Nachum Frage

Composite transparent ceramics offer various advantages over monolithic parts in many photonic applications. Typically, the formation of multicomponent composites requires elaborate processes that involve multiple steps of green body preparation and densification. This study presents a straightforward single‐step fabrication method for composite transparent ceramics using spark plasma sintering. The method utilizes customized compaction tools that enable specific powder arrangements within the graphite die, resulting in a configured green body that can be directly densified. The versatile yet simple process is demonstrated for various composite designs, primarily comprising Nd³⁺:YAG and undoped YAG. The high transparency and precision of these composite ceramics render them suitable for prospective optical components such as high‐power lasers and phosphors. Overall, the proposed method applies not only to multicomponent functional ceramics, but potentially any combination of sinterable materials, with similar densification behavior and thermal expansion coefficients.

Impact response of oriented germanium single crystals and polycrystalline samples obtained by high-pressure spark plasma sintering of pure germanium powder was studied in two series of planar impact tests performed at 300 and 1143 K with samples of different thicknesses and in a series of tests with 2 mm single crystals preheated up to the temperatures 300–1143 K. In all the tests, the samples were shock-loaded by tungsten impactors having velocity 980 ± 40 m/s, while the velocity of the interface between the germanium sample and the fused silica window was continuously monitored by velocity interferometer. Under compression, the cubic diamond (cd) germanium transforms into its high-pressure (β-Sn or liquid) modification. The stress corresponding to the upper bound of the existence of impact loaded cd germanium was found to depart upward from that obtained in the static experiments. At temperatures greater than 900 K, this departure increases due to the initiation of melting in the shock-loaded material. Part of the velocity histories recorded with either single or polycrystalline samples was characterized by a four-wave (instead of the expected three-wave) structure. This “surplus” wave seems to be caused by a short-term existence of an intermediate (nonequilibrium) germanium phase which, however, does not affect the principal germanium Hugoniot.

Additive manufacturing is a novel and breakthrough technology by which parts can be manufactured for various purposes and services. As in any production process, the desired properties of additively manufactured components, particularly in electron beam melting processes, ultimately depend on the manufacturing process parameters. Process parameters should be designed accordingly to manufacture parts with specific and desired characteristics. This study focuses on examining the effect of process parameters, such as beam current and velocity, focus offset, and line offset, at three different values each, on the properties of Ti-6Al-4V alloy. The study on the effect of the process parameters on the as-built material’s performance was performed using the Taguchi approach using an L₉ (3⁴) orthogonal array. The properties of printed parts (density, surface roughness, elastic moduli, hardness, tensile characteristics, fractography, and microstructure) were tested. A wide range of properties was obtained and analyzed; namely, porosity varied from 8% to almost fully dense materials with density higher than 99.9% and a range of yield and ultimate tensile strength values and brittle samples with less than 1% elongation to ductile samples with an elongation greater than 16%. The overall performance of printed parts was determined based on an evaluation criterion. Several parameter combinations were found and yielded the fabrication of parts with high density and relatively fine microstructure. The comparison of the best parameter combinations determined in this study and the parameters recommended by the machine manufacturer showed that improved results were obtained, and even when using the optimal parameters, they can be improved even more. This result highlights the ability of the proposed DOE method to further develop existing results and even for development of manufacturing parameters for new materials.

Spark plasma sintering (SPS) is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various transparent ceramics have been successfully fabricated by SPS, despite the existence of inherent carbon contamination and residual pores. Due to the disk-shape of SPS-processed samples, the technique may be suited for producing thin-disk ceramic laser materials. Nevertheless, an in-depth study of these materials has never been reported. With that goal in mind, the major focus of this study was to characterize the laser performance of Nd:YAG ceramics fabricated by one-stage SPS under conventional (60 MPa) and high (300 MPa) applied pressures. In addition to measuring the lasing slope efficiency and threshold, the passive losses associated with each sample were also evaluated. Surprisingly, it was found that in-line transmittance spectra do not provide accurate predictions of laser performance due to the nature of residual porosity. Moreover, homogeneity and beam quality were assessed, and comparisons were drawn between conventional and high-pressure SPS ceramics. This study lays the groundwork for the future of laser materials fabricated by SPS or similar pressure-assisted techniques.

Creep is a time dependent, temperature-sensitive mechanical response of a material in the form of continuous deformation under constant load or stress. To study the creep properties of a given material, the load/stress and temperature must be controlled while measuring strain over time. The present study describes how a spark plasma sintering (SPS) apparatus can be used as a precise tool for measuring compressive creep of materials. Several examples for using the SPS apparatus for high-temperature compressive creep studies of metals and ceramics under a constant load are discussed. Experimental results are in a good agreement with data reported in literature, which verifies that the SPS apparatus can serve as a tool for measuring compressive creep strain of materials.