Influence of Synthesis-Related Microstructural Features on the Electrocaloric Effect for 0.9Pb(Mg1/3Nb2/3)O3−0.1PbTiO3 Ceramics

Despite having a very similar electrocaloric (EC) coefficient, i.e., the EC temperature change divided by the applied electric field, the 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN-10PT) ceramic prepared by mechanochemical synthesis exhibits a much higher EC temperature change than the columbite-derived version, i.e., 2.37 °C at 107 °C and 115 kV/cm. The difference is due to the almost two-times-higher breakdown field of the former material, 115 kV/cm, as opposed to 57 kV/cm in the latter. While both ceramic materials have similarly high relative densities and grain sizes (>96%, ≈5 µm) and an almost correct perovskite stoichiometry, the mechanochemical synthesis contributes to a lower level of compositional deviation. The peak permittivity and saturated polarization are slightly higher and the domain structure is finer in the mechanochemically derived ceramic. The secondary phases that result from each synthesis are identified and related to different interactions of the individual materials with the electric field: an intergranular lead-silicate-based phase in the columbite-derived PMN-10PT and MgO inclusions in the mechanochemically derived cerami

Multifunctional Cantilevers as Working Elements in Solid-State Cooling Devices

Despite the challenges of practical implementation, electrocaloric (EC) cooling remains a promising technology because of its good scalability and high efficiency. Here, we investigate the feasibility of an EC cooling device that couples the EC and electromechanical (EM) responses of a highly functionally, efficient, lead magnesium niobate ceramic material. We fabricated multifunctional cantilevers from this material and characterized their electrical, EM and EC properties. Two active cantilevers were stacked in a cascade structure, forming a proof-of-concept device, which was then analyzed in detail. The cooling effect was lower than the EC effect of the material itself, mainly due to the poor solid-to-solid heat transfer. However, we show that the use of ethylene glycol in the thermal contact area can significantly reduce the contact resistance, thereby improving the heat transfer. Although this solution is most likely impractical from the design point of view, the results clearly show that in this and similar cooling devices, a non-destructive, surface-modification method, with the same effectiveness as that of ethylene glycol, will have to be developed to reduce the thermal contact resistance. We hope this study will motivate the further development of multifunctional cooling devices

PREPARATION AND CHARACTERIZATION OF GLASS-CERAMICS COMPOSITE FOR APPLICATION IN LTCC TECHNOLOGY

V delu opisujemo študijo kompozitov steklo-keramika, ki so primerni za pripravo večplastnih struktur na osnovi keramike z nizko temperaturo žganja (Low Temperature Co-fired Ceramics – LTCC). Osredotočili smo se na sintezo nizko taljivega stekla, pogoje drobljenja in mletja stekla, homogenizacijo steklaste faze in delcev korunda in določanje pogojev žganja kompozitov. Nizko taljivo silikatno steklo na osnovi SiO2, PbO, Al2O3, CaO, B2O3, Na2O, K2O, Cr2O3, CoO in MgO smo pripravili iz homogenizirane zmesi reagentov, ki smo jo kalcinirali pri temperaturi kalcinacije, T = 750 °C. Vzorec kalcinirane zmesi smo segrevali v segrevalnem mikroskopu in ugotovili, da se tali nad 1050 °C. Steklo, ki smo ga sintetizirali pri temperaturi, T = 1200 °, je bilo rentgensko amorfno, z enakomerno mikrostrukturo in z nizko temperaturo sintranja: začetek krčenja je bil pri temperaturi, T = 660 °C in končni skrček pri temperaturi, T = 780 °C, zato smo ga uporabili za nadaljnje delo. Za pripravo kompozitov smo morali steklasto fazo zdrobiti in zmleti. Na podlagi meritev velikosti in porazdelitve velikosti delcev stekla smo ugotovili, da je manjše vzorce z maso, m = 4 g primerneje drobiti s stresalnim mlinom, večje z maso m = 30 g pa z vibracijskim mlinom. Steklo smo nadalje mleli v planetarnem mlinu v vodnem mediju. Povprečna velikost delcev po 6 urah mletja je bila 2,10 µm. Korundni prah je bil precej grob, povprečna velikost delcev je bila 3,15 µm, zato smo ga mleli v atritorju. Po 90 minutah mletja se je povprečna velikost delcev zmanjšala na 1,32 µm. Nadalje smo pomleti fazi stekla in korunda v utežnem razmerju 50:50 homogenizirali v planetarnem mlinu v acetonu. Vzorce smo segrevali v komorni peči s hitrostjo, rs = 2 in 5 °C/min in pri časih t = 30, 60 in 120 min pri temperaturi T = 850 °C. Posnetki mikrostruktur sintranih vzorcev kažejo, da sta steklo in korund v kompozitih homogeno porazdeljena. V vseh vzorcih so bile prisotne pore mikronske velikosti. Vzorci, žgani krajši čas (rs = 2 °C/min, t = 30, min in rs = 5 °C/min, t = 30, 60 min), so bili rentgensko amorfni. Pri vzorcih, ki so bili žgani pri daljših časih, smo tako z rentgensko fazno analizo kot z energijsko disperzijsko spektroskopijo rentgenskih žarkov potrdili prisotnost faze anortita (CaAl2Si2O8). Lastnosti vzorcev kompozita steklo-korund smo primerjali s komercialnim vzorcem LTCC, pripravljenim pri enakih pogojih, to je pri temperaturi, T = 850 °C in pri časih t = 30, 60 in 120 min, in ugotovili, da imajo podobno gostoto, fazno sestavo in mikrostrukturo.We describe the study of the glass-ceramic composites, which could be used for processing of multilayer structures in Low Temperature Co-fired Ceramics (LTCC) technology. We focused on the synthesis of the low melting point glass, the conditions of its crushing and milling, homogenization of the glassy phase and corundum particles, and sintering and characterization of the composites. The glass based on SiO2, PbO, Al2O3, CaO, B2O3, Na2O, K2O, Cr2O3, CoO and MgO was prepared by homogenization of the reagents and calcination at the temperature, T = 750 °C. The as-calcined mixture melted at/above T = 1050 °, as determined by the heating stage microscope. The glass, synthesized at the temperature, T = 1200 °, was amorphous, with a uniform microstructure, and it sintered at low temperature: namely, the onset and the end of the shrinkage were at T = 660 °C and T = 780 °C, respectively, therefore it was selected for further work. Small (4 g) and large (30 g) batches of the mm-sized glass fragments were further crushed in a shaker mill and vibration mill, respectively. After milling in a planetary mill the mean particle size of the glass phase was 2,10 µm. The as-received corundum powder had the mean particle size of 3,15 µm, and before further use it was milled in an attrition mill, resulting in particles with the mean size of 1,32 µm. The as-milled glass and ceramic phases in the 50 : 50 weight ratio were homogenized in a planetary mill. The powder compacts were heated with the heating rate, r = 2 and 5 °C/min and times t = 30, 60 and 120 min at the temperature, T = 850 °C. The microstructures of the sintered samples reveal a homogeneous distribution of the two phases. Micron sized spherical pores were observed in all samples. The samples, which were heated for shorted periods (r = 2 °C/min, t = 30, min and r = 5 °C/min, t = 30, 60 min), were amorphous. The samples, heated for longer times, contained also the crystalline anorthite (CaAl2Si2O8) phase, as confirmed by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy. We compared the properties of as-sintered glass-ceramic composites with a commercial LTCC material, sintered at the same temperature, T = 850 °C and times, t = 30, 60 and 120 min, and found that they both had quite similar density, phase composition and microstructure

Electrocaloric cooling

Here we explore the effect of electric-energy recovery and heat regeneration on the energy efficiency of an electrocaloric-cooling system. Furthermore, the influence of the polarizationelectric field hysteresis on the energy efficiency of the system is analysed. For the purposes of the analysis, the properties of (1 % x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-100xPT) with x = 0, x = 0.1, and x = 0.35 are characterized. We show that if no heat is regenerated, even small irreversibilities in the electric circuit used to recover the electric energy can cause a significant drop in the achievable energy efficiency. On the other hand, when a heat regeneration process is considered and a realistic value for the degree of electric-energy recovery equal to 80% is assumed, the limit for the energy efficiency of a system employing PMN ceramics is estimated to be equal to 81% of the efficiency of a Carnot heat pump

Thermal properties of polymer-matrix composites reinforced with E-glass fibers

The influence of the fraction of the glass-fiber (EGF) reinforcement, from 0 to 15 wt%, on the thermal properties of the polymer-matrix composites with the CaCO3 mineral filler is studied. The proper ratio of glass-fibers and mineral filler is important for obtaining good mechanical, and at the same time good thermal properties of the composites used in production of high-quality components in electro industry. For this purpose, the thermal stability and the thermal properties of the fiber-reinforced composites are determined from a set of different characterization techniques and linked with the microstructural changes induced by the different fiber content. The composites are stable upon heating to 260 degrees C, and undergo thermal oxidation between 280 and 560 degrees C, as found by thermogravimetric and differential thermal analysis. The specific heat capacity, C-p, of the composites slightly increases with increasing EGF content for 0 to 10 wt% EGF, with respective values 0.899 J/gK and 0.903 J/gK at 20 degrees C. Slightly lower Cp values of the composite with 15 wt% EGF could be related to a non-uniform distribution of EGF and presence of voids evidenced by scanning electron microscopy. Thermal conductivity of the composites decreases with increasing EGF content from 0.960(6) W/mK to 0.878(2) W/mK for 0 and 15 wt% EGF, respectively

Bulk relaxor ferroelectric ceramics as a working body for an electrocaloric cooling device

The electrocaloric effect (ECE), i.e., the conversion of the electric into the thermal energy has recently become of great importance for development of a new generation of cooling technologies. Here, we explore utilization of [Pb(Mg1/3Nb2/3)O3]0.9[PbTiO3]0.1 (PMN-10PT) relaxor ceramics as active elements of the heat regenerator in an ECE cooling device. We show that the PMN-10PT relaxor ceramic exhibits a relatively large electrocaloric change of temperature TEC >1 K at room temperature. The experimental testing of the cooling device demonstrates the efficient regeneration and establishment of the temperature span between the hot and the cold sides of the regenerator, exceeding several times the TEC within a single PMN-10PT ceramic plate

Influence of Synthesis-Related Microstructural Features on the Electrocaloric Effect for 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 Ceramics

Despite having a very similar electrocaloric (EC) coefficient, i.e., the EC temperature change divided by the applied electric field, the 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN-10PT) ceramic prepared by mechanochemical synthesis exhibits a much higher EC temperature change than the columbite-derived version, i.e., 2.37 °C at 107 °C and 115 kV/cm. The difference is due to the almost two-times-higher breakdown field of the former material, 115 kV/cm, as opposed to 57 kV/cm in the latter. While both ceramic materials have similarly high relative densities and grain sizes (>96%, ≈5 μm) and an almost correct perovskite stoichiometry, the mechanochemical synthesis contributes to a lower level of compositional deviation. The peak permittivity and saturated polarization are slightly higher and the domain structure is finer in the mechanochemically derived ceramic. The secondary phases that result from each synthesis are identified and related to different interactions of the individual materials with the electric field: an intergranular lead-silicate-based phase in the columbite-derived PMN-10PT and MgO inclusions in the mechanochemically derived ceramic