Nonstoichiometry Role on the Properties of Quantum-Paraelectric Ceramics

Among the lead-free perovskite-structure materials, strontium titanate (SrTiO3—ST) and potassium tantalate (KTaO3—KT), pure or modified, are of particular importance. They are both quantum paraelectrics with high dielectric permittivity and low losses that can find application in tunable microwave devices due to a dependence of the permittivity on the electric field. Factors as Sr/Ti and K/Ta ratio in ST and KT ceramics, respectively, can alter the defect chemistry of these materials and affect the microstructure. Therefore, if properly understood, cation stoichiometry variation may be intentionally used to tailor the electrical response of electroceramics. The scientific and technological importance of the stoichiometry variation in ST and KT ceramics is reviewed and compared in this chapter. The differences in crystallographic phase assemblage, grain size, and dielectric properties are described in detail. Although sharing crystal chemical similarities, the effect of the stoichiometry is markedly different. Even if the variation of Sr/Ti and K/Ta ratios did not change the quantum-paraelectric nature of ST and KT, Sr excess impedes the grain growth and decreases the dielectric permittivity in ST ceramics, while K excess promotes the grain growth and increases the dielectric permittivity in KT ceramics

Synthesis and characterization of one-dimensional titanate structure

One-dimensional titania structures were synthesized trough a simple hydrothermal process in a highly alkaline conditions. The aim of this work was to elucidate the effect of time on the formation of 1D titanates as well on its structural characteristics (morphology, phase composition, surface area). Apart from that, the effect of heat treatment conditions on the stability of titanate based 1D samples has been investigated. The results have revealed that it is possible to form one-dimensional titanates already after 1 hour of hydrothermal synthesis. Although the composition of titanates is still under debate, the results probably correspond to the layered sodium titanates. The 1D prepared structures show a remarkable stability during heating, remaining the basic morphology and composition even up to 700°C

Role Of Suspension Media For Electrophoretic Deposition: The Case Of Functional Oxides

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Low-Temperature Dielectric Response of Strontium Titanate Thin Films Manipulated by Zn Doping

The voltage dependence of the dielectric permittivity ε' and the low dielectric loss tanδ of incipient ferroelectrics have drawn vast attention to the use of these materials for the development of tuning elements in electronics and telecommunications. Here, we study the DC electric field dependence of low-temperature ε' in ~320 nm thick sol-gel-derived SrTi1-xZnxO3-δ thin films with x = 0.01 and 0.05, deposited on Pt/TiO2/SiO2/Si substrates. Incorporation of Zn onto Ti sites is found to decrease ε' compared to undoped SrTiO3 films, while increasing the relative tunability nr up to ~32.9% under a DC electric field of 125 kV/cm at low temperatures. The hysteresis-free variation in ε' with electric field and tanδ values below 0.6% observed for SrTi1-xZnxO3-δ film with x = 0.01 make this compound more attractive for tunable device applications.publishe

Atmosphere assisted FLASH sintering of KNN

The use of FLASH alternative sintering technique allows a significant decrease in sintering time and temperature, contributing to the sustainable processing of high sintering temperature piezoelectrics. This is the case of potassium sodium niobate, K0.5Na0.5NbO3 (KNN), a relevant lead-free piezoelectric, which, due to alkali evaporation, is difficult to produce by conventional sintering, at T \u3e 1100 ºC. Please click Additional Files below to see the full abstract

Dielectric behavior of FLASH sintered KNN

The market for lead-based piezoceramics, mainly (Pb1-x ZrxTiO3, PZT) - based materials, is higher than $100 billion per year. Due to lead-toxicity, the demand for lead-free piezoceramics is increasing. Potassium Sodium Niobate solid solutions, namely K0.5Na0.5NbO3, KNN, is currently one of the most promising materials for electromechanical applications. However, monophasic conventionally sintered KNN is hard to obtain, due to alkali evaporation during sintering (T\u3e 1100 ºC, t \u3e 2h). Within this context, there is an increasing interest in sustainable sintering techniques, as FLASH, to decrease both sintering time and temperature, avoiding alkali vaporization. However, FLASH applied to bulk ceramics, frequently produces inhomogeneous specimens. Figure 1 – Variation of length with temperature of FLASH sintered KNN, after a 2 h isothermal step. SEM micrograph showing the uniformly dense microstructure. In this work, we propose an experimental approach that allows the production of homogeneous, highly dense, KNN. In this method, the use of FLASH sintering contributed to reduce KNN sintering temperature for more than 200 ºC and the cycle time in ~3h. Uniform densification was achieved by using an isothermal step before the application of the electric field. Scanning Electron Microscopy (SEM) and Specific Surface Area (SSA) measurements were performed to characterize the pre-FLASH sintering microstructure. Please click Additional Files below to see the full abstract

Oxygen vacancies as a link between the grain growth and grain boundary conductivity anomalies in titanium-rich strontium titanate

Titanium-rich (Sr/Ti = 0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625 °C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with each other using a simple “brick-layer” model. In this work we revise the topic and advocate that the deviation from the model comes from the oxygen vacancies localized at GBs of the rapidly-cooled ST ceramics. To verify this, we annealed the ceramics in oxygen and performed their systematic and comparative analysis using impedance spectroscopy. A levelling-off in the GB conductivity activation energy, which increases for ≤1.24 eV, and a four-fold decrease in the GB permittivity are observed after annealing. Thus, we confirm a key role of oxygen vacancies in relation between the grain growth and GB conductivity anomalies of as-sintered Ti-rich ST ceramics.publishe

Understanding Flash Sintering of Semiconductor Oxide Materials at the Nano- Atomic Scale

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ENVIRONMENTALLY FRIENDLY PROCESSING OF LEAD FREE SODIUM POTASSIUM NIOBATE THICK FILMS BY ELECTROPHORETIC DEPOSITION

When looking for industrial applications environmental compatible solutions are highly desired. When looking at microelectronics miniaturization, functionalization and performance are also highly required. When looking at technologies for deposition of thick films electrophoretic deposition (EPD) is attracting increasing interest for a wide range of technical applications due to its high flexibility, low cost and simplicity of management, facility to fabricate intricate shapes and to be scaled-up for large volume fabrication. Within this context in this work aqueous based electrophoretic deposition (EPD) of lead free sodium potassium niobate (KNN) is demonstrated. The goal of our work is to establish the range of conditions in which aqueous deposition of KNN films is possible. By the use of a relatively environmental friendly additive, such as sodium dodecyl sulfate (SDS), high quality crack and defect free KNN films are prepared. The deposition mechanism and the electromechanical performance of these films are presented and discussed. KNN thick films on metallic substrates with thickness varying between 10 - 15 µm have dielectric permittivity of 495, dielectric losses of 0.08 at 1 MHz and a piezoelectric coefficient d33 of ~70 pC/N. At TC (≈405 °C), these films exhibit a relative permittivity of 2166 and loss tangent of 0.11 at 1 MHz. A comparison of the physical properties between these films and their bulk ceramics counterparts demonstrates the impact of aqueous based EPD technique for the preparation of lead free ferroelectric thick films. This opens the door to the possible development of high-performance, lead-free piezoelectric thick films by a sustainable low cost process, expanding the applicability of lead free piezoelectrics. Please click Additional Files below to see the full abstract

Synthesis and structure of nanomaterials in the system K2O-Nb2O5-SiO2

The aim of the present work is synthesis of ferroelectric nanomaterials, in the K2O-Nb2O5-SiO2 system via solgel method and studying the processes of formation and structure of the synthesized ferroelectric nanomaterials. The structure of synthesized materials has been studied by means of the following methods: EDS, XRD, FT-IR, SEM and AFM. The results obtained showed that the structure of the investigated compositions does not depend on the niobium content and all the samples keep their amorphous nature at room temperature. The surface structure shows random distribution of different kinds of aggregates with dimensions about 200–500 nm. The presence of a hybrid nanostructure with well-defi ned nanounits having special geometry is clearly observed