Flash sintering of complex oxides

BiFeO3 is one of the most promising multiferroic (materials combining two or more ferroic properties in the same phase) materials. Preparation of high quality BiFeO3 ceramics is quite challenging. The temperature required for processing is relatively high and partial decomposition takes place damaging the properties of the material. Moreover, the Curie temperature of BiFeO3 is within the same temperature range required for conventional sintering. The volume change during Curie transition produces damage in the obtained pellets. In this work high density ceramics of BiFeO3 have been obtained from nanostructured powders, prepared by direct mechanochemistry, followed by FLASH sintering. For FLASH sintering experiments, the original set up developed in Prof. Raj’s lab has been used using linear heating rate. Effect of Flash sintering conditions, namely applied voltage and current intensity, has been investigated. It has been observed that for applied voltages of 50 Vcm-1 or larger sintering occurs almost instantly once certain temperature is reached, corresponding to a FLASH regime. For lower applied voltages densification takes place progressively in a kind of field assisted or FAST regime. For FLASH experiments, densification takes place at furnace temperatures at low as 480ºC within few seconds, while conventional heating requires temperatures above 850ºC. Interestingly, under FLASH conditions pure phase materials with high density were obtained, while for those materials obtained in the FAST regime mixtures were obtained. This behavior could be explained by the effect of applied voltage on the temperature of the Curie transition. Electrical properties of the samples obtained by FLASH were characterized using complex impedance spectroscopy at different temperatures. It was concluded that resulting materials were electrically insulating. It is worth mentioning that no reduction in the sample was observed, while for samples prepared by spark plasma sintering (SPS) it has been previously reported lower resistivity and activation energy values due to reduction in the samples during the SPS experiment

Reaction flash sintering for producing high quality functional ceramics within seconds

For ceramic materials, it has been recently shown in literature that applying a small electric field and a small DC current through a sample produces sudden sintering (within seconds) at relatively low temperatures. This method is known as Flash Sintering and it has been applied to number of materials. In this work it is shown that both chemical reaction and sintering can be combined into a single flash sintering experiments. This new approach is known as Reaction Flash Sintering. To demonstrate the feasibility of this method, a multiferroic material, BiFeO3, is prepared from a stoichiometric mixture of Bi2O3 and Fe2O3 oxides. Thus, in a single process, dense nanostructured BiFeO3 ceramics are obtained by applying an electric field of 50 V cm-1 and with a current limit of 35 mA mm-2 within seconds at a furnace temperature of about 625 °C. The resulting materials were pure-phase perovskites without any evidence of secondary phases, sillenite or mullite, that are commonly present in materials prepared by conventional procedures. Moreover, samples were electrically insulating, as measured by complex impedance spectroscopy. It is shown here that the synthesis of pure single-phase ceramics of complex oxides from stoichiometric mixtures of single oxides is possible by reaction flash sintering, even for materials difficult to prepare by conventional procedures. This discovery is a breakthrough in materials preparation

Defect chemistry and electrical properties of BiFeO3

BiFeO3 attracts considerable attention for its rich functional properties, including room temperature coexistence of magnetic order and ferroelectricity and more recently, the discovery of conduction pathways along ferroelectric domain walls. Here, insights into the defect chemistry and electrical properties of BiFeO3 are obtained by in situ measurements of electrical conductivity, σ, and Seebeck coefficient, α, of undoped, cation-stoichiometric BiFeO3 and acceptor-doped Bi1−xCaxFeO3−δ ceramics as a function of temperature and oxygen partial pressure pO2. Bi1−xCaxFeO3−δ exhibits p-type conduction; the dependencies of σ and α on pO2 show that Ca dopants are compensated mainly by oxygen vacancies. By contrast, undoped BiFeO3 shows a simultaneous increase of σ and α with increasing pO2, indicating intrinsic behavior with electrons and holes as the main defect species in almost equal concentrations. The pO2-dependency of σ and α cannot be described by a single point defect model but instead, is quantitatively described by a combination of intrinsic and acceptor-doped characteristics attributable to parallel conduction pathways through undoped grains and defect-containing domain walls; both contribute to the total charge transport in BiFeO3. Based on this model, we discuss the charge transport mechanism and carrier mobilities of BiFeO3 and show that several previous experimental findings can readily be explained within the proposed model

Electrical properties of bismuth ferrites:Bi2Fe4O9 and Bi25FeO39

Bi2Fe4O9 was prepared by solid-state reaction and the electrical properties measured by impedance spectroscopy. After annealing in O2 at 900 °C, Bi2Fe4O9 is an electrically-homogeneous insulator. Its high frequency permittivity is constant (∼14.1) over the temperature range 300–400 °C and shows no evidence of incipient ferroelectricity at lower temperatures. On annealing in N2 at 900 °C, the pellets gradually decompose. Bi25FeO39 was prepared by both solid-state reaction and mechanosynthesis. It showed a modest amount of mixed conduction of both oxide ions and holes. Impedance analysis showed a complex response that best fitted an equivalent circuit consisting of a parallel combination of long-range conduction and short range dielectric relaxation elements. The electrical conductivity of both Bi2Fe4O9 and Bi25FeO39 is less than that of BiFeO3 prepared by solid-state reaction, which indicates that any leakage conductivity of BiFeO3 is not due to the possible presence of small amounts of these secondary phases

Inter-Individual variability in insulin response after grape pomace supplementation in subjects at high cardiometabolic risk: role of microbiota and miRNA

Scope Dietary polyphenols have shown promising effects in mechanistic and preclinical studies on the regulation of cardiometabolic alterations. Nevertheless, clinical trials have provided contradictory results, with high inter‐individual variability. This study explores the role of gut microbiota and microRNAs (miRNAs) as factors contributing to the inter‐individual variability in polyphenol response. Methods and Results 49 subjects with at least two factors of metabolic syndrome are divided between responders (n = 23) or non‐responders (n = 26), depending on the variation rate in fasting insulin after grape pomace supplementation (6 weeks). The populations of selected fecal bacteria are estimated from fecal deoxyribonucleic acid (DNA) by quantitative real‐time polymerase chain reaction (qPCR), while the microbial‐derived short‐chain fatty acids (SCFAs) are measured in fecal samples by gas chromatography. MicroRNAs are analyzed on a representative sample, followed by targeted miRNA analysis. Responder subjects show significantly lower (p < 0.05) Prevotella and Firmicutes levels, and increased (p < 0.05) miR‐222 levels. Conclusion After evaluating the selected substrates for Prevotella and target genes of miR‐222, these variations suggest that responders are those subjects exhibiting impaired glycaemic control. This study shows that fecal microbiota and miRNA expression may be related to inter‐individual variability in clinical trials with polyphenols

Shewanella putrefaciens Fish pathogenic strains contain plasmids that are absent in the probiotic strain s. Putrefaciens pdp11

Probiotics are live microorganisms that confer a health benefit to the host when administered in adequate amounts. Shewanella putrefaciens Pdp11 has been described as a probiotic for farmed fish species such as Solea senegalensis and Sparus aurata. In contrast, other strains of S. putrefaciens have been described as pathogenic for other cultured fish damage of the mouth, extensive skin discoloration, exophthalmia, ascites and bad odour. The S. putrefaciens group was recovered from freshwater samples taken at the L′Albufera system, along autumn– winter 2015. Its counts significantly increased in freshwater parallel to hypoxia and temperature rising. Shewanellae strains were identified as S. putrefaciens and S. xiamenensis by 16S rRNA gene sequencing. These isolates recovered from sick eels or freshwater were virulent for European eel by IP challenge (LD50 106 CFU g−1 body weight. The plasmids plays an important role in the genes transfer and insertion then there can been implicated in antibiotics resistance, degradative pathway and pathogenicity characteristics. The genetic variation conducted by plasmid could induce an impact in probiotic proprieties. In this research, we searched the present or absent of plasmids in pathogenic and probiotic strains of S. putrefaciens. As well as, this plasmids implication in development of virulence factors. The probiotic strain S. putrefaciens Pdp11 did not present plasmid, which was only found in two of the five pathogenic strains. The results allowed us to discard the probiotic Pdp11 could present a pathogenic characteristic as the TA type II system as a virulence factor and its self-regulating characteristics, which may be behind its probiotic nature, making the Pdp11 strain unique in comparison to other S. putrefaciens strains