The failure of ceramic materials is caused by the extension of small flaws. Therefore, linear-elastic fracture mechanics can be applied to describe the failure behaviour. The main problem in the application of the simple fracture mechanics relation is the existence of a rising crack growth resistance curve, which is caused by crack bridging forces behind the advancing crack tip or by transformations in front of the crack tip. The increasing crack growth resistance leads to problems in the transformation of results from specimens with macrocracks to components with natural cracks.

Fracture Mechanics of Ceramics

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

/pdf/room-temperature-ferroelectricity-in-strained-srtio3-4u1ustezxf.pdf

Room-temperature ferroelectricity in strained SrTiO3.

Biaxial compressive strain has been used to markedly enhance the ferroelectric properties of BaTiO 3 thin films. This strain, imposed by coherent epitaxy, can result in a ferroelectric transition temperature nearly 500°C higher and a remanent polarization at least 250% higher than bulk BaTiO 3 single crystals. This work demonstrates a route to a lead-free ferroelectric for nonvolatile memories and electro-optic devices.

http://science.sciencemag.org/content/sci/306/5698/1005.full.pdf

Enhancement of ferroelectricity in strained BaTiO3 thin films.

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

Predictions and measurements of the effect of biaxial strain on the properties of epitaxial ferroelectric thin films and superlattices are reviewed. Results for single-layer ferroelectric films of biaxially strained SrTiO3, BaTiO3, and PbTiO3 as well as PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices are described. Theoretical ap- proaches, including first principles, thermodynamic analysis, and phase-field models, are applied to these biaxially strained materials, the assumptions and limitations of each technique are explained, and the predictions are compared. Measurements of the effect of biax- ial strain on the paraelectric-to-ferroelectric transition temperature (TC) are shown, demonstrating the ability of percent-level strains to shift TC by hundreds of degrees in agreement with the predic- tions that predated such experiments. Along the way, important ex- perimental techniques for characterizing the properties of strained ferroelectric thin films and superlattices, as well as appropriate sub- strates on which to grow them, are mentioned.

/pdf/strain-tuning-of-ferroelectric-thin-films-i3r81086fk.pdf

Strain Tuning of Ferroelectric Thin Films

Heteroepitaxial BaTiO3 films of various thicknesses ranging from 12 nm to 79 nm were prepared on SrRuO3/SrTiO3 substrates by radio-frequency magnetron sputtering employing a two-step deposition technique, and the crystallographic and ferroelectric properties of the heteroepitaxial films were evaluated A ferroelectric hysteresis loop was clearly observed in the heteroepitaxial BaTiO3 films even when the thickness was reduced to 12 nm, probably due to improved crystallinity at the interface between the ferroelectric film and the electrodes through optimization of the preparation technique The observation of hysteresis at the temperature of 200°C was explained in terms of modification of the Curie temperature from the inherent 130°C to far above 200°C by lattice misfit strain A possible relationship between the artificially raised Curie temperature and the limited thickness dependence of ferroelectricity was discussed

https://iopscience.iop.org/article/10.1143/JJAP.38.5305/pdf

Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors

The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 Hz, it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

Asymmetric ferroelectricity and conduction of anomalous leakage current were observed in heteroepitaxial BaTiO3 thin films grown by rf magnetron sputtering at 600° C on three different electrode/substrate combinations: SrRuO3/SrTiO3, Pt/MgO and Nb doped SrTiO3. The voltage shift of hysteresis loops of the capacitance was a linear function of the thickness of the BaTiO3 films and became as large as 10 V in the film with a thickness of 410 nm. The asymmetry did not disappear even after a heat treatment carried out at 800° C in air. On the other hand, a steep increase in the leakage current was observed in the heteroepitaxial films when a positive voltage was applied. The origin of both the asymmetric hysteresis and the anomalous conduction is discussed in terms of asymmetric crystal structure caused by misfit dislocations introduced in heteroepitaxial growth.

https://iopscience.iop.org/article/10.1143/JJAP.36.5846/pdf

Asymmetric Ferroelectricity and Anomalous Current Conduction in Heteroepitaxial BaTiO3 Thin Films

A piezoelectric thin film device includes an amorphous metal film disposed on a substrate and a piezoelectric film disposed on the amorphous metal. One of crystal axis of the piezoelectric film is aligned in a direction perpendicular to a surface of the amorphous metal.

Piezoelectric thin film device and method for manufacturing the same

The fundamental corrosion behavior of silicon carbide (SiC) ceramics was investigated after immersion in 290°C water solutions with different pH and dissolved-oxygen concentrations. The weight loss in the oxygenated solution was more than that in the deoxygenated solution and was accelerated by increasing pH. Preferential attack could be found at grain boundaries and around pores on the sample surface immersed in the oxygenated alkaline solution. The weight change, dW, followed the general rate law, (dW)m= kt. The exponent, m, was 1.11 in the alkaline solution and 0.45 in the acidic solution. Based on the above results, the SiC was considered to be directly hydrolyzed to a silica sol, with its dissolution kinetics dependent on the sol stability. This corrosion behavior is quite different from those in high-temperature or vapor-phase hydrothermal oxidation, where the oxidation rate is controlled by oxidant diffusion through the protective silica surface layer.

Takashi Kawakubo

Papers

Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

Asymmetric Ferroelectricity and Anomalous Current Conduction in Heteroepitaxial BaTiO3 Thin Films

Piezoelectric thin film device and method for manufacturing the same

Corrosion Behavior of Silicon Carbide in 290°C Water