The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near-theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis–processing–property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green-body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an expose of the state of the art in transparent polycrystalline spinel fabrication.

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

Conventional sintering of undoped Y 2 O 3 requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y 2 O 3 was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.

Densification behaviour and microstructural development in undoped yttria prepared by flash-sintering

Commercial Y 2 O 3 powder was used to fabricate highly transparent Y 2 O 3 ceramics with the addition of ZrO 2 via slip casting and vacuum sintering. The effects of ZrO 2 addition on the transparency, grain size and lattice parameter of Y 2 O 3 ceramics were studied. With addition of ZrO 2 the transparency of Y 2 O 3 ceramics increased markedly and the grain size of Y 2 O 3 ceramics decreased markedly by cation diffusivity mechanism and the lattice parameter of Y 2 O 3 ceramics slightly decreased. The highest transmittance (at wavelength 1100 nm) of the 5.0 mol% ZrO 2 –Y 2 O 3 ceramic (1.0 mm thick) sintered at 1860 °C for 8 h reached 81.7%, very close to the theoretical value of Y 2 O 3 .

ZrO2-doped Y2O3 transparent ceramics via slip casting and vacuum sintering

The present invention relates to lightweight high strength microsphere containing ceramic particles having controlled microsphere placement and/or size and microsphere morphology, which produces an improved balance of specific gravity and crush strength such that they can be used in applications such as proppants to prop open subterranean formation fractions. Proppant formulations are further disclosed which use one or more microsphere containing ceramic particles of the present invention. Methods to prop open subterranean formation fractions are further disclosed. In addition, other uses for the microsphere containing ceramic particles of the present invention are further disclosed, as well as methods of making the microsphere containing ceramic particles.

Ceramic particles with controlled pore and/or microsphere placement and/or size and method of making same

Highly transparent Yb3+:Y2O3 ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb3+:Y2O3 ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y2O3. The optical spectroscopic properties of Yb3+:Y2O3 transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Sintering in air of a pure yttria powder was investigated on green samples shaped by slip casting. The “relative density/grain size” trajectory has been drawn and hypotheses concerning the mechanisms controlling grain growth and densification were formulated. Samples were fully densified by an additional hot isostatic pressing step on pre-sintered samples. After optimal polishing, optical properties were measured in the UV, visible, and infrared ranges.

Sintering Behavior and Optical Properties of Yttria

A sintered ceramic particle made from a ceramic material having a true density greater than 3.5 g/cc and a composition having no more than 30 weight percent silicon oxide and at least 15 weight percent iron oxide, based on the combined weight of the oxides of aluminum, iron and silicon, is disclosed. A proces that utilizes an externally applied compressive force to make a ceramic particle is also disclosed.

High strength proppants

Disclosed is a process for producing ceramic particles, such as proppants, that have at least 10 percent total porosity. The process includes forming a particle precursor that includes 5 percent to 30 percent of a first ceramic material and at least 40 percent of a second ceramic material. The sintering temperature of the first ceramic material may be lower than the sintering temperature of a second ceramic material. Heating the precursor to a maximum temperature above the sintering temperature of the first material and below the sintering temperature of the second material. Also disclosed is a ceramic article that has a particular combination of chemistry and alumina crystalline phase.

Ceramic article and process for making the same

A sintered refractory product having the form of a block and consisting—of a granulate formed by all the grains having a size larger than 100 μm, referred to as “coarse grains”, and—a matrix binding said coarse grains and consisting of the grains having a size smaller than or equal to 100 μm, the granulate representing between 45% and 90% by mass of the product, said product having a composition such that, in a mass percentage based on the oxides: —Al2O3>80%, —SiO2<15%, —Na2O<0.15% —Fe2O3<0.05%, —CaO<0.1%,—the other oxides forming the remainder up to 100%, the Na2O content in the matrix being greater than 0.010%, in a mass percentage based on the mass of the product.

Laurie San-Miguel

Papers

Sintering Behavior and Optical Properties of Yttria

High strength proppants

Ceramic article and process for making the same

Product having a high alumina content

Ceramic particle comprising an alumina crystalline phase