Electrocaloric Effect in lead-free Relaxor Ceramics

Heutzutage ist die Nachfrage nach umweltfreundlichen, skalierbaren und effizienten Kühltechnologien rasant angestiegen, die alternativ zu konventionellen Kompressionskühltechnologien mit umweltschädlichen Gasen zum Einsatz kommen sollen. Eine anspruchsvolle Variante ist die Festkörper-Kühltechnologie basierend auf den elektrokalorischen Effekt (electrocaloric effect, Abk.: ECE). Der ECE beschreibt die Eigenschaft von polaren Materialien, die adiabatisch ihre Temperatur bzw. isotherm ihre Entropie ändern, wenn ein elektrisches Feld angelegt oder entfernt wird. In der vorliegenden Arbeit wird insbesondere der ECE in bleifreien Relaxorkeramiken untersucht. Relaxoren sind spezielle Ferroelektrika, die im Gegensatz zu herkömmlichen ferroelektrischen Materialien in der paraelektrischen Phase polare Nanocluster bilden. In der Arbeit wurden zuerst zwei Messmethoden entwickelt, womit der ECE auf direkter Weise einerseits isotherm und andererseits quasi-adiabatisch gemessen werden können. Die Ergebnisse der direkten Messmethoden wurden anschließend gegeneinander und mit der häufig verwendeten indirekten Messmethode auf Basis von Maxwell-Gleichungen verglichen, um die Kompatibilität dieser Messmethoden einzuschätzen. Es wurden hierbei Materialien mit unterschiedlichen Dotierungen des bekannten Bariumtitanat- Systems (Formel: BaTiO3, Abk.: BTO) untersucht, wie Ba(ZrxTi1-x)O3 (BZT), Ba(SnxTi1-x)O3 (BSnT) und (1-x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-xBCT). Darüber hinaus wurde der Vergleich des ECE im bleihaltigen System 0.92Pb(Mg1/3Nb2/3)O3–0.08PbTiO3 (PMN-8PT) mit unterschiedlichen Dotierungen hinzugezogen. Im PMN-8PT System wurde der ECE außerdem in Vielschicht-Keramikkondensator Form (multilayer ceramic capacitor, MLC) gemessen. MLC Systeme sind anwendungstechnisch sehr interessant. Sie haben dank dem Aufbau mit mehreren aufeinander liegenden Dickschichten (ca. 50-100 µm dick) die Eigenschaft mit kleinen elektrischen Spannungen (im Vergleich zu Bulk Materialien) akzeptable EC-Werte zu erzielen. Des Weiteren besitzen MLCs im Gegensatz zu einzelnen Dünnschicht-Systemen hohe thermische Massen, was ebenfalls für den thermischen Transport bei möglichen Anwendungen von Bedeutung ist. Bei nahezu allen untersuchten Systemen wurden Maxima des ECE in der Nähe der jeweiligen Phasenumwandlungstemperatur vom ferroelektrischen (FE) in den paraelektrischen (PE) Zustand beobachtet. Dies zeigt, dass durch den feldinduzierten Phasenübergang in der Nähe der Umwandlungstemperatur höhere EC-Entropieänderungen und somit auch höhere EC-Temperaturänderungen erreichbar sind.Nowadays the demand for environmentally friendly, scalable and efficient cooling technologies has increased rapidly, which should be able to replace conventional vapor-compression refrigeration systems, where hazardous gases are used. A promising alternative is the solid-state refrigeration based on the electrocaloric effect (ECE). The ECE describes the property of polar materials which change their temperature in adiabatic condition or the entropy isothermally, when an electric field is applied or removed. In the present work ECE is investigated in lead-free relaxor ceramics. Relaxors are special ferroelectrics that form polar nanoclusters in contrast to conventional ferroelectric materials in the paraelectric phase. At the beginning, two measurement methods have been developed whereby the ECE can be measured directly on the one hand isothermal and on the other quasi-adiabatic. The results of the direct measurements were compared with each other and with frequently used indirect estimations based on Maxwell relations to judge the compatibility of these measurement methods. Materials with different dopings based on the well-known Bariumtitanate (Formula: BaTiO3 ≙ BTO) system were investigated, such as Ba(ZrxTi1-x)O3 (BZT), Ba(SnxTi1-x)O3 (BSnT) and (1-x)Ba(Zr0.2Ti0.8)O3-x (Ba0.7Ca0.3)TiO3 (BZT-xBCT) systems. In addition, ECE measurements were performed for comparison in lead-containing system 0.92Pb(Mg1/3Nb2/3)O3–0.08PbTiO3 (PMN-8PT) with different dopants. In the PMN-8PT system, EC measurements were also performed in multilayer ceramic capacitor design (MLC). From the technical application point of view MLC systems are very interesting. Due to their structure with several superimposed layers (each layer is 50-100 microns thick) they have the property achieving acceptable EC values by applying small voltages (compared to bulk materials). Furthermore, MLCs possess high thermal capacity in contrast to thin-films, which is also important for the thermal transport for potential applications. In almost all studied systems maxima of the ECE were observed in the vicinity of the phase transition temperature from ferroelectric (FE) to the paraelectric (PE) state. This shows that with the field-induced phase transition larger EC entropy changes and thus also higher EC temperature changes can be achieved in the vicinity of the transition temperature

Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics

In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance

ELEVATED TRIGLYCERIDE GLUCOSE INDEX IS RELATED TO THE PRESENCE OF HEART FAILURE

Objective: Previous studies have shown a significant associa-tion between insulin resistance (IR) measured by using different methods and heart failure (HF). In recent years, the triglyceride glucose (TyG) index has been used to measure IR, and there are several reports showing that the TyG index indicates conditions such as metabolic syndrome (MetS) and atherosclerotic process. However, there is no study investigating the association of the TyG index with HF. Therefore, we aimed to evaluate the role of the TyG index in HF presence and its relationship with HF sever-ity in this study

Strong electrocaloric effect in lead-free 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 ceramics obtained by direct measurements

International audienceStrong electrocaloric effect in lead-free 0.65Ba(Zr 0.2 Ti 0.8)O 3-0.35(Ba 0.7 Ca 0.3)TiO 3 ceramics obtained by direct measurements Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8)O 3-x(Ba 0.7 Ca 0.3)TiO 3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr 0.2 Ti 0.8)O 3-0.35(Ba 0.7 Ca 0.3)TiO 3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of DT EC ¼ 0.33 K at $65 C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about$50%. The discrepancy is attributed to the relaxor character of this material. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907774] During the last several years, an increased interest has been paid to the electrocaloric effect (ECE) in ferroelectric materials as a route to develop small, effective, low cost, and environmentally friendly solid-state refrigerators. 1,2 The ECE is defined as an adiabatic and reversible temperature change of a dielectric material when an electric field is applied or removed. 3 If the exchange of heat with the environment is enabled, it defines the change of entropy as a function of the applied electric field under isothermal conditions. 4,5 Since Mischenko et al. reported on the giant electro-caloric effect in PbZr 0.95 Ti 0.05 O 3 thin films in 2006, 6 the ECE has been reported for many different ferroelectric materials such as thick and thin films, 7–10 polymers, 11,12 bulk ceramics, 13,14 and single crystals. 15,16 In general, the ECE peaks are a few degrees above the ferroelectric-paraelectric phase transition. 1 The largest values have been achieved for thin films, 10 where much higher electric fields can be applied than to bulk materials. However, for application, the heating/ cooling capacity is the key factor. Hence, bulk materials, which have large enough heating/cooling capacity, are better suitable for mid-and large-scale cooling applications. 1 To compare the ECE in different materials, the ratio between induced temperature change and applied field, DT EC /DE, called the electrocaloric strength, has been introduced

Direct electrocaloric measurements using a differential scanning calorimeter

The electrocaloric effect (ECE) in ferroelectric materials is a promising mechanism for the development of small, effective, low cost, and environmentally friendly solid state refrigerators. During the last decade, an increased interest has been paid to studies of this effect. Getting reliable values requires direct measurements of the ECE instead of the frequently used indirect estimates based on Maxwell's relation. In this paper, we report on the modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the heat capacitance and thermal losses for the correct ECE measurements is discussed. The ECE was measured for bulk ceramics Ba(Zr0.2Ti0.8)O3. The temperature change reaches a value of ΔTEC= 0.44 K at 305 K under an electric field of 2 kV/mm. The obtained data are compared with results of the evaluation of the indirect ECE

IS SERUM FIBROBLAST GROWTH FACTOR 21 ASSOCIATED WITH THE SEVERITY OR PRESENCE OF CORONARY ARTERY DISEASE?

Background: Recent studies have shown that increased circulating concentrations of fibroblast growth factor 21 (FGF21) are associated with obesity, metabolic disorder, and atherosclerosis. However the relationship between FGF21 and coronary artery disease (CAD) is controversial This study was planned to investigate the role of FGF21 in CAD development and CAD severity

Comparison of direct electrocaloric characterization methods exemplified by 0.92 Pb(Mg_1/3Nb_2/3)O₃-0.08 PbTiO₃ multilayer ceramics

Abstract Electrocaloric device structures have been developed as multilayer ceramics (MLCs) based on fundamental research carried out on PMN-8PT bulk ceramics. Two different MLC structures were prepared with nine layers each and layer thicknesses of 86 μm and 39 μm. The influence of the device design on its properties has been characterized by microstructural, dielectric, ferroelectric, and direct electrocaloric measurement. For direct characterization two different methods, ie temperature reading (thermistor and thermocouple) and heat flow measurement (differential scanning calorimetry), were used. A comparison of results revealed a highly satisfactory agreement between the different methods. This study confirms that MLCs are promising candidates for implementation into energy-efficient electrocaloric cooling systems providing large refrigerant volume and high electrocaloric effect. Due to their micron-sized active layers, they allow for the application of high electric fields under low operation voltages. We measured a maximum electrocaloric temperature change of ΔT = 2.67 K under application/withdrawal of an electric field of ΔE = 16 kV mm⁻¹, which corresponds to operation voltages below 1.5 kV