Matjaz Valant

Bio: Matjaz Valant is an academic researcher from University of Nova Gorica. The author has contributed to research in topics: Dielectric & Permittivity. The author has an hindex of 42, co-authored 205 publications receiving 5643 citations. Previous affiliations of Matjaz Valant include Imperial College London & University of Electronic Science and Technology of China.

Peculiarities of a Solid-State Synthesis of Multiferroic Polycrystalline BiFeO3

Abstract: Studies of a BiFeO3 synthesis were performed to identify reasons for the appearance of secondary phases, the Bi25FeO39- and Bi2Fe4O9-type phases, in the reaction product. X-ray diffraction and microstructural analyses, performed on samples with different concentrations of impurities, showed that the impurities in the starting material crucially influence the phase composition of the reaction product. A fraction of the generated secondary phases strongly depends on the nature and concentration of the impurities. The experimental results can be explained by the theoretical consideration of ternary phase relations between Bi2O3, Fe2O3, and an impurity oxide. Single-phase polycrystalline BiFeO3 was synthesized from ultrapure starting oxides. To avoid using the expensive ultrapure oxides, techniques for reducing the fraction of the secondary phases in the reaction products were developed.

Chemical Compatibility between Silver Electrodes and Low-Firing Binary-Oxide Compounds: Conceptual Study

Abstract: The chemical compatibility of silver electrodes and low-firing ceramics has been considered, in terms of the existence of a tie line between silver (and/or Ag2O) and the binary oxide compound in the corresponding ternary phase diagram. The probability of the existence of the tie line is related to the conditions in the subordinated silver-based isothermal binary systems. Greater probabilities have been calculated for the systems with fewer silver-based binary compounds. Based on the concepts that have been developed, several silver-based isothermal binary systems have been investigated to identify the oxides suitable for the development of low-temperature cofired ceramics. The developed concept has been tested by investigating the phase relations in the Bi2O3–Nb2O5 and Bi2O3–V2O5 ternary systems with silver. X-ray and microstructural investigations of bismuth niobates and bismuth vanadates reveal that, as a result of the inertness of Bi2O3 and the reactivity of Nb2O5 and V2O5 toward silver, compounds that are rich in niobium or vanadium react with silver to form ternary Nb/V-Bi-Ag oxide compounds, whereas for compounds that are rich in bismuth, tie lines to silver and Ag2O do exist.

High-Q Microwave Dielectric Materials Based on the Spinel Mg2TiO4

Abstract: Composite ceramics based on the spinel Mg2TiO4 were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO3. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q×f value of around 150–160 THz was obtained for the undoped Mg2TiO4, whereas a reduced Q×f value was observed for an increase in the Co concentration in the system (1−x)Mg2TiO4−xCo2TiO4. Upon doping with 7 mol% CaTiO3, the Q×f value passed through a maximum with increasing Co concentration. Adding ZnO–B2O3 to the composite system based on Co-doped Mg2TiO4 resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q×f value.

Direct and indirect electrocaloric measurements on 〈001〉-PbMg1/3Nb2/3O3-30PbTiO3 single crystals

Abstract: A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.

Physics and Applications of Bismuth Ferrite

Abstract: BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Caloric materials near ferroic phase transitions

Abstract: A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

Low loss dielectric materials for LTCC applications: a review

Abstract: Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...

Near-complete suppression of surface recombination in solar photoelectrolysis by "Co-Pi" catalyst-modified W:BiVO4.

Abstract: The influence of an earth-abundant water oxidation electrocatalyst (Co-Pi) on solar water oxidation by W:BiVO4 has been studied using photoelectrochemical (PEC) techniques. Modification of W:BiVO4 photoanode surfaces with Co-Pi has yielded a very large (∼440 mV) cathodic shift in the onset potential for sustained PEC water oxidation at pH 8. PEC experiments with H2O2 as a surrogate substrate have revealed that interfacing Co-Pi with these W:BiVO4 photoanodes almost completely eliminates losses due to surface electron–hole recombination. The results obtained for W:BiVO4 are compared with those reported recently for Co-Pi/α-Fe2O3 photoanodes. The low absolute onset potential of ∼310 mV vs RHE achieved with the Co-Pi/W:BiVO4 combination is promising for overall solar water splitting in low-cost tandem PEC cells, and is encouraging for application of this surface modification strategy to other candidate photoanodes.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.