TEM and EDS investigation of heterogeneous interfaces in cofired multilayer ceramic capacitors

Abstract The interfacial microstructure and cofiring migration between Ag Á/Pd electrode and Pb-based relaxor ferroelectric ceramics were directly investigated via transmission electron microscope and energy dispersive X-ray spectroscopy (EDS). Different silver migration abilities of 70Ag Á/30Pd and 90Ag Á/10Pd electrodes were confirmed by EDS microanalysis. This difference is generally considered to be responsible for different effects of inside electrode on the reliability of multilayer cofired devices. Further, Pb diffusion and new phase formation near the interface were detected, incorporating interfacial microstructural observation. A good consistency of direct experimental evidence to the following theoretic analyses was well combined in this paper.

In situ poling X-ray diffraction studies of lead-free BiFeO3–SrTiO3 ceramics

The origin of the large electrostrain in BiFeO3-BaTiO3 (BF-BT) ceramics is controversial and has been attributed to either a field-induced transition to a long-range ferroelectric (FE) state or to multi-symmetry, polar nanoregions within a pseudocubic matrix whose vectors approximately align with the direction of the applied field. The (1-x)BiFeO3-xSrTiO3 (BF-xST) solid solution is structurally and microstructurally similar to BF-BT and provides a further case study to assess the origin of electrostrain. In BF-xST, electrostrain is optimized at x = 0.4 (0.15%) which zero field, room temperature full-pattern X-ray diffraction (XRD) Rietveld refinement and scanning/transmission electron microscopy suggest is composed of 15% rhombohedral (R) cores, surrounded by 85% pseudocubic (PC) shells. In-situ poling synchrotron XRD reveals that all peaks remain singlet and exhibit no change in full width half maximum up to 100 kV cm−1, confirming the absence of long-range FE order and the retention of short-range polar order, despite the large applied field. Strain anisotropy (calculated from individual peaks) of ε220 > ε111 > ε200 and the associated strain orientation distribution however, indicate the existence of local orthorhombic (O), R and tetragonal (T) symmetries. The data therefore imply the existence under poling of multi-symmetry polar nanoregions in BF-0.4ST rather than a long FE phase, supporting the model described by Wang and co-workers (2019) for BF-BT compositions

Superior energy density through tailored dopant strategies in multilayer ceramic capacitors

The Gerson–Marshall (1959) relationship predicts an increase in dielectric breakdown strength (BDS) and therefore, recoverable energy density (Wrec) with decreasing dielectric layer thickness. This relationship only operates however, if the total resistivity of the dielectric is sufficiently high and the electrical microstructure is homogeneous (no short circuit diffusion paths). BiFeO3–SrTiO3 (BF–ST) is a promising base for developing high energy density capacitors but Bi-rich compositions which have the highest polarisability per unit volume are ferroelectric rather than relaxor and are electrically too conductive. Here, we present a systematic strategy to optimise BDS and maximum polarisation via: (i) Nb-doping to increase resistivity by eliminating hole conduction and promoting electrical homogeneity and (ii) alloying with a third perovskite end-member, BiMg2/3Nb1/3O3 (BMN), to reduce long range polar coupling without decreasing the average ionic polarisability. These strategies result in an increase in BDS to give Wrec = 8.2 J cm−3 at 460 kV cm−1 for BF–ST–0.03Nb–0.1BMN ceramics, which when incorporated in a multilayer capacitor with dielectric layers of 8 μm thickness gives BDS > 1000 kV cm−1 and Wrec = 15.8 J cm−3

Mechanism of enhanced energy storage density in AgNbO3-based lead-free antiferroelectrics

The mechanisms underpinning high energy storage density in lead-free Ag1–3xNdxTayNb1-yO3 antiferroelectric (AFE) ceramics have been investigated. Rietveld refinements of in-situ synchrotron X-ray data reveal that the structure remains quadrupled and orthorhombic under electric field (E) but adopts a non-centrosymmetric space group, Pmc21, in which the cations exhibit a ferrielectric configuration. Nd and Ta doping both stabilize the AFE structure, thereby increasing the AFE-ferrielectric switching field from 150 to 350 kV cm−1. Domain size and correlation length of AFE/ferrielectric coupling reduce with Nd doping, leading to slimmer hysteresis loops. The maximum polarization (Pmax) is optimized through A-site aliovalent doping which also decreases electrical conductivity, permitting the application of a larger E. These effects combine to enhance energy storage density to give Wrec = 6.5 J cm−3 for Ag0.97Nd0.01Ta0.20Nb0.80O3

Neutron Spectroscopy Evidence for a Possible Magnetic-Field-Induced Gapless Quantum-Spin-Liquid Phase in a Kitaev Material α-RuCl3

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, α-RuCl3 has received a great deal of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of α-RuCl3 under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the M point and a continuum near the Γ point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field μ0Hc = 7.5 T, the former ones vanish and only the latter ones are left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the Γ point become more intense. By following the gap evolution of the excitations near the Γ point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly gapless QSL phase near μ0Hc, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive α-RuCl3 into a long-sought QSL state near the critical field