Piezoelectric ceramics are widely used in electronic devices such as communication devices, oscillators, various actuators and sensors and so on. In the future IoT (Internet of Things) society, huge number of electric devices will be connected each other, and the piezoelectric devices will increase their importance. However requirements to piezoelectric materials has become more difficult than before. Electronic devices should be placed in more harsh conditions such as high temperatures and they should have enough reliabilities against the harsh conditions. Moreover they should be harmonized with natural environment. Harmful substances should be removed from the materials used in electronic devices. Structural designs of piezoelectric ceramics are very important for improvements of their properties which is difficult to improve by ordinary compositional modifications.
Developments of Lead-free piezoelectric ceramic materials are also important as environment-benign technologies. In this review, some examples of the structure-derived functionalities and lead-free piezoelectric R&D are introduced and their future perspectives are described.
This study is reported for the monolithic thermoelectric generator (TEG) which consists of p-type semiconductor, n-type semiconductor, and insulator layers. We used Ni0.9Mo0.1 alloy as a p-type thermoelectric material, (Sr1-x,Lax)TiO3 as an n-type thermoelectric material, and Y2O3-ZrO2 as an insulator. A firmly integrated TEG after co-firing was obtained without delamination by controlling the sintering behavior of the three different materials. The TEG has 36 pairs of p-i-n junction and generates about 100 μW at temperature difference 10°C. After the reliability test (high-temperature storage test, pressure cooker test, and heat shock test), power change rate of the TEG was less than 5%. Wireless sensor node (WSN) using the TEG as a power supply was manufactured. The TEG under a load connected to a circuit was continued generating power for 4600 h at a 120°C heat source and air cooling. During the test, signals were sent from WSN once every 10 s.
In recent years, lead-free has been promoted by the RoHS Directive which bans the use of lead in electronic devices, and nano Ag die attach pastes has attracted attentions. Nano Ag particles show high electrical and thermal conductivity, but generally require Au or Ag plating on the surface of the substrate. Therefore, demands for Cu substrate without the plating are also increasing to reduce costs. This paper deals with the fundamental study on nano Ag pastes newly developed with a unique approach. The technology provides a low-temperature sintering capability without pressure during cure process. In addition, resin reinforcing technology and lowering modulus technology have been developed to improve the mechanical properties. By adding epoxy resins, the porous area is filled with the resin and the sintered structure is reinforced. Additionally, the reliability can be more improved by thermoplastic resin particles. To meet the demand for the applications without Ag or Au plating, nano Ag pastes for bare Cu substrate have newly been developed. By using several types of Cu substrates without plating, surface analysis and bonding properties of the pastes were investigated in this study.
In various kinds of functional material, its properties can be controlled by microstructure refinement in order to optimize its performance. Especially for a thermoelectric material, the reduction of thermal conductivity by the microstructure refinement can enhance its utility for thermoelectric energy conversion devices. For the fabrication of a fine-grain sintered body, a powder metallurgical technique is commonly used. In particular, electric current sintering is an effective means of microstructure refinement. Recently, flash sintering, which is defined as sintering with a duration in the order of seconds, is expected as an energy efficient sintering technique. In addition, from an application viewpoint, the high production rate of flash sintering is expected to bring improved productivity to mass production of sintered parts. In this paper, flash sintering of thermoelectric compound was conducted by the sintering apparatus specially designed for flash sintering. With a current feed duration of 1 s, Sb2Te3 sintered sample having a thermoelectric performance comparable to that of a sample prepared by a conventional current sintering can be obtained. This significant reduction in the sintering time can result in the energy consumption for the sintering process being decreased to less than 1% of that required for conventional current sintering.
Piezoelectric materials are currently applied in various electronic devices. Multilayer technology has become indispensable for the miniaturization of electronic devices. In this case, the development of materials that do not contain toxic elements has been receiving considerable attention because of environmental issues. This tutorial paper describes the processing of reduction-resistant BaTiO3-based piezoceramics with improved properties for multilayer-type component using base metal internal electrodes. Nonreducible BaTiO3-based ceramics that exhibited a high piezoelectric constant equivalent to that of samples prepared in air were successfully fabricated at temperatures below the melting point of Ni metal. Grain-oriented ceramics were prepared by the reactive templated grain growth method using platelike perovskite oxide particles. The electrical properties of the BaTiO3-based ceramics, sintered in the reducing atmosphere, were markedly improved as a result of fabricating grain-oriented samples. A desired reducing atmosphere was achieved by controlling the oxygen partial pressure with a suitable mixture of Ar, H2, and CO2 gases. Optimizing the amount of Li2CO3 added to composition modified BaTiO3-based ceramics sintered under a low oxygen partial pressure resulted in improved piezoelectric properties while maintaining the high sintered density. Reduction-resistant BaTiO3-based ceramics developed in this study are promising lead-free materials for applications in multilayer piezoelectric components.
In authors group, functional ceramics materials e.g., Li ion battery materials (domain atomic structure and its influence on battery properties), ferroelectric materials (ferroelectricity in wurtzite crystal structure) were studied using modern first-principle calculations. In this paper, our recent studies were explained. The possibilities and challenges of next generation materials informatics were also discussed.
Rapid advances in modern technology have led to increasing demands for control of thermal expansion. Recently, it has become necessary to control thermal expansion in a local region at the micrometer level, especially in the field of electronic devices. To meet this demand, attempts have been made to produce micrometer or submicrometer scale fine particles showing negative thermal expansion (NTE). Here, recent studies on spry-dry synthesis of β-Cu1.8Zn0.2V2O7 ceramic particles are reviewed. This class of vanadates shows large NTE linear to temperature over a wide temperature range. This NTE originates from the microstructures consisting of voids and anisotropic thermal deformation of crystal grains in a ceramic particle. By reducing the size of the microstructures that produce NTE, large NTE equivalent to bulk was realized even for ceramic particles of about 2 μm sizes. Comparison with the conventional method shows that this method provides the benefit of obtaining fine particles of narrow particle size distribution with a shape close to a sphere. This achievement is expected to pave the way for the use of NTE materials to control thermal expansion of a local region, for example, internal components of advanced electronic devices such as three-dimensional integrated circuit.
The sensor devices operated in low frequency ultrasonic region usually contain the piezoelectric element driven by flexure vibration mode. Two successful examples of these devices as piezoelectric vibration gyroscope and SMD type ultrasonic transducer are reviewed. In both cases, reversibility of piezoelectricity has play an important role for simplifying the structure and cost reduction. Designed structure to minimize the leak of vibration generated by flexure vibration is key factor for improved performance and reliability which must be critical in miniature size.