Jong-Sook Lee

A zero-thermal-quenching phosphor

Phosphor-converted white light-emitting diodes (pc-WLEDs) are efficient light sources used in lighting, high-tech displays, and electronic devices. One of the most significant challenges of pc-WLEDs is the thermal quenching, in which the phosphor suffers from emission loss with increasing temperature during high-power LED operation. Here, we report a blue-emitting Na3-2xSc2(PO4)3:xEu2+ phosphor (λem = 453 nm) that does not exhibit thermal quenching even up to 200 °C. This phenomenon of zero thermal quenching originates from the ability of the phosphor to compensate the emission losses and therefore sustain the luminescence with increasing temperature. The findings are explained by polymorphic modification and possible energy transfer from electron-hole pairs at the thermally activated defect levels to the Eu2+ 5d-band with increasing temperature. Our results could initiate the exploration of phosphors with zero thermal quenching for high-power LED applications.

A carbon-coated Li3V2(PO4)3 cathode material with an enhanced high-rate capability and long lifespan for lithium-ion batteries

A facile sol–gel approach combined with a carbon-coating technique via high-temperature thermally decomposing C2H2 has been developed for the synthesis of a Li3V2(PO4)3/C (LVP/C) cathode material employing the biomass of phytic acid as an eco-friendly phosphorus source. The effects of the carbon-coating on the structural, morphological and electrochemical properties of LVP have been investigated. Compared with pristine LVP, the LVP/C composite presents a higher discharge capacity of 127 mA h g−1 at 0.1 C, better rate capability and long-term cyclability in the voltage range of 3.0–4.3 V. Even at a high charge–discharge rate of 5 C, it can still deliver a reversible capacity of 107 mA h g−1 over 400 cycles without obvious fading, demonstrating great potential as a superior cathode material for lithium-ion batteries.

Transport and Phase Transition Characteristics in AgI : Al2 O 3 Composite Electrolytes Evidence for a Highly Conducting 7‐Layer AgI Polytype

The extreme conductivity enhancement (by a factor of up to 10 4 ) in AgI:Al 2 O 3 composites as well as the anomalous phase transition to the α-phase with a large hysteresis of 50°C was confirmed by conductivity, differential scanning calorimetry, and in situ X-ray diffractometry (XRD). All the composites exhibited an activation energy of 0.29 ± 0.02 eV in conductivity which was identified with the migration enthalpy for transport in the disordered close-packed layers in AgI system. A close examination of the XRD patterns shows the existence of a seven-layer polytype (7H) for Agl with the stacking sequence ABCBCAC. This may account for anomalies in the composites in contrast to the ideal space charge effect in AgCl:Al 2 O 3 and AgBr:Al 2 O 3 composites. Since the stacking arrangement can be regarded as a heterostructure of β-Agl (ABABAB...) and γ-Agl (ABCABC...) with a sub-Debye length spacing, the conductivity enhancement may be attributed to the mesoscopic multiphase effect predicted earlier, The hysteresis can be attributed to the elastic effects in the martensitic transformation between α-AgI and 7H-AgI.

Space-charge concepts on grain boundary impedance of a high-purity yttria-stabilized tetragonal zirconia polycrystal

A detailed impedance analysis using the brick-layer model is performed on a high-purity yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). Space-charge impedance is generally formulated and expressions for the respective space-charge models are therefrom derived depending on whether dopant ions are mobile or immobile. Pronounced yttrium segregation in Y-TZP is also considered in the analysis in that the dopant profile is assumed to be frozen from a high-temperature equilibrium distribution. Comparison with experimental observations shows that the electrically measured grain-boundary thickness corresponds to the Schottky-barrier width, slightly modified by the dopant segregation. The grain-boundary resistance is not consistent with any space-charge models and the strong defect interaction due to the yttrium enrichment is suggested to be mainly responsible.

Defect chemistry and transport characteristics of β-AgI

Abstract The defect chemistry and d.c. transport characteristics of β-AgI are reconsidered by taking into account (i) two structurally different interstitial positions, (ii) short-range interactions via associations, (iii) anisotropy of the wurtzite structure, (iv) long-range defect-defect interactions via Coulomb forces, and (v) formation of highly conducting layers perpendicular to the c -axis via a disordered interface structure with stacking faults. Besides microstructural characterization the analysis relies on the ionic conductivity data by impedance spectroscopy with conventional as well as micro-electrodes, and utilizes recent reports on the defect concentration and defect energies of β-AgI by molecular dynamics simulations and on AgI:Al 2 O 3 composites. Static valence sum calculations were performed to elucidate the ion conduction pathways and related migration barriers.

Ion conduction across nanosized CaF2∕BaF2 multilayer heterostructures

The authors investigate the ion conduction of molecular beam epitaxy grown CaF2∕BaF2 multilayers perpendicular to the interfaces. Unlike previous measurements along the heterostructure boundaries, the more resistive contributions dominate here; the detailed analysis allows for a complementary insight into the charge carrier distribution. The features of perpendicular conductivites in both semi-infinite and mesoscopic situations can be qualitatively as well as quantitatively explained by the same defect chemical model used for parallel ion conduction. The authors can distinguish three different size regimes. For large interfacial spacings (l>50nm), the conduction is dominated by bulk parts of the CaF2 layers, showing only a slight increase with decreasing layer thickness. For very small spacings, i.e., l<30nm, the conductivity increases steeply and tends toward a saturation value, corresponding to the space charge overlap situation with the overall value that can be attributed to Fi′ accumulated in CaF2. T...

A mesoscopic heterostructure as the origin of the extreme ionic conductivity in AgI:Al2O3

Abstract A 7H polytype of AgI (characterised by stacking fault arrangement) is detected at the interface of β-AgI and γ-Al2O3, which may even form the major (or even the only) AgI-constituent at pronounced Al2O3-contents. The driving force for the formation of the 7H phase is assumed to be the interaction of Ag+ with the basic alumina phase similar as in the AgCl and AgBr composites in which due to the relative stability of the rock salt structure only ideal semi-infinite space charge layers occur. Considering the 7H structure as a heterostructure of γ- and β-phase (β/γ/β/γ/…) leads to an explanation that is consistent with all the features observed: The extreme Ag+-conductivity (that can hardly be explained by semi-infinite space charges) and the peculiar phase transition behaviour on one hand, and the qualitative similarities with AgCl:Al2O3 and AgBr:Al2O3 on the other hand. Since the layer separation is in the sub-Debye length range we expect a mesoscopic effect on the ionic conductivity as predicted earlier. This is supported by conductivity anomalies in β/γ two phase mixtures. This mesoscopic heterolayer is discussed in the context of nano-ionics (ion conductivity in nanocrystalline materials and nano-composites) and nano-electronics (quantum wells, wires and dots).

Evaluation of the AC response of Li-electrolytic perovskites Li0.5(LnxLa0.5-x)TiO3 (Ln = Nd, Gd) in conjunction with their crystallographic and microstructural characteristics

Abstract We performed an in-depth electrochemical study on the ion-blocking electrochemical cells Pt/Li 1/2 (Ln x La 1/2− x )TiO 3 /Pt (Ln=Nd, Gd) over a wide frequency, impedance magnitude, and temperature range, and proposed an equivalent circuit which is original in its low frequency interpretation. The electrical properties of the bulk and the grain boundary were analyzed closely in relation to the crystallography and the characteristic microstructure of the system.

EFFECT OF SUBSTITUTION OF K FOR Na ON THE FERROELECTRIC STABILITY AND PROPERTIES OF (Bi1/2Na1/2)TiO3-BaTiO3-(K0.5Na0.5)NbO3

The effect of an isovalent chemical variation in A site cations on the stability of ferroelectric order and consequent changes in the electrical properties of ferroelectric 0.93(Bi1/2Na1/2)TiO3-0.05BaTiO3-0.02(K0.5Na0.5)NbO3 (0.93BNT-0.05BT-0.02KNN) lead-free piezoceramic at the morphotropic phase boundary (MPB) was investigated by substituting K for Na. X-ray diffraction analysis reveals that all the studied compositions undergo an irreversible phase transition from a pseudocubic to a mixture of rhombohedral and tetragonal phases during poling. In spite of the absence of an obvious structural difference, both electric-field-induced strain and polarization hysteresis loops show a clear distinction. It was found that the effect of substitution is so drastic that about 0.5 at% substitution renders the material softer and more than 1.0 at% results in a significant destabilization of the ferroelectric order. However, no notable difference in the temperature dependent permittivity measurements was detected, which suggests that the depoling of these materials has some other origin than a symmetry-breaking phase transition.

Direct measurement of partial ionic conductivity of Co1−δO via impedence spectroscopy combined with dc relaxation

Abstract For an electron-blocking cell ZrO 2 (+8m/0Y 2 O 3 )/Co 1−δ O/ZrO 2 (+8m/0Y 2 O 3 ), a full impedance spectrum has been comstructed for a wide frequency range comprising diffusion, electrode and bulk by combining impedance data directly measured using a commercial impedance analyzer and the Fourier-Laplace transform of the dc relaxation into the frequency domain which carries the (ionic) information in the low frequency range including the dc limit. The ionic conductivity of Co 1−δ O was then extracted from the finite length Warbung impedance at the low end of the full impedance spectrum, which was in a good agreement with the literature. Disturbing artifacts associated with gas/solid reactions and the limitation of the method are discussed.