Anna-Karin Axelsson

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

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.

Statistical mechanical lattice model of the dual-peak electrocaloric effect in ferroelectric relaxors and the role of pressure

Despite considerable effort, the microscopic origin of the electrocaloric (EC) effect in ferroelectric relaxors is still intensely discussed. Ferroelectric relaxors typically display a dual-peak EC effect, whose origin is uncertain. Here we present an exact statistical mechanical matrix treatment of a lattice model of polar nanoregions forming in a neutral background and use this approach to study the characteristics of the EC effect in ferroelectric relaxors under varying electric field and pressure. The dual peaks seen in the EC properties of ferroelectric relaxors are due to the formation and ordering of polar nanoregions. The model predicts significant enhancement of the EC temperature rise with pressure which may have some contribution to the giant EC effect.

Mechanism of photocatalytic oxidation of 3,4-dichlorophenol on TiO2 semiconductor surfaces

Abstract In this paper, the results of a detailed study of the kinetics of photocatalytic oxidation of 3,4-dichlorophenol at various concentrations in oxygenated solutions on TiO2 are presented. Electron–hole recombination is often suggested to be decreased by increasing the molecular oxygen concentration; but our study indicates that this is not the primary rate enhancing property of the dissolved oxygen. It is argued here that a hydroxyl radical is ejected from the catalyst surface by photo-excitation onto a repulsive excited electronic state leading to a translationally hot OH( 2 π ) radical. In the presence of molecular oxygen, a simple hydroxyl addition to the dichlorophenol occurs. In the absence of an adsorbed oxygen molecule, the electron transfer to the aromatic ring from a Ti3+ site causes partial dechlorination of the dichlorophenol. Subsequently, hydroxyl radical addition to the aromatic ring may occur. Hence, we find that dissolved molecular oxygen has two important roles in the photo-catalytic oxidation of 3,4-dichlorophenol on the semiconducting TiO2 surfaces. One of these is as a H-atom acceptor required in direct hydroxyl radical addition to the phenyl ring while the other is as an electron transfer inhibitor when adsorbed at defective Ti3+ sites. A theoretical model of the kinetics is proposed which is able to account semi-quantitatively for the overall features of the reaction state space. Significantly, monitoring of the intermediate species produced by these two routes shows that the relative yields can be inverted by changing the dissolved oxygen concentration which significantly is accord with the theoretical predictions.

Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics

The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.

Microwave dielectric loss in oxides: Theory and experiment

We present a model that provides a description of the microwave dielectric loss in oxides. The dielectric loss (tan δ) in single crystal and polycrystalline MgO and Al2O3 is measured over the temperature range 70–300 K. We are able to model the dielectric loss in terms of a two-phonon difference model. There are two key parameters in this model: The third derivative, φ3, of the lattice potential and the linewidth, γ, of the thermal phonons. In polycrystalline samples, rather than considering the different mechanisms of extrinsic loss, it is assumed that the main effect of extrinsic factors is a modification of the linewidth of the thermal phonons. By varying γ(T), it is shown that the model can describe the loss in both single crystals and polycrystallines materials. In single crystal and polycrystalline MgO, we use γ as a fitting parameter. In single crystal and polycrystalline Al2O3, we obtain γ(T) by Raman spectroscopy. The theory gives the right order of magnitude of the measured loss.

Microscopic interpretation of sign reversal in the electrocaloric effect in a ferroelectric PbMg1/3Nb2/3O3-30PbTiO3 single crystal

With increasing temperature, PbMg1/3Nb2/3O3-30PbTiO3 (PMN-30PT) crystals change from pseudo-rhombohedral to tetragonal to cubic phases. In addition to the usual positive electrocaloric effect (ECE), a negative ECE, whose origin is uncertain, is observed. Here, these two types of the ECE contributions in PbMg1/3Nb2/3O3-30PbTiO3 crystals are modelled theoretically using a one dimensional statistical mechanical lattice model, which is solved by an exact matrix method. The quasi one-dimensional model reproduces the trends in the experimental behaviour and attributes the electrocaloric sign reversal to free energy changes induced by the electric field.

Microscopic theory of the electrocaloric effect in the paraelectric phase of potassium dihydrogen phosphate

Here we present a microscopic theory of the electrocaloric effect in potassium dihydrogen phosphate, KH2PO4, based on Slater’s lattice model. The model reproduces the essential features of the experimentally observed behavior and also gives a remarkably accurate description of the electric field dependence of the electrocaloric effect. The basic principle of the theory also gives guidelines for a theoretical analysis of other dielectrics and for the further development of materials with an enhanced electrocaloric effect.

Effect of Ce doping on the electrocaloric effect of SrxBa1−xNb2O6 single crystals

The electrocaloric effect (ECE) of SrxBa(1−x)Nb2O6 (SBN100x) single crystals, with a tetragonal tungsten bronze structure and a high ECE near room temperature, is studied by direct measurements. It is shown that although the onset of the ECE peak is closer to room temperature in SBN80 than in SBN75, the effect of the increase of the strontium content is very detrimental to the ECE performances with a decrease to ΔTEC = 0.23 K for SBN80 from the reported value of ΔTEC = 0.42 K under 10 kV/cm for SBN75 [F. Le Goupil et al., “Anisotropy of the electrocaloric effect in lead-free relaxor ferroelectrics,” Adv. Energy Mater. (published online)]. However, when 1.40% of cerium is introduced in SBN61, the temperature of depolarisation is shifted below 30 °C, while an ECE above 0.6 K is maintained over more than 70 K for a low electric field of 28 kV/cm. The maximum ECE ΔTEC = 0.85 K is measured at 61 °C. In addition to having an ECE peak close to room temperature, the ECE measured in Ce-doped SBN61 is comparable wi...

Microwave Dielectric Properties of Bi₂ O₃ -TiO₂ Composite Ceramics

Bi₂ O₃ -TiO₂ composite dielectric ceramics have been prepared by a conventional solid state ceramic route. The composite ceramics were prepared with starting materials of different origin and the microwave dielectric properties were investigated. The sintered ceramics were characterised by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray microanalysis, Raman and microwave methods. Structural and microstructural analyses identified two separate phases: TiO₂ (rutile) and Bi₂Ti₄O 11 . The separate grains of titania and bismuth titanate were distributed uniformly in the ceramic matrix. The composition 0.88TiO₂-0.12Bi₂Ti₄O 11 was found to have a Q×f of 9,300 GHz (measured at a frequency of 3.9 GHz), a temperature coefficient of frequency, τ cf , near zero and a high relative permittivity, e r , of 83. The microwave dielectric properties were measured down to 20°K K. The quality factor increased on cooling the ceramic samples.

Oxygen transport during formation and decomposition of AgNbO3 and AgTaO3

A thermogravimetric method was used to analyze intermediate processes involved in the formation and decomposition of AgNbO 3 and AgTaO 3 perovskites. Critical parameters that control the kinetics of the formation are associated with oxygen transport. The Nb 2 O 5 crystal structure has a capacity to trap molecular oxygen, which evolves during the decomposition of Ag 2 O that is present in a starting mixture. The formation of the perovskite phase involves a simultaneous reaction of three species: O 2 , Ag, and Nb 2 O 5 /Ta 2 O 5 . As the trapped molecular oxygen is in the immediate vicinity of the reaction site, the kinetics of the reaction is significantly accelerated. An absence of the molecular oxygen in the solid-state phase cannot be compensated for with an increase in a partial pressure of oxygen in the gas phase, that is, application of oxygen atmosphere.