The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

Polycrystalline transparent magnesium aluminate spinel processed by a combination of spark plasma sintering (SPS) and hot isostatic pressing (HIP)

A two-stage processing approach combining spark plasma sintering (SPS) and hot isostatic pressing (HIP) was employed for the fabrication of relatively large (30?mm diameter) and thick (up to 8?mm) samples of transparent polycrystalline magnesium aluminate. The effects of sample thickness, heating rate during SPS, and the temperature and duration of HIP treatments were investigated. It was established that the heating rate during SPS had a major influence on discoloration due to carbon contamination, which increased with sample thickness. HIP treatment allowed for the elimination of cloudiness due to samples porosity, although carbon contamination present after the SPS step could not be reduced by HIP treatment, regardless of the temperature and duration applied. Highly transparent specimens with thicknesses of 4 and 8?mm exhibiting an in-line transmittance of 85.2 and 83.2% at 600?nm, respectively, were fabricated.     

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

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 15 min and an applied pressure of 350–400 MPa. 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).     

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co²⁺:MgAl₂O₄ saturable absorbers for laser passive Q-switching. Densification of commercial MgAl₂O₄ powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co₉S₈ inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co²⁺:MgAl₂O₄. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co²⁺, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ_gs) and excited (σ_es) cross-sections of 3.5×10^-19 and 0.8×10^-19 cm², respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co²⁺:MgAl₂O₄ ceramics.     

Polycrystalline infrared-transparent MgO fabricated by spark plasma sintering

In the present research, polycrystalline magnesium oxide (MgO) bodies were fabricated using spark plasma sintering (SPS) at different temperatures and times from MgO nanopowder. Microstructural development, densification, and optical properties were investigated during SPS. The critical pressure of plastic deformation of the MgO compacts during sintering was also analyzed. The results showed that the plastic deformation phenomenon had a profound effect on the grain size and optical properties. In addition, the optical properties and microstructure of MgO bodies were strongly dependent on sintering temperature and time. Full-dense infrared-transparent magnesium oxide with a relative density of 99.99% was prepared at 1200 °C for 5 min under the pressure of 80 MPa. The spark plasma sintered MgO demonstrated the highest infrared transmittance of 72% in the 3–7 μm wavelength range, which was comparable with the values reported for MgO single crystal.     

Influence of the heating rate on grain size of alumina ceramics prepared via spark plasma sintering (SPS)

The dependence of grain size on the heating rate has been investigated for alumina ceramics prepared via spark plasma sintering (SPS). For this purpose, the local grain size has been determined via position-dependent microscopic image analysis, using two independent grain size measures (mean chord length and Jeffries grain size). For alumina ceramics prepared with heating rates between 5 and 100 °C/min (pressure 80 MPa, maximum temperature 1300 °C) it is found that for higher heating rates the grain size is smaller. However, the microstructural non-uniformity is so large that any grain size determination that does not take this non-uniformity into account becomes meaningless, because grain size gradients from the specimen periphery to the center are larger than the differences in grain size due to different heating rates. Temperature and pressure gradients are discussed as the most plausible reasons for the microstructural non-uniformity.     

Post-annealing effect on transparent Mg-Zn aluminate solid solutions fabricated by spark plasma sintering

In MgAl₂O₄ (Mg aluminate) fabricated by pressure sintering, carbon contamination, oxygen vacancy, and other defects exist and cannot be completely removed even after post-annealing. ZnAl₂O₄ (Zn aluminate) has the same crystallographic structure as Mg aluminate and excellent light transmission by post-annealing. In this study, the transmission of (Mg_1-XZn_X)Al₂O₄ spinel, which is a solid solution between Mg aluminate and Zn aluminate, was investigated. After post-annealing, the transmission of (Mg_1-XZn_X)Al₂O₄ spinel was higher than Zn aluminate and Mg aluminate, which means that the absorption center disappeared. Besides, the real in-line transmission is improved by up to 30% by post-annealing, which is related to the driving force to increase the configurational entropy and form a more stable solid solution phase. In solid solution spinel, as the Zn content increases, the optical properties can be improved by post-annealing, but the mechanical properties are slightly decreased.     

Effect of segregation on particle size stability and SPS sintering of Li2O-Doped magnesium aluminate spinel

Magnesium aluminate spinel (MAS) was prepared using the simultaneous precipitation method by varying the concentration of Li₂O from 0 to 5 mol%. No residual chlorine from the LiCl precursor was detected in the final powders while Li achieved the target concentration in all samples and contributed to stabilizing nanoparticles smaller than 10 nm. Li segregation to both interfaces (surfaces and grain boundaries) occurred and tended to be more pronounced at the grain boundaries stabilizing this type of interface during processing rather than surfaces. Spark plasma sintering (SPS) was used to consolidate the nanopowders into fully dense nanostructured pellets. The increase in Li content facilitated the sintering process and pore elimination occurred at 850–900 °C, a much lower temperature range as compared to conventional sintering (1650 °C). Samples containing 5 mol% Li sintered at 850 °C exhibited a medium grain size of ?25 nm, microhardness of ?24 GPa and ?50% in-line optical transmission at the 800 nm.     

Transparent magnesium aluminate spinel: Effect of critical temperature in two-stage spark plasma sintering

The discolouration of magnesium aluminate spinel caused by carbon contamination is a main drawback of fabricating transparent bodies by spark plasma sintering (SPS). In this study, a two-stage heating rate profile was used to produce transparent MgAl₂O₄ without using sintering aids by SPS at 1250°C. The effect of critical temperature (Tc), at which the heating rate is decreased, on transparency and carbon contamination was investigated: higher critical temperature resulted in higher contamination. Non-uniform densification indicated that fast heating results in a hot-zone formation in the centre of sintered pellets; the higher temperature of centre favoured reaction of graphite die with spinel and formation of disordered carbon structures in residual pores.     

Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration

The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.     

Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °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 °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 °C under 600 MPa.     

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.     

Consolidation by spark plasma sintering (SPS) of polyetheretherketone

The present study deals with the consolidation of an ultra‐high performance polymer, the poly(ether ether ketone) (PEEK), for structural applications, using the powder metallurgy (PM) way, and more precisely the Spark Plasma Sintering (SPS) processing. The effects of SPS parameters such as temperature, pressure, and dwell time on density and mechanical properties of PEEK were investigated via a Design of Experiments (DoE). A temperature of 250 °C, a pressure of 40 MPa, and a dwell time of 20 min have been identified as the optimal SPS process parameters. In these conditions, a density of 1.31 g / cm³ was reached and homogeneous mechanical properties in the volume determined by means of compression tests were found with a compressive modulus of 2.75 GPa, a yield strength of 134 MPa, and a maximum compressive strain of 43%. These results are better than those of commercial products obtained by injection molding. The pressure appears to be a significant parameter on PEEK properties and plays positive or negative roles according to the responses of DoE studied. To our knowledge, it is one of the first studies based on the application of the PM techniques for PEEK consolidation showing the possibility to process below its melting point. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44911.     

Highly dense spinel ceramics with completely suppressed grain growth prepared via SPS with NaF as a sintering additive

For the preparation of transparent spinel ceramics it is common practice to use LiF as a sintering additive to achieve transparency. However, it is well known that in this case the grain size exhibits a significant increase compared to pure spinel ceramics, which can lead to a deterioration in mechanical properties. The findings of this paper indicate that when NaF is used as a sintering additive for the preparation of spinel ceramics via spark plasma sintering (SPS) translucent materials with a high level of densification can be obtained without observable grain growth. It is shown that the grain size after SPS at 1500 °C with 1 h dwell is essentially the same as the primary particle size of the spinel powder, whereas pure spinel ceramics prepared via SPS under the same conditions exhibit grain growth by approximately a factor 5.     

The effect of pre-annealing and post-annealing on the transparency of MgAl2O4, prepared by slip casting and spark plasma sintering (SPS)

The aim of this paper is to investigate the effect of slip casting process and the annealing before and after sintering to achieve a transparent MgAl₂O₄. To remove contaminants such as carbon from the structure of shaped spinel bodies, at first, the samples were annealed at temperature of 800?°C, 900?°C and 1000?°C for 2?h and then sintered at 1400?°C. By annealing the sample before sintering at 900?°C, the transmission increased (15% at IR region and 10% at visible region). Although by annealing the samples, the amount of carbon contamination reduced. Annealing the samples after sintering also had some desirable results. The samples annealed at temperature of 1200?°C for a time of 3, 5 and 10?h. The darkness of samples reduced due to the removal of carbon impurities and the sample was annealed at 1200?°C for 5?h had the most transparency in the visible and infrared regions.     

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.     

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.     

Sub-micrometre grained UO2 pellets consolidated from sol gel beads using spark plasma sintering (SPS)

UO₂ beads from the sol supported precipitation method were calcined at a low temperature in order to obtain porous micro-beads, composed of nanometric particles. The sintering behaviour of the beads in spark plasma sintering was investigated. The powder had a good sinterability and the final grain size of the pellets could be tailored by varying the processing conditions, in order to resemble the microstructure of the traditionally fabricated UO₂ pellets (i.e. grains of several µm size), or to achieve sub-micrometre size as observed in the high burnup structure. Dense UO₂ pellets with a grain size as small as 300 nm were obtained by sintering at 835 °C without dwell time, whereas 3 µm grained pellets were obtained at 1000 °C and a 5 min dwell time.     

Mechanical properties and microstructures of TiCN/nano-TiB2/TiN cermets prepared by spark plasma sintering

The effects of TiN and nano-TiB₂ additions to titanium carbonitride (TiCN-WC-Cr₃C₂-Co)-based cermets processed by spark plasma sintering (SPS) are identified. The TiN and nano-TiB₂ additions were varied from 0 to 15?wt% to ascertain their combined effects on the mechanical properties. Scanning electron microscopy (SEM) revealed the combined chemical composition of the new phases formed during sintering. The hardness and fracture toughness values were recorded. Increase in the fracture toughness value with TiN addition was more compared with the nano-TiB₂ addition. In contrast, the hardness values were higher for the cermets formed with the nano-TiB₂ addition. Sintered bodies were made as tool inserts that meet SNGN120408 standard tool configuration. Using these tools, EN24 work-piece was turned at different cutting speeds of 11.87, 29.68, 71.46, 163.88?m/min under conditions of dry cutting. The performance was evaluated. Cutting force as well as surface roughness of the work-piece after machining was measured. For all cutting tools, initially the cutting force was high but it tended to decrease at higher cutting speeds. In addition, for all the tools, at higher cutting tools the surface roughness values were uniformly minimal. The cermet with a composition 55TiCN-15WC-10Co-5Cr₃C₂–15nanoTiB₂ (all in wt%), in particular, showed a balanced enhancement in both fracture toughness (6.8?MPa?m^1/2) and Vickers hardness (18?GPa) values. The surface finish of the work-piece was also the best after machining when a tool of the above composition was used.     

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.