A reflection on recent efforts in optimization of cooling capacity of electrocaloric thin films

Despite the advantages of electric field efficiency and miniaturization, the limited operating temperature range and mediocre cooling efficiency of electrocaloric thin films represent the key obstacles to their practical applications in cooling advanced electronics. In this review, we discussed the current efforts and challenges facing the development of high-performance electrocaloric thin films and explored universal approaches along with their physical mechanisms for optimizing the electrocaloric response in thin films. We first emphasize the significance of the indirect method for determining the electrocaloric effect (ECE) in thin films and restate the conditions for the application of Maxwell’s equations. Particularly, we flag a couple of common artifacts of the electrocaloric results induced by the indirect method in recent attempts at the optimization of the ECE. We then cover chemical modification, interface engineering, and strain engineering as effective routes to improve the adiabatic temperature change (ΔT), reduce the driving electric field (E), and widen the operating temperature range (Tspan). At last, we propose that slush relaxors can be exploited as the base system for simultaneously achieving large ΔT, broad Tspan, and low E. Furthermore, we also discuss that the employment of high-entropy oxide perovskites is a feasible approach for greatly raising the dipolar entropy change under low electric fields. At last, we stress the significance and pressing need to measure the EC parameters of thin films with reliable direct methods. We hope that the high-performance electrocaloric thin films and the design rationale discussed in this review could inspire more facile and novel methods to achieve a better electrocaloric response

Oleylamine surface functionalized FeCoyFe2-yO4 (0.0 <= y <= 1.0) nanoparticles

In this study, oleylamine (OAm) capped FeCoyFe2-yO4 (0.0 <= y <= 1.0) nanocomposites (NCs) were prepared via a polyol route. Effect of Co3+ ion substitution on structure, morphology and magnetic properties of Fe3O4 nanoparticles was investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analyzer (TGA), scanning and transmission electron spectroscopy (SEM and TEM), vibrating sample magnetometer (VSM) and Mossbauer analyzer. All XRD patterns show the single phase spinel ferrite without any impurity. The crystallite size of the samples is within the range of 7.1-21.7 nm. FT-IR analysis showed that all products were successfully packed by OAm. Both SEM and TEM results confirmed that products have spherical morphology with small agglomeration. When Co3+ ions were substituted to the Fe3O4, Ms continued to decrease up to Co3+ content of y = 0.4. It was reported that Co3+ ions prefer to replace Fe2+ ions on octahedral side up to some concentration. Although the Mossbauer spectra for the all samples were composed of magnetic sextets, superparamagnetic particles are also formed for FeCo0.6Fe1.4O4, FeCo0.8Fe1.2O4 and FeCoFe2O4 samples. (C) 2016 Production and hosting by Elsevier B.V. on behalf of King Saud University

The Effect of Cr3+ Substitution on Magnetic Properties of CoFe2O4 Nanoparticles Synthesized by Microwave Combustion Route

CoCr (x) Fe2-x O-4 (0.0 aecurrency sign x aecurrency sign 1.0) nanoparticles were synthesized by a microwave combustion method and the effect of Cr3+ substitution on structural, morphological, and magnetic properties of CoFe2O4 was studied. The structural, morphological, and magnetic properties of the products were determined and characterized in detail by X-ray diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometer (VSM). Cation distribution of calculations confirmed the B site (octahedral site) preference of substituted Cr3+ ions. X-ray analysis showed that all compositions crystallize with a cubic spinel-type structure. The lattice parameter decreased from 8.384 to 8.362 with increasing Cr content. The average crystallite size was found in the range of 30.6-45.4 nm. Magnetization measurements showed that saturation magnetization of products decreases with the increase of the Cr substitution (x).Fatih University under BAPFatih University [P50031504_B]This study is supported by the Fatih University under BAP grant no. P50031504_B

Influence of Cr3+ ion on the structural, ac conductivity and magnetic properties of nanocrystalline Ni-Mg ferrite

Nanocrystalline samples of the Ni–Mg ferrite with the general chemical formula Ni0.5Mg0.5CrxFe2�xO4(0rxr1.0, in the step of0.2) have been synthesized by the citrate-gel auto combustion route. The synthesized samples were characterized by means of XRD,EDAX, FT-IR, UV–visible, dielectric, ac impedance spectroscopy, and AFM, SEM, TEM and FC/ZFC measurements. X-ray powderdiffraction of all the samples were carried out at room temperature to check the formation of the required products and structuralrelated properties. The elemental analysis as obtained from the EDAX measurement is in close agreement with the expected compositionfrom the stoichiometry of the reactant solutions. IR studies confirm two main absorption bands in the frequency range of 400–800 cm�1, arising due to the tetrahedral (A) and octahedral [B] stretching vibrations. Impedance spectroscopy techniques have beenused to study the effect of grain and grain boundary on the electrical properties of the prepared samples. The samples were zero fieldcooled (ZFC) to 100 K. Typical blocking temperature effects were observed below the temperature of about 129 K. The obtained resultshave been discussed so as to bring out the role of chromium substitution on the structural, dielectric and magnetic properties of Ni–Mgferrites

Dynamical magnetic behavior of anisotropic spinel-structured ferrite for GHz technologies

We have fabricated a high quality magnetic Ni0.5Zn0.5Fe2O4 ferrite powder/polymer composite sheet consisting of common and environmentally friendly elements only. The sheet was then tested for its dynamic permeability by irradiating with electromagnetic waves with frequencies up to 50 GHz. Two different originally developed methods were used for the high-frequency permeability measurements, a short-circuited microstrip line method and a microstrip line-probe method. It is challenging to measure the dynamic permeability of magnetic thin films/sheets beyond 10 GHz because of the low response signal from these materials. However, the two methods produced essentially equivalent results. In the frequency dependent permeability profile, the maximum position of the profile, μmax″, shifted towards higher frequencies upon increasing an applied (strong) static external magnetic field, Hdc. A linear relationship between μmax″ and Hdc for the entire range of Hdc was observed even at small Hdc. In general, the spinel-structured Ni-based ferrites exhibit low magnetic anisotropy, but the present sample showed a uniaxial-anisotropic behavior in the parallel direction of the sheet. Our Ni0.5Zn0.5Fe2O4 powder/polymer composite sheet thus exhibits high performance at GHz frequencies, and should be applicable e.g. as an anisotropic electromagnetic wave-interference material.Peer reviewe

Synthesis and characterizations of Ni2+ substituted cobalt ferrite nanoparticles

Nanocrystalline Co-Ni ferrites bearing chemical formula Co0.5Fe2-xNi0.5+1.0xO4 for x ranging from 0.0 to 0.4 with the step increment of 0.1 were successfully synthesized by sol gel auto-combustion method. The energy dispersive X-ray analysis (EDAX) results give relevant information for the homogenous mixing of the Co, Fe, and Ni atoms as expected from the synthesis. The phase identification of the materials by XRD reveals single phase with cubic symmetry. The presence of functional group was identified by Fourier transform infrared spectroscopic studies. The dielectric parameters such as dielectric constant ((epsilon'), dielectric loss (epsilon '') & dielectric loss tangent (tan delta) have been studied at room temperature in the frequency range 42 Hz-5MHz and is explained in the light of interfacial polarization, arising from the heterogeneous nature of ferrite structure. The decrease in DC resistivity with increasing Ni concentration is attributed to the Verwey mechanism between Fe2+ Fe3+, Co2+ Co3+ and Ni2+ Ni3+. Ni-doped nanocrystalline cobalt ferrite samples exhibit a very large value for dielectric constant of the order of 10(13). Complex impedance analysis has been used to separate grain and grain boundary in the studied samples. It is observed that saturation magnetization (Ms) decreases with increase in nickel contents which is attributed to the substitution of magnetic Fe3+ ions of 5 mu(B) by less magnetic Ni2+ ions of 2 mu(B). The analysis of the Mossbauer spectra shows the hyperfine field, relative % area and isomer shift decreases whereas quadruple splitting and line width increases at A- and B-sites on increasing the substitution of Ni2+ ions

Random site occupancy induced disordered N&#233;el-type collinear spin alignment in heterovalent Zn²⁺–Ti⁴⁺ ion substituted CoFe₂O₄

CoFe2O4, cobalt ferrite (CFO) nano-particles with composition CoZnxTixFe2-2xO4 (0 &#8804; x &#8804; 0.4) were synthesized by sol–gel autocombustion method. The effect of Zn2+–Ti4+ substitution on the structural, magnetic and frequency dependent permeability properties of the CFO nano-particles were investigated by X-ray diffraction, 57Fe M&#246;ssbauer spectroscopy, vibrating sample magnetometry, transmission electron microscopy and permeability analysis. The Rietveld refinement of XRD patterns confirm the single spinel phase and the crystallite size is found in the range of 22–32 nm. Cation distribution was estimated by refining the XRD pattern by Rietveld method, and shows Zn2+ ions at the tetrahedral A-sites, and Co2+ and Ti4+ ions at octahedral B-sites. The saturation magnetization (Ms) increased from 58 to 75 emu g−1 for up to x = 0.2 and then decreased, while the coercivity decreased continuously with Zn2+–Ti4+ substitution. Two distinct composition ranges with Zn2+–Ti4+ substitution are identified for which Ms variation with x is explained by the N&#233;el and Yafet–Kittel models. The room temperature M&#246;ssbauer spectra are analyzed in detail for probing the magnetic properties of Fe based Zn2+–Ti4+ substituted CFO. The effect of Zn2+–Ti4+ substitution on various M&#246;ssbauer parameters, viz. hyperfine field distribution, isomer shift, quadrupole splitting, and line width, has also been studied. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of A–B and B–B supertransferred hyperfine interactions. The CFO nanoparticle is considered to possess a fully inverse spinel structure with a N&#233;el-type collinear spin alignment, whereas the Zn2+–Ti4+ substitution in CFO is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. This study also demonstrates that one can tailor the magnetic properties of CFO particles by optimizing the Zn2+–Ti4+ substitution. The increase in the permeability, saturation magnetization and lower loss factor makes the synthesized materials suitable for applications in microwave devices and deflection yokes

Synthesis and Structural and Magnetic Characterization of BaZn (x) Fe12-x O-19 Hexaferrite: Hyperfine Interactions

To study the effect of Zn substitution on structural magnetic properties and hyperfine interactions of barium hexaferrite, BaFe12-x Zn (x) O-19 (0.0aexae0.3) hexaferrites were synthesized via sol-gel auto-combustion technique. Rietveld analysis of XRD powder patterns confirmed the formation of single-phase hexaferrites for all products. Due to the larger ionic size of Zn2+ as compared with Fe3+, while x increases, the lattice constant parameters increase to a small degree. Nanoplate morphology of the products is presented by SEM analyses. It was observed that both saturation magnetization and coercivity decrease in almost the same manner with zinc concentration for all substitutions. Cation distribution calculations showed that Zn2+ occupies 12k, 4 f (2), 4 f (1), and 2b sites and at the same time pushes Fe3+ ions towards 2a and 12 k (1) sites. From(57)Fe Mossbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting, and hyperfine magnetic field values on Zn2+ substitution have been determined

Magnetic and microstructural features of Dy3+ substituted NiFe2O4 nanoparticles derived by sol-gel approach

This study explored the microstructural and magnetic features of NiFe2-xDyxO4 (x <= 0.10) NPs (nanoparticles) that were synthesized by sol-gel auto-combustion method. The single phase of spinel ferrite has been verified for all samples without any impurity. The cubic morphology of the products was also showed by SEM. Room temperature (300 K) and 10 K magnetization curves were recorded applying a dc magnetic field up to +/- 50 kOe and it was observed that magnetic features of NiFe2O4 NPs significantly changed by the substitution of Dy3+ ion. Magnetization measurements showed low order of 300 and 10 K magnetic parameters (such as K-eff, coercivity and anisotropy field values), revealing soft ferrimagnetic behaviors of all pristine and doped NiDyxFe2-xO4 (0.00 <= x <= 0.10) NPs at both 300 and 10 K. Pristine NiFe2O4 has maximum magnetic moment and saturation magnetization values among all samples. Dy3+ substitution showed a slight decrement in magnetization values compared with pristine sample. A slight increase in coercivity was noticed with Dy3+ substitution. Squareness ratios (SQRs) have a range between 0.144 and 0.324. These values are smaller than the theoretical limit of 0.50, implying the multi-domain nature for NPs. Blocking temperature (T-B) was calculated as 28 K for NiFe2O4 NPs