Hisaaki Takao

Lead-free piezoceramics.

Lead has recently been expelled from many commercial applications and materials (for example, from solder, glass and pottery glaze) owing to concerns regarding its toxicity. Lead zirconium titanate (PZT) ceramics are high-performance piezoelectric materials, which are widely used in sensors, actuators and other electronic devices; they contain more than 60 weight per cent lead. Although there has been a concerted effort to develop lead-free piezoelectric ceramics, no effective alternative to PZT has yet been found. Here we report a lead-free piezoelectric ceramic with an electric-field-induced strain comparable to typical actuator-grade PZT. We achieved this through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution, and the development of a processing route leading to highly 〈001〉 textured polycrystals. The ceramic exhibits a piezoelectric constant d33 (the induced charge per unit force applied in the same direction) of above 300 picocoulombs per newton (pC N-1), and texturing the material leads to a peak d33 of 416 pC N-1. The textured material also exhibits temperature-independent field-induced strain characteristics.

High Performance Lead-free Piezoelectric Ceramics in the (K,Na)NbO3-LiTaO3 Solid Solution System

A new solid solution system, namely {(K 0.5 Na 0.5 ) 1− x Li x }(Nb 1− y Ta y )O 3 , with full density, a large piezoelectric constant and high stability has been developed. The system has a morphotropic phase boundary (MPB) between orthorhombic and tetragonal. The MPB gives the largest piezoelectric d constant at {(K 0.5 Na 0.5 ) 0.97 Li 0.03 }(Nb 0.8 Ta 0.2 )O 3 , which exhibits a piezoelectric actuator constant d 33 of 230 pm/V, 2.34 times that of pure (K 0.5 Na 0.5 )NbO 3 (KNN). Also, it gives a piezoelectric coupling Kp value (0.505), a dielectric constant (1256), and an elastic compliance (10.9 TPa−1 ) 1.51, 2.93, and 1.14 times those of pure KNN, respectively. The long-term stability of the dielectric loss was demonstrated to be far superior to that of pure KNN.

Synthesis of Platelike {100} SrTiO3 Particles by Topochemical Microcrystal Conversion and Fabrication of Grain-Oriented Ceramics

Platelike SrTiO3 particles with a cubic perovskite structure were synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of layer-structured SrBi4Ti4O15 at 950 °C. SrTiO3 particles preserved the shape of precursor particles, and had a thickness of approximately 0.5 µm and a width of 5–10 µm. X-ray diffraction analysis revealed that in the TMC reaction, the crystallographic {001} plane of SrBi4Ti4O15 is converted into the {100} plane of SrTiO3. Using the platelike SrTiO3 particles as a template in templated grain growth method, dense {100} grain-oriented SrTiO3 ceramics having a {100} orientation degree (Lotgerings factor) higher than 91% could be fabricated at sintering temperatures between 1350 and 1550 °C. The maximum orientation factor reached 99.3%.

Evolution of Crystallographic Grain Orientation of (K0.5Na0.5)NbO3 Piezoelectric Ceramics

The microstructural development of crystalline-oriented (K0.5Na0.5)NbO3 (described as KNN hereafter)-based piezoelectric ceramics during sintering was investigated. The addition of CuO as a sintering aid was found to be effective for fabricating highly oriented and dense KNN ceramics. KNN specimens with 0.5-1.0 mol% CuO sintered at 1100°C for 1 h have both a relative density and a pseudo-cubic {100} orientation degree of 95% or higher. In the early stages of sintering, KNN is formed in the reaction between complementary reactants NaNbO3 and KNbO3, after which oriented grain growth proceeds at a relative density of more than 90%. In addition, the results of TEM observation showed that textured KNN ceramics have a unique pectinate-like domain structure, in which the domain walls consist of {101} planes.

Improvement of cross-tension strength using concave electrode in resistance spot welding of high-strength steel sheets

In the spot welding of ultrahigh-strength steel sheets, the generation of expulsion could be suppressed and large-diameter nuggets could be formed by using a concave electrode. The expulsion was suppressed because the clearance for expansion was formed when using the concave electrode. Thus, nuggets with a diameter of ≥4√t (lowest for securing joint strength, t = 1 mm) were obtained without expulsion in a greater welding current range than the range when using a conventional electrode. Furthermore, nuggets with a diameter of approximately 6.5 mm were formed with a large current of 8 kA, which could not be set when using the conventional electrode. In addition, a transition from partial interfacial fracture to pullout fracture occurred as the nugget diameter increased, and the cross-tension strength (CTS) tended to increase. A value of about 8 kN was obtained for a nugget with a diameter of 6.5 mm, which is approximately 1.5 times larger than the value (approximately 5.5 kN) obtained with the conventional ...

Effect of material flow on joint strength in activation spot joining of Al alloy and steel sheets

A new joining method for dissimilar metal sheets was developed where a rotated consumable rod of Al alloy is pressed onto an Al alloy sheet at the part overlapped with a mild steel sheet. The metal flow in the joining region is increased by the through-hole in the Al sheet and consumable Al rod. The rod creates the joint interface and pads out of the thinly joined parts through pressing. This produces a higher joint strength than that of conventional friction stir spot welding. Measurements of the joint interface showed the presence of a 5-10 nm thick amorphous layer consisting of Al and Mg oxides.

Synthesizing of Platelike {100} SrTiO 3 Particle by Topochemical Microcrystal Conversion Method

Cubic perovskite-structured platelike SrTiO₃ particles were synthesized by topochemical microcrystal conversion (TMC) method from platelike precursor of layer-structured SrBi₄Ti₄O₁₅ particles. SrTiO₃ particles inherited the shape of precursor particles having a thickness of about 0.5 microns and a width of 5-10 microns. X-ray diffraction measurement of cast particles on glass substrate elucidated that in the TMC reaction, the crystallographic {001} plane of SrBi₄Ti₄O₁₅ is converted into the {100} plane of SrTiO₃. With use of the platelike SrTiO₃ particles as a template by templated grain growth method, dense {100} grain-oriented SrTiO₃ ceramics having a {100} orientation degree (Logerrings factor) larger than 91% were fabricated at sintering temperatures between 1350degC to 1550degC. The maximum orientation factor was 99.3%. The textured ceramics had a unique microstructure of well aligned brick wall-like grains in the side face to tape-cast plane observed by scanning electron microscopy.