Suppression of carbon contamination in SPSed CaF2 transparent ceramics by Mo foil

Transparent calcium fluoride (CaF₂) ceramics were fabricated by spark plasma sintering (SPS) using Mo foil to separate from graphite die, and the effect of Mo foil on transparency, microstructure and carbon contamination was investigated. The discoloration was removed and the transmittance increased from 8% to 54% at the wavelength of 300 nm and 63% to 86% at 1100 nm by using Mo foil. The average grain size of CaF₂ ceramics SPSed at 1100 °C was 16 μm by using graphite sheet, whereas it was 260 μm by using Mo foil, indicating that ceramics with large grain size are possible to reach high transmittance. Raman spectroscopy and X-ray photoelectron spectroscopy revealed that carbon contamination in the CaF₂ ceramics using Mo foil decreased 78% than that shielded by graphite sheet.     

Influence of spark plasma sintering conditions on microstructure,carbon contamination,and transmittance of CaF2 ceramics

Commercial CaF₂ powder was applied to fabricate transparent CaF₂ ceramics by spark plasma sintering under various sintering conditions. The low sintering temperatures and high pressures caused serious carbon contamination, while the soak time had less influence on the carbon concentration in the ceramics. The highest carbon contamination occurred to the CaF₂ ceramics sintered at 800 °C. A low sintering pressure suppressed carbon contamination but led to high porosity and large pore size. A high pre-loading pressure led to relatively high porosity and carbon concentration. Furthermore, the relatively fast densification in the edge region of the plates may cause the non-uniform distribution of porosity, thereby affecting the distribution of carbon concentration. The low pre-loading pressure and the high sintering pressure reduced porosity and carbon concentration to obtain dense transparent ceramic with uniform microstructure and high transmittance.     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Conventional HP sintering of asymmetric hexagonal structure Yb3+-doped Sr5(PO4)3F transparent ceramic without additives

The 2 at.% Yb³⁺:Sr₅(PO₄)₃F (S-FAP) polycrystalline transparent ceramic with asymmetric hexagonal structures has been synthesized by vacuum hot-pressing the nanoparticles prepared via coprecipitation method. X-ray diffraction results of powder and ceramic indicate that their phase peaks are well matched to the crystal structure of S-FAP. The average particle size of 35.5 nm has been exhibited by powder scanning electron microscopy images, and subsequent images of the ceramic cross section and surface morphology revealed a homogenous and compact microstructure with an average grain size of around 220 nm. The relationship between the optical loss caused by the scattering of anisotropic ceramic grains and the optical transmittance of ceramics was revealed in the hexagonal S-FAP transparent ceramics with different thicknesses. The in-line transmittance of hot-pressed ceramics with 1.5-mm thickness achieved 79.95% at 1100-nm wavelength, and the room-temperature absorption and emission spectra of Yb³⁺ in S-FAP polycrystalline ceramic matrix were measured using a spectrofluorometer.     

Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er³⁺-doped CaF₂–silica glass ceramics were prepared by the direct physical introduction of Er³⁺ doped CaF₂ nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er³⁺ doped CaF₂ nanocrystals in the silica sols. The CaF₂–silica sol mixture went through gelation and heat-treatment to form Er³⁺-doped CaF₂–silica glass ceramics. The morphology of Er³⁺ doped CaF₂ in silica glass did not change after heat-treatment at 600 °C for 10 h. The experimental results showed that Er³⁺ doped CaF₂ in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er³⁺:⁴F_9/2→⁴I_15/2 transition and long lifetime of ⁴F_9/2 level (1.73 ms) under 980 nm excitation.     

Translucent Yttria‐ and Silica‐Doped Mullite Ceramics with Anisotropic Grains Produced by Spark Plasma Sintering

Translucent, high‐performance, mullite ceramics with anisotropic grains were prepared by the spark plasma sintering (SPS) of a powder mixture consisting of commercial mullite powder, which already contained small amounts of alumina (θ and α) and silica (cristobalite) (≤3 wt% in total), to which 2 and 1 wt% of yttria and amorphous silica was admixed, respectively. The combination of low‐viscosity Y₂O₃–Al₂O₃–SiO₂ transient liquid formation and SPS sintering provided enhanced densification, also provoking anisotropic grain growth (which became exaggerated after 20 min of SPS dwell time), at a relatively low sintering temperature of 1370°C. In this way, it was possible to meet the conflicting demands for obtaining a dense mullite ceramic with anisotropic grains, ensuring good mechanical properties, while preserving a noticeable light transmittance. In terms of mechanical and optical properties, the best results were obtained when SPS dwell times of 5 and 10 min were employed. The as‐sintered samples possessed densities in the range 3.16–3.18 g/cm³, anisotropic grains with an aspect ratio (AR) of 7 and a grain thickness of approximately 0.45 μm, a flexural strength between 350 and 420 MPa, a Vickers indentation toughness and a hardness of approximately 2.45 MPa·m^1/2 and 15 GPa, respectively, and an optical transmittance of between 30% and almost 50% in the IR range.     

Effect of sintering temperature on the microstructure and transparency of Nd,Y:CaF2 ceramics

1 at% Nd, 3 at% Y doped CaF₂ transparent ceramics were obtained by hot pressing at the sintering temperature varing from 500 to 800 °C under vacuum environment with co-precipitated CaF₂ nanopowders. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grain size around 26 nm. Scanning electron microscopy (SEM) observations of the Nd, Y: CaF₂ ceramics indicated that the mean grain size of the ceramic sintered at 800 °C was about 748 nm. The influence of the temperature on the grain size, microstructure and optical transmittance was investigated. For the ceramic sintered at 800 °C, the transmittance was 85.49% at the wavelength of 1200 nm. The room temperature emission spectra of Nd: CaF₂ and Nd, Y: CaF₂ ceramics were measured and discussed.     

New KNbTeO6 transparent tellurate ceramics

New transparent defect pyrochlore KNbTeO₆ ceramics were successfully prepared by Spark Plasma Sintering (SPS) of same composition polycrystalline powders elaborated by classic solid-state reaction from oxide precursors (K₂CO₃, Nb₂O₅, TeO₂) and followed by high energy milling powders. As such precursors are not available as commercial nanopowders, a suitable process has been developed by combining solid-state reactions and high energy milling. The determination of appropriate consolidation conditions and sintering parameters of the green body such prepared, are described in this paper. The resulting ceramic is transparent in both the visible and near infrared range (up to 5.5 μm). The maximum of transmittance is reached in the near infrared region around 2500 nm with a value of 78 % (1 mm thick sample), close to the maximum theoretical value of transmittance. This transparent KNbTeO₆ ceramic demonstrates a homogeneous and dense microstructure with an average grain size less than 500 nm. A small content of secondary phase has been detected by nanoscale observations without drastic effects on transparency. This ceramic exhibits very good mechanical properties similar to the Y₂O₃ transparent ceramic, as well as interesting dielectric properties in the microwave range. This innovative method should drive the development of new transparent materials with technologically relevant applications.     

Influence of Yb Concentration on the Optical Properties of CaF2 Transparent Ceramics Codoped with Er and Yb

Er, Yb:CaF₂ nanoparticles with different Yb concentrations were synthesized by a coprecipitation method using nitrates as raw materials. X‐ray powder diffraction and transmission electron microscopy analysis showed that the nanoparticles were single fluorite phase and the nanoparticle size was found to decrease with increasing Yb concentrations. The obtained nanoparticles were hot‐pressed at 800°C under 30 MPa under vacuum environment to fabricate Er, Yb:CaF₂ transparent ceramics. The influence of Yb ion concentrations on the optical transmission, microstructure, and luminescence properties of Er, Yb:CaF₂ transparent ceramics were investigated. The addition of Yb ions was found effectively to reduce grain size and has a positive effect on improving the optical transmission of Er, Yb:CaF₂ transparent ceramics. The highest transmittance in the near‐infrared spectral region of the Er, Yb:CaF₂ transparent ceramic reached about 90%. The green, red, and near‐infrared emission intensities were found to increase with increasing Yb concentration.     

Fabrication of LaGdZr2O7 transparent ceramic

A simple combustion method was used to synthesize LaGdZr₂O₇ powder and LaGdZr₂O₇ transparent ceramic was prepared by vacuum sintering at 1850 °C for 6 h. The final transparent ceramic, with a density of 6.46 g/cm³, has an in-line transmittance of 70.7% at 1000 nm and a refractive index of 2.08 at 632.8 nm.     

Scalable and tunable Y2O3-MgO composite for infrared transparency applications

The process-structure-property correlationships in yttria-magnesia (YM) composite have been investigated. YM composite was synthesized using commercial powders via ball-milling route with three different grinding balls (Si₃N₄, Al₂O₃, ZrO₂) having two different sizes (2 and 5 mm diameter). The alteration in grinding ball material and size produces sintered ceramic having different grain sizes (420–560 nm) and degree of phase mixing homogeneity (0.40–0.70). The contamination induced by the milling ball resulted in changes in Y₂O₃ and MgO defect chemistry, which influenced the grain growth behavior in the YM composite. The hot-pressed composite prepared using 2-mm Si₃N₄ ball-milled powders exhibited the finest grain size (420 nm) and better phase mixing homogeneity (0.63). The subsequent impact was seen on transmittance efficiency (71%) over the 3–7-μm wavelength range, which is ∼85% of the theoretical limit. The findings show that the selection of the right size and type of grinding ball for milling commercial powder is a simple and cost-effective way for scalable production of YM composite with high transmittance efficiency for infrared windows and dome applications.     

Pressureless sintering of highly transparent aluminum oxynitride ceramic by precipitation-coating with sintering aid

AlON transparent ceramic is an ideal material for photoelectric windows, domes, and transparent armor. This ceramic is commonly fabricated by ball-mill mixing of AlON powder with appropriate sintering aids; however, it is difficult to homogeneously disperse trace sintering aids using this method and contamination is easily introduced, which may generate pores and flaws in the ceramic, resulting in degraded properties. To address this problem, we adopted a wet-chemical method to precipitation-coat sintering aids on the surface of AlON powder to achieve homogeneous distribution and enable nano-size sintering aids. Pressureless sintered AlON with 0.5 mass% Y₂O₃-coating gave in-line transmittance of 83.5% at a wavelength of 1064 nm and flexural strength of 326.3 MPa; the corresponding values for the ball-milled product were 79.3% and 304.6 MPa. Our results demonstrate that this method of sintering-aid coating has potential to fabricate AlON ceramic with excellent performance.     

High-entropy transparent fluoride laser ceramics

For the first time, a transparent high-entropy fluoride laser ceramic has been prepared and characterized. X-ray diffraction (XRD) analysis of a CeNdCaSrBaF₁₂ (CNCSBF) transparent ceramic consolidated by vacuum hot pressing (VHP) reveals that Ce³⁺, Nd³⁺, Ca²⁺, Sr²⁺, and Ba²⁺ have formed a single-phased fluorite solid solution, with a lattice constant of 5.826 Å. Bulk density measurements produced a value of 6.15 g/cm³. Scanning electron microscopy (SEM) analysis of the ceramic revealed a uniform distribution of grain sizes in the material, with the average grain size being approximately 20 μm. The material exhibits a maximum in-line transmittance of approximately 60% at 1000 nm. A near-infrared range photoluminescence (PL) emission band was observed at 1057 nm, with a visible-range PL emission band being located at 440 nm.     

Effects of Nd Concentration on Microstructure and Optical Properties of Nd: CaF2 Transparent Ceramics

Highly transparent Nd‐doped calcium fluoride (Nd: CaF₂) ceramics with different Nd‐doped concentrations were fabricated by hot‐pressed method using Nd: CaF₂ nanopowders synthesized by coprecipitation method. SEM observations indicated that the average grain size of nanopowders was about 16–30 nm and the average grain size of the ceramics was between 200 nm and 1 μm. The grain boundaries of the ceramics were clean and no pores or impurities were detected. For 2‐mm‐thickness sample, the transmittance of the as‐fabricated 5 at.% Nd: CaF₂ ceramic at 1200 nm was about 85%. The absorption coefficient and emission intensity of the Nd: CaF₂ ceramics were measured and discussed. From the Nd: CaF₂ ceramics fluorescent spectra and the decay curves, it was found that the fluorescent quenching effect became more evident with the increase in the Nd³⁺ ions‐doped concentration.     

Fabrication and microstructure characterizations of transparent Er:CaF2 composite ceramic

A novel layered transparent Er:CaF₂ composite ceramic was proposed in the present study. Er:CaF₂ nanoparticles were synthesized by a chemical coprecipitation method. The crystal structures and morphologies of synthesized nanoparticles were performed by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) measurements, respectively. Transparent composite ceramic was fabricated by the combination of multistep dry pressing and hot-pressed sintering method without any sintering aids or binders. The average grain size of 2% Er-doped and 5% Er-doped layers were about 30 and 55 μm, respectively. The thickness of interfacial between two different Er-doped layers was 150-200 μm. For a 1.5 mm thickness transparent Er:CaF₂ composite ceramic, the optical transmittance reached 44.9% at 500 nm and 53.6% at 1200 nm. The luminescence spectra and thermal conductivities of transparent ceramic specimens were also discussed.     

Energy transfer and controllable colors of upconversion emission in Er3+ and Dy3+ co-doped CaF2 transparent ceramics

x at. % Er³⁺, 3 at. % Dy³⁺: CaF₂ transparent ceramics (x=1-5) with good transparency were fabricated by hot-pressed sintering. The phase composition of nanoparticles and transparent ceramics, microstructure, in-line transmittance, upconversion spectra and lifetime of transparent ceramics, as well as energy transfer mechanism between Er³⁺ and Dy³⁺ were investigated. The mean grain sizes of nanoparticles decreased from 33.0 nm to 26.2 nm with the Er³⁺ doping concentration increasing from 1 to 5 at.%. The microstructure of ceramic samples presented nearly dense microstructure and EDS analysis indicated Er³⁺ and Dy³⁺ were uniformly incorporated into CaF₂ lattice. Under 900 nm excitation, the emission intensity for ⁴F_9/2→⁶H_15/2 transition of Dy³⁺ decreased and for ⁴S_3/2→⁴I_15/2 transition of Er³⁺ increased, the lifetime for the ⁴F_9/2 level of Dy³⁺ decreased while the ⁴F_7/2 level of Er³⁺ increased with the raise of Er³⁺ doping concentration. The energy transfer mechanism was proved to be the dipole-dipole interaction. The upconversion luminescence color was tuned from orange through yellow to green by changing the Er³⁺/Dy³⁺ ratio. In addition, the Vickers hardness, fracture toughness, and the thermal conductivity of Er³⁺, Dy³⁺: CaF₂ transparent ceramics were discussed. All the results showed the Dy³⁺ could be used as a sensitizer for Er³⁺: CaF₂ transparent ceramic in the upconversion field.     

Fine-grained transparent MgAl2O4 spinel obtained by spark plasma sintering of commercially available nanopowders

A high transmittance/small grain size combination for pure spinel ceramics from commercially available nanopowders without sintering aids can be obtained by SPS sintering. By using a low heating rate ≤10 °C/min and a sintering temperature ≤1300 °C, a transparent polycrystalline MgAl₂O₄ spinel was fabricated by SPS with an in-line transmission of 74% and 84% for 550 nm (visible) and 2000 nm (NIR) wavelengths respectively. A small average grain size of about 250 nm was obtained and the pores located at the multiple grain junctions have a mean size of about 20 nm. The high in-line transmission is linked not only to the low residual porosity but particularly to the very small size of pores.     

Preparation of nano - coating powder CaF2@Al(OH)3 and its application in Al2O3/Ti(C,N) self-lubricating ceramic tool materials

Nano CaF₂ particles of different sizes were prepared by direct precipitation. The diameters of nano CaF₂ particles prepared in mixed solvent can reach 5–7 nm, and can be effectively dispersed. The surface of nano CaF₂ was modified and coated by heterogeneous nucleation method. A shell layer of Al(OH)₃ was coated on the surface of nano CaF₂, and the structure and coating mechanism of the coated powder were analyzed. Under varying preparation conditions, the surface morphology of CaF₂@Al(OH)₃ was analyzed using TEM and SEM. The results showed that the coating powder showed good dispersion in mixed solvents, and the particle size of the composite powder was about 20 nm. Self-lubricating ceramic tool materials were prepared by adding coating particles to the Al₂O₃/Ti(C,N) matrix. The coating powder shell and the matrix material melt during sintering, so that CaF₂ forms nanostructures in the particles. thereby improving the mechanical properties of the material. Cutting experiments show that the addition of coating particles can effectively reduce the temperature, cutting force and friction coefficient in the cutting process of the tool, thus improving the cutting performance of the tool material.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.