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Register a new userInfluence of P2O5 and SiO2 addition on the phase, microstructure, and electrical properties of KNbO3
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In this contribution, the effect of P2O5 and SiO2 addition on the phase, microstructure, and electrical properties of KNbO3 was studied. Sample powders with the general formula (1-x)KNbO3.xP2O5 (x = 0.03, 0.05) and (1-x)KNbO3.xSiO2 (x = 0.1) were prepared via mixed-oxide route. The thermal behavior of the mixed-milled powder was investigated by TG/DTA which revealed an overall weight loss of 33.4 wt % in the temperature range of 30 < T < 1200 C and crystallization exotherm occurring at about 795 C. The present results indicated that P2O5 acted as a sintering aid and lowered the sintering temperature by about 30 C and promoted densification of KNbO3. Sample compositions at various stages of processing were characterized using X-ray diffraction. Samples sintered at T < 1020 C revealed mainly KNbO3 together with a couple of low-intensity K3NbO4 peaks as a secondary phase. The SEM images of (1-x)KNbO3.xSiO2 (x = 0.1) samples showed a slight increase in the average grain size from 3.76 um to 3.86 um with an increase in sintering temperature from 1000 C to 1020 C. Strong variations in dielectric constant and loss tangent were observed due to P2O5 and SiO2 addition as well as frequency of the applied AC signals.
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Polycrystalline Yb substituted NiZn nanoferrites with the compositions of Ni0.5Zn0.5YbxFe2-xO4 (x= 0.00, 0.04, 0.08, 0.12, 0.16 and 0.20) have been synthesized using sol gel auto combustion technique. Single phase cubic spinel structure has been confirmed by the X ray diffraction (XRD) patterns. Larger lattice constants of the compositions are found with increasing Yb3+ concentration while the average grain size (52 to 18 nm) has noticeable decrease as Yb3+ content is increased. The presence of all existing elements as well as the purity of the samples has also been confirmed from energy dispersive X ray spectroscopic (EDS) analysis. Frequency dependent dielectric constant, dielectric loss, dielectric relaxation time, AC and DC resistivity of the compositions have also been examined at room temperature. The DC resistivity value is found in the order of 10 to power 10 (omega-cm) which is at least four orders greater than the ferrites prepared by conventional method. This larger value of resistivity attributes due to very small grain size and successfully explained using the Verwey and deBoer hopping conduction model. The contribution of grain and grain boundary resistance has been elucidated using Cole Cole plot. The study of temperature dependent DC resistivity confirms the semiconducting nature of all titled compositions wherein bandgap (optical) increases from 2.73 eV to 3.25 eV with the increase of Yb content. The high value of resistivity is of notable achievement for the compositions that make them a potential candidate for implication in the high frequency applications where reduction of eddy current loss is highly required.
Quantum dot heterostructures with excellent low-noise properties became possible with high purity materials recently. We present a study on molecular beam epitaxy grown quantum wells and quantum dots with a contaminated aluminum evaporation cell, which introduced a high amount of impurities, perceivable in anomalies in optical and electrical measurements. We describe a way of addressing this problem and find that reconditioning the aluminum cell by overheating can lead to a full recovery of the anomalies in photoluminescence and capacitance-voltage measurements, leading to excellent low noise heterostructures. Furthermore, we propose a method to sense photo-induced trap charges using capacitance-voltage spectroscopy on self-assembled quantum dots. Excitation energy-dependent ionization of defect centers leads to shifts in capacitance-voltage spectra which can be used to determine the charge density of photo-induced trap charges via 1D band structure simulations. This method can be performed on frequently used quantum dot diode structures.
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The prediction of ferromagnetism at room temperature in Co-ZnO thin films has generated a large interest in the community due to the possible applications. However, the results are controversial, going from ferromagnetism to non-ferromagnetism, leading to a large debate about its origin (secondary phase, Co clusters or not). By carefully studying the micro-structure of various Co-ZnO films, we show that the Co2+ partly substitutes the ZnO wurtzite matrix without forming Co clusters. Surprisingly, the ferromagnetism nature of the films disappears as the Co content increases. In addition, our results suggest that the observed ferromagnetism is likely associated to a large amount of defects- close to the interface and strongly depending on the growth temperature- which may explained the spreading of the results.
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