Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

Fundamentals, processes and applications of high-permittivity polymer–matrix composites

Organometal halide perovskite solar cells have evolved in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) reached 20.1% late last year. The key disquiet was stability, which has been limiting practical application, but now the state of the art is promising, being measured in thousands of hours. These improvements have been achieved through the application of different materials, interfaces and device architecture optimizations, especially after the investigation of hole conductor free mesoporous devices incorporating carbon electrodes, which promise stable, low cost and easy device fabrication methods. However, this work is still far from complete. There are various issues associated with the degradation of Omh-perovskite, and the interface and device instability which must be addressed to achieve good reproducibility and long lifetimes for Omh-PSCs with high conversion efficiencies. A comprehensive understanding of these issues is required to achieve breakthroughs in stability and practical outdoor applications of Omh-PSCs. For successful small and large scale applications, besides the improvement of the PCE, the stability of Omh-PSCs has to be improved. The causes of failure and associated mechanisms of device degradation, followed by the origins of degradation, approaches to improve stability, and methods and protocols are discussed in detail and form the main focus of this review article.

Organometal halide perovskite solar cells: degradation and stability

This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...

Low loss dielectric materials for LTCC applications: a review

A novel nitrate–citrate gel process was used to synthesize NiCuZn ferrite powders. The thermal decomposition and combustion process of nitrate–citrate gels were investigated by using DTA-TG, IR spectra and XRD techniques. The results revealed that the nitrate–citrate gels exhibit self-propagating behavior after ignition in air at room temperature. NiCuZn ferrite powders with a particle size of 20–50 nm were directly formed after combustion. The ratio of metal nitrates to citric acid in the starting solution affects the combustion process, and then determines the particle size of the as-synthesized powder. The magnetic structure of powders was characterized using Mossbauer spectroscopy.

Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

Synthesis of nanocrystilline ferrites by sol-gel combustion process: the influence of pH value of solution

Ferroelectric characteristics of ytterbium-doped barium zirconium titanate ceramics

Preparation and characterization of sol-gel derived (100)-textured Pb(Zr, Ti)O3 thin films: PbO seeding role in the formation of preferential orientation

The bulk dense nanocrystalline BaTiO3 (BT) ceramics ranging from 20 to 100 nm have been successfully prepared by the spark plasma sintering (SPS) method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometer scale for BT ceramics. The results reveal that the SPS technique provides exceptional opportunity to compact ceramics to full density with nanograin size. It is also demonstrated that all structural modifications in nanocrystalline BT and low-symmetry structures still exist in 20 nm nanograin BT ceramics. The ferroelectric properties of crystalline structures were investigated by scanning force microscopy in piezoresponse mode. Piezoelectric hysteresis loop was recorded, demonstrating that 20 nm BT ceramics has a remanent polarization and is switchable by an electric field. Thus, if a critical grain size exists for ferroelectricity, it is less than 20 nm for polycrystalline BT ceramics.

Zhilun Gui

Papers

Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

Synthesis of nanocrystilline ferrites by sol-gel combustion process: the influence of pH value of solution

Ferroelectric characteristics of ytterbium-doped barium zirconium titanate ceramics

Preparation and characterization of sol-gel derived (100)-textured Pb(Zr, Ti)O3 thin films: PbO seeding role in the formation of preferential orientation

Phase Transitions in Nanocrystalline Barium Titanate Ceramics Prepared by Spark Plasma Sintering