Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

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Review: liquid phase sintering

Systematic trends of the 57Fe Mössbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (→ Fe) (where X is O or F and T any element with a formal positive charge)

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

The positive temperature coefficient of resistivity in barium titanate

The grain growth of donor-doped BaTiO3 at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO3 in the presence of a TiO2-rich liquid phase is proposed.

Oxygen Partial Pressure and Grain Growth in Donor-Doped BaTiO3

Microstructural changes occurring during oxidation of the reduced form of donor-doped BaTiO3 (Ba1−XDX.Ti1−X4+TiX3+O3) and during reduction of the oxidized form of donor-doped BaTiO3 (Ba1−XDX.Ti1−X/44+(VTi)X/4O3) were studied using TEM. Samples of both types of solid solutions, containing different La concentrations (from 2 to 20 mol% La), were prepared by sintering under reducing conditions and in air, respectively. The reduced form of donor-doped BaTiO3 was oxidized by annealing at high temperatures (1150° and 1350°C) in air, while the oxidized form was reduced by annealing under reducing conditions. Because of oxidation of the reduced phase of donor-doped BaTiO3, the Ti-rich phases Ba6Ti17O40 and BaLa2Ti4O12 were precipitated. Reduction of the oxidized form caused precipitation of the Ba-rich phase Ba2TiO4 preferentially inside the matrix grains. All precipitates had well-defined orientational relationships with the perovskite matrix.

Microstructural Changes during the Reduction/Reoxidation Process in Donor‐Doped BaTiO3 Ceramics

The release of oxygen during the sintering of Sb2O3-doped BaTiO3 ceramics containing excess TiO2 was measured using a mass spectrometer. The amount of oxygen released is proportional to the dopant concentration in the product phase. The evolution of oxygen during sintering was attributed to dissolution of the oxidized form of doped BaTiO3 in the reacting mixture and simultaneous re-crystallization of the- reduced form.

Release of Oxygen During the Sintering of Doped BaTiO3 Ceramics

A new monoclinic phase in the Fe2O3TiO2 system. I. Structure determination and Mössbauer spectroscopy

A new monoclinic phase in the Fe2O3TiO2 system. II. The defect structure

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Release of Oxygen During the Sintering of Doped BaTiO3 Ceramics

A new monoclinic phase in the Fe2O3TiO2 system. I. Structure determination and Mössbauer spectroscopy

A new monoclinic phase in the Fe2O3TiO2 system. II. The defect structure