BaTiO3-based Multilayer Capacitors Research Articles

Measurement of the dynamic temperature response of electrocaloric effect in solid ferroelectric materials via thermoreflectance

ABSTRACT We propose a characterization method based on modulated thermoreflectance technique for measuring directly the electrocaloric (EC) effect. First, the EC response in BaTiO3-based multilayer capacitors (MLC) with Y5V specification was measured using thermoreflectance set-up. Further, the direct measurement method was implemented on P(VDF-TrFE) 55/45 thin films deposited on flexible PET substrate, where the frequency dependence of the EC response in these films was investigated. It was noted that the P(VDF-TrFE) EC temperature responses depend on the operation voltage frequency, thus we considered three frequency ranges using square voltage waveform. Three distinct types of EC temperature variation responses were observed depending on the waveform frequency range. In addition, the EC temperature variation of the thin film with a thickness of 1.4 µm was measured via thermoreflectance. The data reveal a large EC effect occurring at room temperature where a temperature change ΔT = 1.14°C was induced under a field of 82 MV/m.

Enhanced electrocaloric efficiency via energy recovery

Materials that show large and reversible electrically driven thermal changes near phase transitions have been proposed for cooling applications, but energy efficiency has barely been explored. Here we reveal that most of the work done to drive representative electrocaloric cycles does not pump heat and may therefore be recovered. Initially, we recover 75–80% of the work done each time BaTiO3-based multilayer capacitors drive electrocaloric effects in each other via an inductor (diodes prevent electrical resonance while heat flows after each charge transfer). For a prototype refrigerator with 24 such capacitors, recovering 65% of the work done to drive electrocaloric effects increases the coefficient of performance by a factor of 2.9. The coefficient of performance is subsequently increased by reducing the pumped heat and recovering more work. Our strategy mitigates the advantage held by magnetocaloric prototypes that exploit automatic energy recovery, and should be mandatory in future electrocaloric cooling devices.

Non-volatile magnetoelectric edge effects observed using magnetic force microscopy

In BaTiO3-based multilayer capacitors (MLCs) that show volatile magnetoelectric (ME) effects [C. Israel et al., Appl. Phys. Lett. 93, 173501 (2008)], a magnetic force microscopy study of the Ni electrode edges reveals non-volatile ME effects, due to the inhomogenous stress associated with an electric-field distribution that we model using finite-element analysis. Our findings imply that volatile ME effects in thin films may be rendered non-volatile by lithographic patterning, thus suggesting a route for the design of electric-write magnetic-read data-storage elements.

Effect of Cr additions on the microstructural stability of Ni electrodes in ultra-thin BaTiO3 multilayer capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCC) is one of the present day challenges to maintain an increase in capacitive volumetric efficiency. This present paper opens a series of investigations aimed to engineer the stability of ultra-thin Ni electrodes in BaTiO3-based multilayer capacitors using refractory metal additions to Ni. Here, pure Ni and Ni–1 wt.% Cr alloy powders are used to produce 0805-type BME MLCCs with 300 active layers and with dielectric and electrode layer thickness around 1 μm. To investigate the continuity of Ni electrodes, both MLCC chips with pure and doped electrodes were sintered at different temperatures for 5 h. It is found that the continuity of Ni electrodes is improved most likely due to the effect of Cr on the low-melting point (Ni,Ba,Ti) interfacial alloy layer formation. The interfacial alloy layer is not observed when Cr is segregated at Ni-BaTiO3 interface in the Cr-doped samples, while it is found in all undoped samples. The interfacial alloy layer is believed to increase mass-transfer along the Ni-BaTiO3 interfaces facilitating an acceleration of Ni electrodes discontinuities.

Dependence of breakdown field on dielectric (interelectrode) thickness in base-metal electroded multilayer capacitors

The authors have empirically determined the dependence of breakdown field EB versus dielectric thickness d (electrode separation) for a large number (4100) of BaTiO3-based multilayer capacitors with Ni base-metal electrodes. The data averaged over lateral area show for 6<d<22μm that EB=const×d−n, where n=0.50±0.06, a result compatible with macroscopic “thermal” dc breakdown mechanisms. More precisely, however, the results nearly perfectly fit a situation of breakdown occurring in connected defects from collision ionization resulting from field emission from the cathode.

Scientific and Engineering Issues of the State-of-the-Art and Future Multilayer Capacitors.

BaTiO3-based multilayer capacitors are rich with the science of crystal chemistry, defect chemistry, phase transition theory, ferroelectricity, and processing. State-of-the-art multilayer capacitors are fabricated with base metals, submicron grains and have thickness of layers approaching one micron. There are, however, needs to keep improving the volumetric efficiency of capacitance, improve reliability in thin layers, and increase the number of active layers. This paper summarizes and reviews some of the studies our group is performing to scientifically aid progress in this fast moving field. Specifically, we will review size effect issues, the role of rare-earth doping in improving degradation resistance, and the potential of electrophoretic deposition to produce multilayer structures with nanopowders.

Fine Grained Relaxor Dielectric Ceramics Prepared by Hydrothermally Synthesized Powder

In response to the demand for small grain size in thin dielectric layers of multilayer capacitors (MLCs), relaxor dielectric powder was hydrothermally synthesized. The sintering temperature for the hydrothermally synthesized powder was reduced to 950° C from 1200° C for the conventional solid phase reaction synthesized powder. As a result, the grain size of sintered specimens decreased from 3.9 µm for the solid phase reaction synthesized powder as a starting powder to 1.3 µm for hydrothermally synthesized powder: this is equivalent to the smallest grain size of a commercially available X7R-designated barium titanate-based dielectric fired at 1300° C. Using the hydrothermally synthesized powder, Y7R-designated 7.4 µF MLCs with 3.2 µm-thick 240 dielectric layers of EIA1206 size (3.2×1.6×1.5 mm) were fabricated. The MLCs were fired at 1000° C for 2 h and the grain size was 1.3 µm. The capacitance per unit volume of these MLCs is approximately 900 µF/cm3, which is comparable with that for tantalum capacitors. In addition, the new MLCs have a low dissipation factor of 1.3% and stable dependence of capacitance on dc bias voltage as compared with BaTiO3-based MLCs.

Improving sintering behaviour of BaTiO3 by small doping additions

The dielectric properties of BaTiO3 ceramics are associated with its microstructure, which depends on the stoichiometric ratio, dopant nature, and ceramic processing. Optimum densification at low temperatures to reduce the amount of noble metals in electrodes for Multilayer Ceramic Capacitors (MLC) requires the use of liquid phase sintering. However, depending on the nature and amount of such a liquid phase, exaggerated grain growth is produced. In order to increase the capacitance in BaTiO3-based multilayer capacitors, increasing the dielectric constant by careful and homogeneous development of the microstructure, is necessary. The correct distribution of dopant on the particles' surface leads to a modification of their features by reacting with surface cations such as Ba2+. Such a modification allows densification at low temperatures with very small amounts of liquid phase. As a consequence, good control of grain size is obtained which enhances the dielectric properties of the BaTiO3 ceramics. The controlled microstructure also allows the reduction of the layer width in multilayer arrangements.