Doped transparent ceramics have potential for a wide range of optical applications derived from their easily controllable fluorescence properties. The aim of this contribution is to present and discuss an effective fabrication method for doped transparent yttrium aluminum garnet ceramics. The powder synthesis route was adapted from common co-precipitation methods and the powder densification was performed by spark plasma sintering. Parameters of the fabrication process were optimized for different lanthanide dopants in order to obtain highly transparent single- and multi-doped ceramics. The photoluminescence properties of the samples were measured and discussed. The use of lithium fluoride as a sintering additive was confirmed to be favorable and post-sintering heat treatments were shown to be necessary for certain dopants. Finally, the photoluminescence lifetime of dopants in a multi-doped sample were measured and the energy transfer efficiency between dopants was determined.

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 degrees C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 degrees C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 degrees C under 600 MPa.

Polycrystalline magnesium aluminate (MgAl2 O4 ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for lithography, electroinsulation, and structural windows for both VIS and IR region radiation and armor applications. Conventional two-stage processing of PMAS involves prolonged pressureless sintering followed by hot isostatic pressing. The costly processing of high-quality transparent PMAS ceramic is the main reason for the limited usage of this material in industry. Spark plasma sintering (SPS) is a relatively novel one-stage, rapid, and cost-effective sintering technique, which holds great potential for producing high-quality optical materials. Here, recent advances in the fabrication of transparent PMAS by the SPS approach, the influence of sintering parameters on microstructure evolution during densification, and their effects on the optical and mechanical properties of the material are reviewed.

High‐pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min and holding time of 60 min displayed submicrometer microstructure and elevated mechanical properties, including resistance to thermal shock. Optical properties (i.e., spectral transmittance, fluorescence emission spectra and fluorescence lifetime) of the HPSPS‐processed specimens were close to those obtained with specimens fabricated by conventional sintering procedure. Specifically, remarkable differences in threshold power and laser slope efficiency were found and attributed to the variance in Nd concentration in the specimens tested. The results of this study indicate that the low cost and timesaving HPSPS process allows for the fabrication of polycrystalline Nd:YAG specimens with optical properties suitable for laser applications.

TiB2–TiB composites with a TiB2 content that varied between 100 and 36 wt% were synthesized from TiB2 and Ti powder mixtures by the Spark Plasma Sintering (SPS) technique. The residual porosity, microstructure, phase composition and static mechanical properties of the composites (bending strength, hardness and fracture toughness) were determined using standard characterization techniques. The impact response of the composites has been studied in two series of planar impact experiments. In the first, the samples were shock loaded to an ultimate compressive stress of about 25 GPa in order to determine the materials Hugoniot elastic limit (HEL). In the second series, the composites were shock loaded by a modest, 3–4 GPa, compressive pulse with the purpose to define their spall strength.

Reaction-bonded boron carbide was manufactured by infiltrating porous boron carbide preforms at 1273K with a Mg-Si eutectic alloy. The resulting composite material consists, in addition to the original B4C, of SiC, Mg2Si, and a Mg-rich complex boride/carbide Mg-x(Al,Si)(y)(B,C)(z) phase. The composites display high hardness (1700HV), Young's modulus (356MPa) and a moderate bending strength (230MPa). The ballistic efficiency (of about 6.7), as determined by the depth of penetration method, is much higher than that of alumina and similar to that of silicon-infiltrated reaction-bonded composites.

The study deals with the effect of the SPS parameters and LiF doping on the mechanical and optical properties polycrystalline magnesium aluminate spinel (PMAS) with emphasis on the grain size of the final product. Sintering at 1300°C of undoped powder yielded fully dense submicrometer (0.4–0.6 μm) samples with elevated mechanical properties (1600HV and 300MPa bending strength). Doped samples had a larger, 40 μm grain size, lower, 1450HV, hardness and 150MPa bending strength. The transmittance of the doped samples (80% at 500 nm wavelength) was higher than that of the undoped ones. Thus, the required functionality of the ceramic dictates the choice of parameters for the fabrication of dense transparent PMAS.

The present study deals with a comprehensive comparison of the mechanical and functional properties of Nd:YAG single crystals with those of the polycrystalline ceramics (PCs), undoped and LiF-doped, processed by Spark Plasma Sintering (SPS). The polycrystalline ceramics have higher mechanical properties (hardness, bending strength and thermal shock resistance) than the single crystals. The optical transmittance of the LiF-doped PC Nd:YAG is significantly higher than that of the undoped one and is close to that of the single crystal. With respect to other optical and thermal properties, i.e. refraction index, absorption coefficient, extinction ratio, thermo-optic coefficient, fluorescence and thermal conductivity, no significant differences were observed between the single crystals and the polycrystalline ceramic. (C) 2014 Elsevier B.V. All rights reserved.

Fully dense submicron grain size alumina samples were manufactured from alumina nano-powder using Spark Plasma Sintering and tested in two kinds of VISAR-instrumented planar impact tests. In the first kind, samples were loaded by 1-mm tungsten impactors, accelerated to a velocity of about 1 km/s. These tests were aimed at studying the Hugoniot elastic limit (HEL) of Spark Plasma Sintering (SPS)-processed alumina and the decay, with propagation distance, of the elastic precursor wave. In the tests of the second kind, alumina samples of 3-mm thickness were loaded by 1-mm copper impactors accelerated to 100–1000 m/s. These tests were aimed at studying the dynamic tensile (spall) strength of the alumina specimens. The tensile fracture of the un-alloyed alumina shows a monotonic decline of the spall strength with the amplitude of the loading stress pulse. Analysis of the decay of the elastic precursor wave allowed determining the rate of the irreversible (inelastic) strains in the SPS-processed alumina at the initial stages of the shock-induced inelastic deformation and to clarify the mechanisms responsible for the deformation. The 1-% addition of Cr2O3 decreases the HEL of the SPS-processed alumina by 5-% and its spall strength by 50% but barely affects its static properties.

ABSTRACTThe present paper is concerned with the fabrication of fully dense B4C‐Mg, B4C‐AZ91 alloy, and B4C‐Mg,Si (eutectic) composites at a significantly lower (750–1000 °C) temperature range. The key feature of the novel approach is based on the Mg‐vapor atmosphere, under which the infiltration process is carried out. This approach has also been used to fabricate B4C‐Al composites at a low temperature. The microstructure and the mechanical properties of the composites are described and discussed. It was concluded that the formation of the deleterious aluminum carbide (Al4C3) depends on the presence of free carbon in the boron carbide powders. The suggested approach for low temperature fabrication of reaction bonded composites under Mg vapor can be further expanded to other boride and carbide phases.

Fully dense alumina samples with 0.6 μm grain size were produced from alumina powder using Spark Plasma Sintering and tested in two types of VISAR-instrumented planar impact tests. In the tests of the first type the samples of 0.28 to 6-mm thickness were loaded by 1-mm tungsten impactors accelerated up to a velocity of about 1 km/s. These tests were aimed to study the Hugoniot elastic limit (HEL) of the SPS-processed alumina and the decay of the elastic precursor wave with propagation distance. In the second type of test the samples of ~3-mm thickness were loaded by 1-mm copper impactors accelerated up to velocities 100-1000 m/s. These tests were aimed to study the dynamic tensile (spall) strength of the alumina. The data on tensile fracture of the alumina demonstrate a monotonic decline of the spall strength with the amplitude of the loading stress pulse. The data on the decay of the elastic precursor wave allows for determining the rates of the irreversible (inelastic) strains in the SPS-processed alumina at the initial stages of shock-induced inelastic deformation and, thus, to derive some conclusions concerning the mechanisms responsible of the deformation.

A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200°C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200°C at a heating rate of 5°/min, a holding time of 15min and an applied pressure of 350–400MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).

The fully dense composites were obtained by vacuum infiltrating boron carbide compacts (80% green density) with molten AZ91 magnesium alloy (850°C) and with the melt of a 50/50 AZ91- silicon mixture (1050°C). The densities composites were, 2.44 g/cm3 and 2.54 g/cm3, respectively. The impact response of the composites was studied in a series of VISAR -instrumented planar impact experiments with velocities of W and Cu impactors ranging from 100 to 1000 m/s. The velocity history recorded for the composites produced by infiltration with the Mg-Si alloy contains a distinct elastic precursor front followed by a plastic ramp. In contrast, the velocity history of the composite infiltrated with AZ91 does not display any step-like front; the amplitude of the elastic wave grows gradually from zero level and transforms smoothly into the plastic front. The influence of the composites microstructure on their compressive and tensile behavior is discussed.

Composites with a high fraction of the ceramic phase were fabricated by infiltration of 80% dense preforms with liquid Mg or Mg alloys, under an Mg vapor atmosphere. The infiltration was performed at 1123K in a semi-hermetically closed container, from which gas had been evacuated. The Mg vapor atmosphere was achieved by heating subsequently to the relatively high temperature of the Mg infiltration process. The specific weight of the composites is about 2.44g/cm3. The microstructure of the composite consists of the newly formed MgB2 and ternary carbide MgB2C2 phases that connect the initial B4C particles and some residual metal. A thermodynamic analysis of the interaction between B4C and liquid Mg was conducted and its results are in good agreement with the experimental observations. The mechanical properties of the composites were investigated and discussed.

The Pb0.25Sn0.25Ge0.5Te compound is a promising p-type thermoelectric material. It is single phased at elevated temperature and undergoes upon cooling an allotropic transformation from the cubic Geβ, to the rhombohedral Geα structure. In addition, a phase separation takes place in the course of aging treatments within the miscibility gap at lower temperature. The phase separation in this pseudo-ternary system takes place by a discontinuous precipitation process, giving rise to a heterogeneously nucleated coupled structure of Pb- and Ge-rich lamellae, with 102nm to 2μm wide spacing. The presence of Sn atoms in the structure tends to suppress the spinodal decomposition and is characteristic of the low Sn content compounds. The phase separation by discontinuous precipitation follows the Johnson–Mehl–Avrami kinetics. In the course of further lengthy aging treatments at 390°C, the microstructure consists of spheroidized Ge-rich precipitates in a Pb-rich telluride matrix. The thermoelectric transport properties undergo significant changes in the initial stages of the phase separation process, leading to a stable and relatively elevated figure of merit, ZT=0.95±0.07 at 400°C.

Transparent 1at.% Nd:YAG ceramics were fabricated by spark plasma sintering (SPS) from nanometric Nd:YAG powders, both undoped and pre-mixed with 0.25wt.% LiF additive. The mechanical and optical properties of the consolidated samples were determined as a function of the processing parameters, namely holding time, peak sintering temperature and heating rate. The presence of LiF accelerates densification and grain growth. Hardness and bending strength are decreased in the presence of the LiF additive, in consistence with the increase of the grain size. The optical transmittance in the doped samples sintered at 1400°C, reaches 97% of the theoretical transmission and is significantly higher than that of the undoped samples. The increased optical transmittance of the doped samples is attributed to pore elimination by enhanced mass transport and cleansing of the carbon contamination by the fluorine component of the LiF additive. The presence of the latter has no effect on the absorption spectrum of the Nd:YAG ceramic.

Reaction bonded boron carbide composites are among the hardest ceramics with low specific gravity values, a combination that makes them eminently suitable for light armour applications. The present review covers the main issues involved in the reaction bonding process based on the infiltration of boron carbide preforms with molten silicon. The importance of achieving high green density values before infiltration is emphasised. Separate sections deal with the morphology of the composite material, the thermodynamics of its microstructure formation, the static mechanical properties and the dynamic mechanical properties at strain rates up to 10(5) s(-1). All along the review, emphasis is placed on describing the effect of free carbon presence in the initial preform on the morphology and properties of the final composite.

The solubility of boron in the molten alloy and the interaction of silicon dissolved in the melt with the boron carbide substrate determine the interface structure and the wetting behavior in the B4C/(Me-Si) systems. The equilibrium contact angle at the B4C/(CU-Si) interface is affected by the composition of the near-surface layer of boron carbide that has shifted to a higher boron content. Wetting in the B4C/(Au-Si) and B4C/(Sn-Si) systems reflect the formation of a SiC interlayer, and the wetting behavior of the SiC/(Au-Si) and SiC/(Sn-Si) systems.

The unique combination of SiC properties opens the ways for a wide range of SiC-based industrial applications. Dense silicon carbide bodies (3.18±0.01g/cm3) were obtained by an SPS treatment at 2050°C for 10min using a heating rate of 400°C/min, under an applied pressure of 69MPa. The microstructure consists of fine, equiaxed grains with an average grain size of 1.29±0.65μm. TEM analysis showed the presence of nano-size particles at the grain boundaries and at the triple-junctions, formed mainly from the impurities present in the starting silicon carbide powder. The HRTEM examination revealed high angle and clean grain boundaries. The measured static mechanical properties (HV=32GPa, E=440GPa, σb=490MPa and KC 6.8MPam0.5) and the Hugoniot Elastic Limit (HEL=18GPa) are higher than those of hot-pressed silicon carbide samples.