Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

Thermochemical model on the carbothermal reduction of oxides during spark plasma sintering of zirconium diboride

Carbon was used to reduce oxides in spark plasma sintered ZrB₂ ultra-high temperature ceramics. A thermodynamic model was used to evaluate the reducing reactions to remove B₂O₃ and ZrO₂ from the powder. Powder oxygen content was measured and carbon additions of 0.5 and 0.75 wt% were used. A C–ZrO₂ pseudo binary diagram, ZrO₂–B₂O₃–C pseudo ternaries, and Zr–C–O potential phase diagrams were generated to show how the reactions can be related to an open system experiment in the tube furnace. Scanning transmission electron microscopy identified impurity phases composed of amorphous Zr–B–O with lamellar BN and a Zr–C–O ternary model was calculated under SPS sintering conditions at 1900°C and 6 Pa to understand how oxides can be retained in the microstructure.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Ultraviolet emission transparent Gd:YAG ceramics processed by solid-state reaction spark plasma sintering

Transparent 1% Gd-doped YAG and YAG ceramics were synthesized via solid-state reaction spark plasma sintering using commercially available powder and TESO as a sintering additive. The highest in-line transmission values achieved were 77.1% at 550 nm and 80.6% at 800 nm in the 1% (at.%) Gd-doped YAG transparent ceramic with 99.90% relative density. Ultraviolet emission at 312.5 nm was observed in 1% Gd-doped YAG ceramic via photoluminescence excitation, making it a promising material for applications in solid-state UV devices.     

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al₂O₃ and AlN phases during heating. These α-Al₂O₃ and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al₂O₃ and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. This effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

Fabrication of translucent AlN ceramics with MgF2 additive by spark plasma sintering

Translucent AlN ceramics with 0‐2 wt.% MgF₂ additive were prepared by spark plasma sintering. AlN powder was heated temporarily up to 2000°C, before holding at 1850°C for 20 minutes in N₂ gas. The sintered ceramics consisted of a single phase of hexagonal AlN, and showed a transgranular fracture mode. The total transmittance was improved remarkably by the additive, to reach 74% at a wavelength of 800 nm for 1 wt.% MgF₂. For 2 wt.% MgF₂, the transmittance was slightly lower than that for 1 wt.% MgF₂, and an absorption band was observed apparently at around 400 nm. The addition of MgF₂ along with the temporary heating at higher temperatures than the sintering temperature contributed to improve the transmittance remarkably.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

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.     

Defect dipoles induced high-energy storage density in Mn-doped BST ceramics prepared by spark plasma sintering

Ba_0.5Sr_0.5TiO₃ ceramics with different Mn-doping amount (Ba_0.5Sr_0.5Ti_1-xMn_xO₃, x = 0, 0.1%, 0.3%, 0.5%) were prepared by spark plasma sintering method. The single phase with cubic structure symmetry was confirmed and a gradual increase in lattice parameter with increasing x was observed. Fine grains with dense microstructure were revealed from the SEM images, while an obvious increase in grain size was detected when x = 0.5%. An optimized doping amount of 0.3% was determined, showing high dielectric constant (ε_r ≈ 2190), low dielectric loss (tanδ ≈ 2.78 × 10⁻³), enhanced breakdown strength (290 kV/cm), and high-energy storage density (1.69 J/cm³) at room temperature. A possible mechanism, namely defect dipoles formation mechanism, was employed to explain the optimization of energy storage performance, and further confirmed from the variation in AC conductivity.     

Transparent Er2O3 ceramics fabricated by high-pressure spark plasma sintering

Fabrication of transparent Er₂O₃ ceramics was carried out by high-pressure spark plasma sintering (HP-SPS). The color and in-line transmittance of these ceramics was highly sensitive to the sintering parameters. Samples exhibited a strong pink or wine color after sintering at 1150 °C under 600 MPa or 1250 °C under 250 MPa, respectively. This was confirmed to be a result of oxygen vacancies created during the sintering process and high sensitivity of Er₂O₃ to the strong reducing atmosphere in the SPS apparatus. Post-sintering annealing in an air furnace led to elimination of oxygen vacancies and increased transparency. Additionally, the photoluminescence intensity and phosphorescence lifetime of annealed (pink) samples was higher and shorter, respectively, compared to that of the reduced (wine-colored) samples.     

Processing and properties of Bi0.98R0.02FeO3 (R = La,Sm, Y) ceramics flash sintered at ~650°C in <5 s

We show that flash sintering produces single-phase, nanograin-sized polycrystals of isovalent-substituted multiferroic ceramics of complex compositions. Single-phase polycrystals of Bi_0.98R_0.02FeO₃ (R = La, Sm, Y) were produced at a furnace temperature of ~650°C in a few seconds by the application of an electric field of 50 V cm⁻¹, with the current limit set to 40 mA mm⁻². The dielectric and insulating properties compared favorably with expected values. Impedance spectroscopy suggests electrically homogenous microstructure, except for the sample Bi_0.98La_0.02FeO₃ that shows a small grain boundary contribution to the impedance. These results reinforce the enabling nature of flash sintering for ceramics which pose difficulties in conventional sintering because they contain low melting constituents or develop secondary phases during the sintering protocol.     

Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

Synthesis of dense NiZn ferrites by spark plasma sintering

Dense NiZn ferrites were fabricated by spark plasma sintering (SPS) at 900 °C and 20 MPa in short periods. The powder was densified to 98% of the theoretical density by the SPS process. The SPS disks exhibited a higher saturation magnetization (Ms), up to 272 emu/cm³, than did the disks sintered by the conventional process. A higher coercivity (Hci) was obtained when the green bodies were sintered by the SPS process for 5 min. A modest holding time is essential to obtain fine grain and uniformity in the SPS process. Secondary crystallization, inhomogeneous microstructure and intragranular pores were found as a result of the rapid sintering and relatively longer holding time in the SPS process. Infrared (IR) spectra were also measured in the range from 350 to 700 cm⁻¹ to study the efforts of the SPS process on NiZn ferrites.     

Sintering behavior and mechanical properties of spark plasma sintering SiO2-MgO ceramics

SiO₂-MgO ceramics containing different weight fractions (0, 0.5, 1, 2, and 4 wt%) of SiO₂ powder were prepared by mixing nano MgO powder, and the powder mixtures were densified by spark plasma sintering (SPS). The effect of SiO₂ addition and SPS method on the sintering behavior, microstructure and mechanical properties were investigated. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The highest relative density, flexural strength and hardness of 2 wt% SiO₂-MgO ceramics reached 99.98%, 253.99 ± 7.47 MPa and 7.56 ± 0.21 GPa when sintered at 1400 °C by SPS, respectively. The observed improvement in the sintering behavior and mechanical properties are mainly attributed to grain boundary "strengthening" and intragranular "weakening" of the MgO matrix. Furthermore, the spark plasma sintering temperature could be decreased by more than 100 °C as compared with the HP method, SPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Onset of selective laser flash sintering of AlN

Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria-stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450–670 K in 2–0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33–300 m/s, laser powers of 10–30 W, and an applied electric field of 3000 V/cm.     

Microstructure and properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering

In this study, WC-8Co cemented carbides were prepared by spark plasma sintering. When the samples sintered at 1300℃ were cooled to room temperature, the samples were sintered multiple times at 1250℃. The changes in microstructure and mechanical properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering were studied. The hardness of cemented carbides increased in the first two sintering, reaching 16.5 GPa. However, the hardness decreased seriously in the last two sintering. The attenuation rates of hardness were 6.2% and 2.5% due to the abnormal coarse grains. Furthermore, the crack path along the grain boundary was almost straight, causing a decrease in the indentation fracture toughness of cemented carbides. Additionally, the grains of cemented carbides were abnormally coarsened, and the morphologies of grains became unstable due to multiple sintering.     

Improved thermal stability of the piezoelectric properties of (Li,Ag)‐co‐modified (K,Na) NbO3‐based ceramics prepared by spark plasma sintering

High‐performance lead‐free piezoelectric ceramics 0.94(K_0.45Na_0.55)_1?xLi_x(Nb_0.85Ta_0.15)O₃–0.06AgNbO₃ (KNNLTAg‐x) were successfully prepared by spark plasma sintering technique. The doping effect of Li on the structural and electrical properties of KNNLTAg‐x (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10) ceramics was studied. The lattice structure, ferroelectric and piezoelectric properties of the KNLNTAg‐x ceramics are highly dependent on the Li doping level. In particular, the Li dopant has a great impact on both Curie temperature T_c and orthorhombic‐tetragonal transition temperature T_O‐T. The 4% Li‐doped sample exhibited relatively high T_O‐T of 95°C, leading to a stable dynamic piezoelectric coefficient (d₃₃*) of 220‐240 pm/V in a broad temperature range from 25°C to 105°C. Additionally, the 2% Li‐doped sample shows a high d₃₃* of 320 pm/V at 135°C, suggesting its great potential for high‐temperature applications.     

Microstructure and magnetic properties of nickel-zinc ferrite ceramics fabricated by spark plasma sintering

Dense nickel-zinc (NiZn) ferrite ceramics were successfully fabricated within tens of seconds via spark plasma sintering. The phase composition and microstructure of the sintered samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The static magnetic properties at room temperature and Curie temperature of the samples were investigated by vibrating sample magnetometry. The results indicated that the main phase of the sintered samples was Ni_0.75Zn_0.25Fe₂O₄ with spinal structure, and the sintering temperature and heating rate observably affected the microstructure and density, then the magnetic properties of the sample. The Joule heat generated by NiZn ferrite during spark plasma sintering was very important for the rapid preparation of the sample with high density and small grain size. The low sintering temperature and heating rate would be helpful to obtain samples with small grain size, high density, and then good magnetic properties. The samples sintered at 900 °C with the heating rate of 5–10 °C/s were characterized of the relative density above 95%, 4πM_s value beyond 4000 Gs and coercivity below 27.7 Oe.