancy A.; Vogel, Victor G.; Kanbour-Shakir, Amal; Kelley, Joseph L.; Johnson, Ronald R.; Grant, Stephen G.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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The molecular etiology of breast cancer has proven to be remarkably complex. Most individual oncogenes are disregulated in only approximately 30% of breast tumors, indicating that either very few molecular alterations are common to the majority of breast cancers, or that they have not yet been identified. In striking contrast, we now show that 19 of 19 stage I breast tumors tested with the functional unscheduled DNA synthesis assay exhibited a significant deficiency of DNA nucleotide excision repair (NER) capacity relative to normal epithelial tissue from disease-free controls (n = 23). Loss of DNA repair capacity, including the complex, damage-comprehensive NER pathway, results in genomic instability, a hallmark of carcinogenesis. By microarray analysis, mRNA expression levels for 20 canonical NER genes were reduced in representative tumor samples versus normal. Significant reductions were observed in 19 of these genes analyzed by the more sensitive method of RNase protection. These results were confirmed at the protein level for five NER gene products. Taken together, these data suggest that NER deficiency may play an important role in the etiology of sporadic breast cancer, and that early-stage breast cancer may be intrinsically susceptible to genotoxic chemotherapeutic agents...

‣ Drug therapy for hereditary cancers

Imyanitov, Evgeny N; Moiseyenko, Vladimir M
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
Publicado em 06/08/2011 Português
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Tumors arising in patients with hereditary cancer syndromes may have distinct drug sensitivity as compared to their sporadic counterparts. Breast and ovarian neoplasms from BRCA1 or BRCA2 mutation carriers are characterized by deficient homologous recombination (HR) of DNA, that makes them particularly sensitive to platinum compounds or inhibitors of poly (ADP-ribose) polymerase (PARP). Outstandingly durable complete responses to high dose chemotherapy have been observed in several cases of BRCA-related metastatic breast cancer (BC). Multiple lines of evidence indicate that women with BRCA1-related BC may derive less benefit from taxane-based treatment than other categories of BC patients. There is virtually no reports directly assessing drug response in hereditary colorectal cancer (CRC) patients; studies involving non-selected (i.e., both sporadic and hereditary) CRC with high-level microsatellite instability (MSI-H) suggest therapeutic advantage of irinotecan. Celecoxib has been approved for the treatment of familial adenomatous polyposis (FAP). Hereditary medullary thyroid cancers (MTC) have been shown to be highly responsive to a multitargeted tyrosine kinase inhibitor vandetanib, which exerts specific activity towards mutated RET receptor. Given the rapidly improving accessibility of DNA analysis...

‣ DNA-Platinum Thin Films for Use in Chemoradiation Therapy Studies

Rezaee, Mohammad; Alizadeh, Elahe; Hunting, Darel; Sanche, Léon
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
Português
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Dry films of platinum chemotherapeutic drugs covalently bound to plasmid DNA (Pt-DNA) represent a useful experimental model to investigate direct effects of radiation on DNA in close proximity to platinum chemotherapeutic agents, a situation of considerable relevance to understand the mechanisms underlying concomitant chemoradiation therapy. In the present paper we determine the optimum conditions for preparation of Pt-DNA films for use in irradiation experiments. Incubation conditions for DNA platination reactions have a substantial effect on the structure of Pt-DNA in the films. The quantity of Pt bound to DNA as a function of incubation time and temperature is measured by inductively coupled plasma mass spectroscopy. Our experiments indicate that chemical instability and damage to DNA in Pt-DNA samples increase when DNA platination occurs at 37°C for 24 hours, the condition which has been extensively used for in vitro studies. Platination of DNA for the formation of Pt-DNA films is optimal at room temperature for reaction times less than 2 hours. By increasing the concentration of Pt compounds relative to DNA and thus accelerating the rate of their mutual binding, it is possible to prepare Pt-DNA samples containing known concentrations of Pt while reducing DNA degradation caused by more lengthy procedures.

‣ DNA-PK Mediates AKT Activation and Apoptosis Inhibition in Clinically Acquired Platinum Resistance12

Stronach, Euan A; Chen, Michelle; Maginn, Elaina N; Agarwal, Roshan; Mills, Gordon B; Wasan, Harpreet; Gabra, Hani
Fonte: Neoplasia Press Inc. Publicador: Neoplasia Press Inc.
Tipo: Artigo de Revista Científica
Publicado em /11/2011 Português
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Clinical resistance to chemotherapy is a frequent event in cancer treatment and is closely linked to poor outcome. High-grade serous (HGS) ovarian cancer is characterized by p53 mutation and high levels of genomic instability. Treatment includes platinum-based chemotherapy and initial response rates are high; however, resistance is frequently acquired, at which point treatment options are largely palliative. Recent data indicate that platinum-resistant clones exist within the sensitive primary tumor at presentation, implying resistant cell selection after treatment with platinum chemotherapy. The AKT pathway is central to cell survival and has been implicated in platinum resistance. Here, we show that platinum exposure induces an AKT-dependent, prosurvival, DNA damage response in clinically platinum-resistant but not platinum-sensitive cells. AKT relocates to the nucleus of resistant cells where it is phosphorylated specifically on S473 by DNA-dependent protein kinase (DNA-PK), and this activation inhibits cisplatin-mediated apoptosis. Inhibition of DNA-PK or AKT, but not mTORC2, restores platinum sensitivity in a panel of clinically resistant HGS ovarian cancer cell lines: we also demonstrate these effects in other tumor types. Resensitization is associated with prevention of AKT-mediated BAD phosphorylation. Strikingly...

‣ Human MLH1 status can potentially predict cisplatin sensitivity but not microsatellite instability in head and neck squamous cell carcinoma cells

ADACHI, MAKOTO; IJICHI, KEI; HASEGAWA, YASUHISA; NAKAMURA, HIDEAKI; OGAWA, TETSUYA; KANEMATSU, NOBUTAKE
Fonte: D.A. Spandidos Publicador: D.A. Spandidos
Tipo: Artigo de Revista Científica
Português
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Resistance to platinum-based chemotherapy frequently poses a serious problem in the treatment of head and neck squamous cell carcinoma. In this study, we isolated cisplatin-resistant cells from a head and neck squamous cell carcinoma cell line. The mismatch repair (MMR) system is known as one of the cisplatin-resistant mechanisms. When the expression levels of hMLH1 and hMSH2, a mismatch repair gene and its gene product, were analyzed, the hMLH1 mRNA and protein expression levels were significantly decreased in the cisplatin-resistant cell lines compared with a cisplatin-sensitive cell line. In addition, the microsatellite instability (MSI) phenotype was examined for the absence of MMR. Our data support the hypothesis that hMLH1 mRNA and protein expression levels are predictors of cisplatin sensitivity, but MSI was not involved in cisplatin resistance. The status of hMLH1 predicts the sensitivity of head and neck squamous cell carcinoma to platinum-based chemotherapy.

‣ NFκB mediates cisplatin resistance through histone modifications in head and neck squamous cell carcinoma (HNSCC)☆

Almeida, Luciana O.; Abrahao, Aline C.; Rosselli-Murai, Luciana K.; Giudice, Fernanda S.; Zagni, Chiara; Leopoldino, Andreia M.; Squarize, Cristiane H.; Castilho, Rogerio M.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em 30/12/2013 Português
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Cisplatin-based chemotherapy is the standard treatment of choice for head and neck squamous cell carcinoma (HNSCC). The efficiency of platinum-based therapies is directly influenced by the development of tumor resistance. Multiple signaling pathways have been linked to tumor resistance, including activation of nuclear factor kappa B (NFκB). We explore a novel mechanism by which NFκB drives HNSCC resistance through histone modifications. Post-translational modification of histones alters chromatin structure, facilitating the binding of nuclear factors that mediate DNA repair, transcription, and other processes. We found that chemoresistant HNSCC cells with active NFκB signaling respond to chemotherapy by reducing nuclear BRCA1 levels and by promoting histone deacetylation (chromatin compaction). Activation of this molecular signature resulted in impaired DNA damage repair, prolonged accumulation of histone γH2AX and increased genomic instability. We found that pharmacological induction of histone acetylation using HDAC inhibitors prevented NFκB-induced cisplatin resistance. Furthermore, silencing NFκB in HNSCC induced acetylation of tumor histones, resulting in reduced chemoresistance and increased cytotoxicity following cisplatin treatment. Collectively...

‣ Modeling Platinum Sensitive and Resistant High-Grade Serous Ovarian Cancer: Development and Applications of Experimental Systems

Cunnea, Paula; Stronach, Euan A.
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 17/04/2014 Português
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High-grade serous ovarian cancer remains the most common sub-type of ovarian cancer and, characterized by high degrees of genomic instability and heterogeneity, is typified by a transition from early response to acquired resistance to platinum-based chemotherapy. Conventional models for the study of ovarian cancer have been largely limited to a set of relatively poorly characterized immortalized cell lines and recent studies have called into question the validity of some of these as reliable models. Here, we review new approaches and models systems that take into account advances in our understanding of ovarian cancer biology and advances in the technology available for their generation and study. We discuss primary cell models, 2D, 3D, and organotypic models, and “paired” sample approaches that capture the evolution of chemotherapy failure within single cases. We also overview new methods for non-invasive collection of representative tumor material from blood samples. Adoption of such methods and models will improve the quality and clinical relevance of ovarian cancer research.

‣ A Genomic Instability Score in Discriminating Nonequivalent Outcomes of BRCA1/2 Mutations and in Predicting Outcomes of Ovarian Cancer Treated with Platinum-Based Chemotherapy

Zhang, Shaojun; Yuan, Yuan; Hao, Dapeng
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 01/12/2014 Português
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Detecting mutation in BRCA1/2 is a generally accepted strategy for screening ovarian cancers that have impaired homologous recombination (HR) ability and improved sensitivity to PARP inhibitor. However, a substantial subset of BRCA-mutant ovarian cancer patients shows less impaired or unimpaired HR ability, resulting in nonequivalent outcome after ovarian cancer development. We hypothesize that genomic instability provides a lifetime record of DNA repair deficiency and predicts ovarian cancer outcome. Based on the multi-dimensional TCGA ovarian cancer data, we developed a biological rationale-driven genomic instability score integrating somatic mutation and copy number change in a tumor genome. The score successfully divided BRCA-mutant ovarian tumors into cases of significantly improved outcome and cases of unimproved outcome. The score was also capable of discriminating HR-deficiency indicated by BRCA1 epigenetically silencing, EMSY amplification and homozygous deletion of core HR genes. We further found that the score was positively correlated with the complete response rate of chemotherapy and the rate of platinum-sensitivity, and predicted improved outcome of ovarian cancer, regardless of BRCA-mutation status. The score may have important value in outcome prediction and clinical trial design.

‣ Pan-cancer analysis of genomic scar signatures associated with homologous recombination deficiency suggests novel indications for existing cancer drugs

Marquard, Andrea M; Eklund, Aron C; Joshi, Tejal; Krzystanek, Marcin; Favero, Francesco; Wang, Zhigang C; Richardson, Andrea L; Silver, Daniel P; Szallasi, Zoltan; Birkbak, Nicolai J
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
Português
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Background: Ovarian and triple-negative breast cancers with BRCA1 or BRCA2 loss are highly sensitive to treatment with PARP inhibitors and platinum-based cytotoxic agents and show an accumulation of genomic scars in the form of gross DNA copy number aberrations. Cancers without BRCA1 or BRCA2 loss but with accumulation of similar genomic scars also show increased sensitivity to platinum-based chemotherapy. Therefore, reliable biomarkers to identify DNA repair-deficient cancers prior to treatment may be useful for directing patients to platinum chemotherapy and possibly PARP inhibitors. Recently, three SNP array-based signatures of chromosomal instability were published that each quantitate a distinct type of genomic scar considered likely to be caused by improper DNA repair. They measure telomeric allelic imbalance (named NtAI), large scale transition (named LST), and loss of heterozygosity (named HRD-LOH), and it is suggested that these signatures may act as biomarkers for the state of DNA repair deficiency in a given cancer. Results: We explored the pan-cancer distribution of scores of the three signatures utilizing a panel of 5371 tumors representing 15 cancer types from The Cancer Genome Atlas, and found a good correlation between scores of the three signatures (Spearman’s ρ 0.73–0.87). In addition we found that cancer types ordinarily receiving platinum as standard of care have higher median scores of all three signatures. Interestingly...

‣ Sierra Leone : Tapping the Mineral Wealth for Human Progress, A Break with the Past

World Bank
Fonte: Washington, DC Publicador: Washington, DC
Português
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Political instability and lack of an enabling and internationally competitive environment conducive to attracting investments particularly affected the mining sector. Excessive government control and attempts to raise higher foreign exchange and fiscal revenues from artisanal diamond activities were detrimental to artisanal mining development. The study provides an assessment and analysis of the economic and social importance of the mining sector in Sierra Leone and its impact on economic outcomes. It makes recommendations and identifies an action plan for mining sector revival and development. The Government presented this report to the stakeholders, including its international development partners, at a workshop in May 2003. The outcome of the discussions and recommendations is reflected in the report. The resulting report was subsequently discussed in a second national workshop during January 2004. Since January 2003, the Ministry of Mineral Resources has focused on a number of fronts including: formation of a core mineral policy; deepening the policy work through a review of the mineral and regulatory framework; efforts to improve the development of basic mapping geo-information; completion of the initial work required for improved administration and management of mineral rights through a cadastre system; improvements in productivity...

‣ The Contribution of the Mining Sector to Socioeconomic and Human Development

McMahon, Gary; Moreira, Susana
Fonte: World Bank, Washington, DC Publicador: World Bank, Washington, DC
Português
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Many low and middle-income mineral-rich countries have experienced strong growth for a decade or longer, propelled by a rapid expansion of their mineral exports and a rise in prices of these commodities. This sustained strong economic performance goes against the accepted wisdom that even though the mining sector, like other extractive industries, can generate foreign exchange and fiscal revenues, it contributes little to sustained economic growth and, by extension, human development. Through the presentation of trends and patterns of various indicators, this paper shows that in addition to economic growth, countries rich in minerals other than oil have experienced significant improvements in their human development index (HDI) scores that are on average better than those experienced by countries without minerals. In a sample of five low and middle-income countries with relatively long histories of mining, benefits came from foreign direct investment (FDI), export revenues, and fiscal revenues. The overall impact of the mining sector was much stronger if there were infrastructure benefits and strong linkages to other industries...

‣ Thermal instability of decahedral structures in platinum nanoparticles

Schebarchov, D.; Hendy, S. C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/08/2006 Português
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We conduct molecular dynamics simulations of 887 and 1389-atom decahedral platinum nanoparticles using an embedded atom potential. By constructing microcanonical caloric curves, we identify structural transitions from decahedral to fcc in the particles prior to melting. The transitions take place during phase coexistence and appear to occur via melting of the decahedral structure and subsequent recrystallisation into the fcc structure.; Comment: 11 pages, 5 figures

‣ Dynamics and stability of icosahedral Fe-Pt nanoparticles

Jochym, Paweł T.; Łażewski, Jan; Sternik, Małgorzata; Piekarz, Przemysław
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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The structure, dynamics and stability of Fe-Pt nanoparticles have been investigated using DFT-based techniques: total energy calculations and DFT molecular dynamics. The investigated systems included multi-shell and disordered nanoparticles of iron and platinum. The study is concerned with icosahedral particles with magic number of atoms (55): iron-terminated Fe$_{43}$Pt$_{12}$, platinum-terminated Fe$_{12}$Pt$_{43}$, and disordered Fe$_{27}$Pt$_{28}$. Additionally, the Fe$_6$Pt$_7$ cluster has been investigated to probe behaviour of extremely small Fe-Pt particles. Molecular dynamics simulations have been performed for a few temperatures between $T=150-1000$ K. The calculations revealed high structural instability of the Fe-terminated nanoparticles and a strong stabilising effect of the Pt-termination in the shell-type icosahedral particles. The platinum termination prevented disordering of the particle even at $T=1000$ K indicating very high melting temperatures of these Fe-Pt icosahedral structures. The analysis of evolution of the radial distribution function has shown significant tendency of Pt atoms to move to the outside layer of the particles -- even in the platinum deficient cases.; Comment: 8 pages, 7 figures

‣ Charge Density Wave Instability and Soft Phonon in $A$Pt$_3$P ($A$=Ca, Sr, and La)

Chen, Hui; Xu, Xiaofeng; Cao, Chao; Dai, Jianhui
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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The electronic and phonon properties of the platinum pnictide superconductors $A$Pt$_3$P ($A$=Ca, Sr, and La) were studied using first-principles calculations. The spin-orbit coupling effect is significant in LaPt$_3$P but negligible in CaPt$_3$P and SrPt$_3$P, although they all share the same anti-pevroskite structure. Moreover, SrPt$_3$P has been demonstrated to exhibit an unexpected weak charge-density-wave(CDW) instability which is neither simply related to the Fermi-surface nesting nor to the momentum-dependent electron-phonon coupling alone. The instability is absent in CaPt$_3$P and can be quickly suppressed by the external pressure, accompanied with gradual decreases in the phonon softening and BCS $T_c$. Our results suggest SrPt$_3$P as a rare example where superconductivity is enhanced by the CDW fluctuations.

‣ The effect of chloride as catalyst layer contaminant on the degradation of PEMFCs

Paiva, T. I.; Hashimoto, T.; Plancha, Maria João; Thompson, G. E.; Rangel, C. M.
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em //2013 Português
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In this work, the effect of chloride as a catalyst contaminant was studied on the performance and durability of a low power open-cathode fuel cell, intended for passive management of water. In an ex-situ study, cyclic voltammetry was used to assess the redox behaviour of platinum in chloride contaminated solutions at room temperature.The cell was operated integrating a range of relative humidity (RH) from ~30 to 80% and temperatures from 5 to 55 ºC. Results indicated that 60% RH is associated to maximum fuel cell performance over the studied temperature range. An examination of the fuel cell components after 100 h of operation revealed that chloride contamination has produced cathode failure associated to catalyst migration favored by operation conditions that allowed platinum particles to break free from their carbon backing and migrate toward the polymer electrolyte. Migration resulted in precipitation with larger mean particle size distribution within the solid electrolyte when compared to the original catalyst layer, rendering a very significant loss of thickness in the cathode material.