Pre-sintered Samples Research Articles

Effect of grain size on the dielectric and magnetic properties of YFeO3 ceramics densified by microwave sintering and spark plasma sintering from powders synthesized via reverse chemical co-precipitation

YFeO3 (YFO) ceramics were synthesized by reverse co-precipitation at different pH values followed by microwave sintering (MS) and spark plasma sintering (SPS) techniques. Also, the effect of the grain size on the magnetic and dielectric properties of the sintered ceramics was examined using various characterization methods. Moreover, FESEM micrographs showed that the particle size of the calcined samples has declined by increasing the pH value. Furthermore, the magnetization curves exhibited weak ferromagnetic behavior in the pre-sintered powder samples. Besides, the Ms value was enhanced from 1.26emu/g to 2.3emu/g by an increase in pH from 8.5 to 10.5 owing to reducing the particle size and subsequently raising the amount of uncompensated surface spins. Furthermore, the powder samples showed an increase in the Hc and Mr values from 15Oe to 28Oe and 0.127emu/g to 0.296emu/g, respectively, with a reduction in particle size from 0.632 μm to 0.272 μm. Additionally, the specimens processed through the sintering techniques demonstrated a remarkable reduction in magnetic characteristics. Therefore, a decrease in magnetic saturation values from 2.3emu/g to 0.43emu/g for the MS sample and to 0.17emu/g for the SPS specimen was attributed to a marked reduction in the amount of free surfaces and subsequently uncompensated surface spins. At last, the observed dielectric characteristics of all the sintered pellets had a common dielectric behavior. This signifies that by increasing the frequency the ε′ and tanδ decline at lower frequencies and become frequency-independent at higher frequencies based upon the Maxwell-Wagner polarization theory. On the other hand, the ε′ value for the SPS sample produced at pH = 10.5 underwent a decline of 2.5 times at the low frequency of 200 Hz compared to the MS one due to the significant decrement of the grain boundaries on account of the coarsening of grains.

Liquid phase migration in cemented carbides – Experiments and modelling

Liquid phase migration (LPM) and its potential impact on the phenomenon known as cobalt capping in cemented carbides have been investigated through experiments on fully densified, pre-sintered samples and simulations. A model for LPM was developed based on interface energies, grain size, and contiguity, and was implemented and integrated with Thermo-Calc software to make predictions about the final microstructure. The model's predictions, when applied to cases with gradients in grain size and volume fraction of the binder phase, were found to somewhat match experimental observations. Additionally, it was obsereved that LPM can cause a phenomenon resembling cobalt capping due to gradients in carbon activity.

DIFFUSE REFLECTION SPECTRA OF FROZEN SALT MELTS OF THE CeF3-EuF3-NaCl-KCl SYSTEMS

The nature of the interaction was determined by the method of diffuse reflection (DR) spectroscopy and conclusions were made about the possible composition of the reaction products in the CeF3-EuF3-NaCl-KCl system. As a result of high-temperature (800 and 1100 °C) heat treatment of a mixture of CeF3 with EuF3 in the ratios 1: 1 and 1: 2, a significant change in the nature of the spectral curves of DR in UV and near IR-spectral ranges is observed. In the first of them, intense absorption was found in the entire spectral range, and more pronounced at higher EuF3 content. On the other hand, significant deformation and a sharp decrease in the intensity of the absorption band are observed on the spectral curves of the DR in the near IR range of the spectrum. The DR spectra of sintered samples in the UV range indicate the emergence of various valent states of Ce (III, IV) and Eu (II, III) with a predominance of Ce4+ and Eu2+ ions compared to Ce3+ and Eu3+. The decrease in the absorption intensity of Eu3+ ions due to 4f‑4f electronic transitions in the near-IR spectra is a direct confirmation of the oxidation-reduction processes occurring in the solid state system. These processes become even more pronounced when processing samples of systems in NaCl-KCl melt, even without pre-sintering. Thus, in the sample, which is a mechanical mixture of CeF3-EuF3 after treatment in the salt melt, the absorption in the near IR range on the DR spectra either disappears altogether (ratio 1: 1), or is a slight residue of F(R) at about 10–3 a. u.). The spectra of DR in the UV range also change dramatically, indicating a predominant content of Eu (II) compared to Eu (III). A similar effect on the nature of the DR spectrum is exerted by the processing of pre-sintered samples in the salt melt, namely, there is a change in the nature of the DR spectral curves both in the UV and in the near IR ranges of the spectrum. This indicates the deepening of the oxidation-reduction processes in the samples of the CeF3-EuF3-NaCl-KCl system with the participation of the salt melt. The increase in the ratio of CeF3-EuF3 from 1: 1 to 1: 2 contributes to above mentioned processes.

Powder Injection Molding of Ti-Al-W Nano/Micro Bimodal Powders: Structure, Phase Composition and Oxidation Kinetics

Products from the materials of the Ti-Al system are difficult to manufacture. This often restricts the use of such materials despite their outstanding properties. Some of the promising methods for manufacturing products, which allows working with materials of the Ti-Al system, are powder injection molding (PIM) and material extrusion additive manufacturing (MEAM) technologies. In the present study, powder composites Ti-48Al-4W with different powder size distribution, obtained by the electric explosion of wire (EEW) method, were investigated. The powder was used in PIM technology to produce bulk samples. After polymer debinding, PIM samples were sintered in a vacuum and using hot isostatic pressing (HIP) at the same temperatures and isothermal holding times. The results show the influence of size distribution and sintering method on the structure, phase composition, mechanical properties and oxidation resistance of pre-sintered PIM samples. It is found that both the size distribution and sintering method affect the mechanical properties. The smaller the particle size of the powder in the material, the greater the resistance to oxidation of such samples.

Sintering anisotropy of binder jetted 316L stainless steel: part II – microstructure evolution during sintering

ABSTRACT Green density of binder jetted parts are typically equal or lower than the powder tap density. Also, anisotropic green porosity distribution is expected because of the characteristics of the binder jetting (BJ) printing process. In this study, the microstructure evolution in terms of phases and porosity characteristics was studied. A transition from irregular-shape interconnected porosity in pre-sintered samples to closed quasi-spherical porosity for samples sintered at 1370°C was observed. EBSD phase map showed ∼2.73% of δ-ferrite in sample sintered at 1370°C. The anisotropic porosity distribution was revealed by a higher area fraction of aligned large pores (>35 µm), within the cross-section perpendicular to the building direction. Chemical analysis showed an increase of C, O and N on the green sample, while a strong decrease was found after sintering when compared with the powder chemistry. δ-ferrite onset, from phase equilibrium calculations, varies from ∼1250°C (sintered sample chemistry) to ∼1350°C (powder chemistry).

A simple solution to recycle and reuse dental CAD/CAM zirconia block from its waste residuals.

Purpose To seek a simple solution that can recycle and regenerate dental CAD/CAM zirconia green blanks from its waste residuals.Methods Waste residuals (3M® Lava™ Plus HT) were pulverized after dry milling and cutting, and subsequently sieved before pickling in a 0.5 M nitric acid. These powders were then dry-pressed and pre-sintered into blocks at seven different temperatures in the range 800-1100 °C. New zirconia blocks flagged with the same batch numbers were used as control. These blocks were cut into bars before subjected them to manufacturer-recommended sintering at 1450 °C. Crystalline phases (by XRD), elemental compositions (by EDX), surface morphologies (by SEM), machinability, linear shrinkage rate, relative density, and Knoop microhardness were evaluated before and after sintering, and four-point flexural strengths were also evaluated for the sintered zirconia bars.Results Only tetragonal phases were found in both pre- and fully-sintered recycled zirconia blocks. SEM results showed that pre-sintered samples at 950 °C had smooth and flat surfaces with evenly distributed particles. Recycled and control zirconia blocks had similar elemental compositions. Results from machined surface, linear shrinkage rate, relative density, and Knoop microhardness established that 950 °C and 1000 °C were suitable pre-sintering temperatures for recycling zirconia. Pre-sintered recycled zirconia had no significant differences in flexural strengths, however, samples pre-sintered at 1000 °C exhibited the closest value (897 MPa) compared to that of the control (904 MPa).Conclusions Dental CAD/CAM zirconia can be recycled and reused from its waste residuals by adopting a simple method that requires a pre-sintering at 950 or 1000 °C.

Sintering Behavior of AL<sub>2</sub>O<sub>3</sub> Ceramics Doped with Pre-Sintered NB<sub>2</sub>O<sub>5</sub> and LiF

A study was carried out on the behavior during the sintering of Al2O3 ceramics with pre-sintering of the additives Nb2O5 and LiF at different temperatures. It was observed through XRD and DSC the formation of the LiNb3O8, Nb3O7F and LiNbO3 phases during the pre-sintering of the additive powders and the formation of the LiAl5O8, AlNbO4 phases during the sintering of the samples The samples showed densities around 91% of the theoretical density, and pre-sintered samples showed low growth in grain size.

Towards Creation of Ceramic-Based Low Permeability Reference Standards

Low-permeable materials, either artificial or natural, are essential components of the current technological development. Their production or processing requires a comprehensive characterization method based on confident reference standards. Permeability standards for values below 10−15 m2 are lacking. In this study, we explored the feasibility of using the ceramic sintering process to reach low, but measurable values of gas permeability in Al2O3 samples, as one of the potential materials for reference standards. The studied samples were produced with a ceramic 3D printer, which enables the manufacturing of low-permeable samples having complex geometrical arrangements. A series of preliminary laboratory testing indicated the available gas permeability range from 2.4 × 10−15 m2 for the pre-sintered samples to 1.8 × 10−21 m2 for the sintered samples. The verification of the permeability reduction was carried out using a unique unsteady state fast and accurate measurement method. The results confirm the possibility of developing a technology for materials manufacturing with low porosity and permeability. Such materials open many areas for application, including manufacturing of ceramics with controlled transport properties and low-permeability standards for calibrating laboratory equipment for geosciences, construction industries, biomedical, and other relevant fields.

Oxidation kinetics of Ti3GeC2 MAX phase

Present work reports the oxidation kinetics of Ti3GeC2 MAX phase. Two different methods were used: thermogravimetric analysis, and differential scanning calorimetry. To study the oxidation kinetics, pre-sintered samples were heated up to 1473 K in air at different heating rates. The samples were nearly stable up to about 735 K. It exhibits a non-linear oxidation behavior, i.e. different weight gains at different temperature regions. The data were analyzed separately using conventional kinetic models. Oxidation of Ti3GeC2 compound was characterized with different activation energies for different temperature ranges. Process was found to initiate with the formation of TiO2 followed by CO and GeO2.

Effect of temperature and pre-sintering on phase transformation, texture and mechanical properties of silicon nitride ceramics

Silicon nitride composite ceramics with excellent mechanical properties were prepared by hot pressing. The effects of temperature on the sintering process, β-Si3N4 phase transformation ratio (β%) and mechanical properties were investigated. By calculating the diffraction intensity (I), the β-Si3N4 phase transformation ratios of the three samples considered in this study were 57.2% (pre-sintered samples, PSN), 63.5% (urea homogeneous precipitation method, USN) and 66.6% (mechanical milling method, MSN) at 1550 °C, respectively. These results showed that the Si3N4 composites coated by using Y-Al precursor (USN) have a lower ratio of the phase transformation from α to β. Meanwhile, from the analysis of SEM images of the etched surface, the grain sizes were 0.47 µm (PSN), 0.39 µm (USN), and 0.43 µm (MSN) at 1650 °C, respectively. Compared to the other samples, the USN sample offered superior sintering behaviour and mechanical properties. The optimal flexural strength, hardness and fracture toughness of the Si3N4 composites coated by using the Y-Al precursor were 911 ± 69 MPa, 16.23 ± 0.24 GPa and 4.9 ± 0.35 MPa/m2 at 1650 °C, respectively.

Densification and microstructure evolution of reactively sintered transparent spinel ceramics

Reactive sintering is an effective and simple method to prepare transparent spinel ceramics. In this research, transparent MgO·nAl2O3 (0.98 ≤ n ≤ 2) spinel ceramics were prepared via reactive sintering in air followed by hot isostatic press (HIP), using MgO and γ-Al2O3 powders as raw materials. The influence of composition on densification and microstructure evolution was systemically investigated. More importantly, the relationship between microstructure of presintered samples and final properties of transparent ceramics was singled out. Thermodynamically stable large pores were easily generated in magnesia-rich and stoichiometric samples after presintering in air, causing severe abnormal grain growth during the HIP treatment and poor optical quality of the resulting samples. The presintering temperature of alumina-rich samples widely varied with composition. No large pores were observed in the presintered sample, which was beneficial for the elimination of residual pores in the following HIP process. Highly transparent spinel ceramics with n = 1.1 and 1.3 were successfully fabricated with the transmittance above 84% even at the short wavelength of 400 nm, close to the theoretical value.

Flexural strength evaluations and fractography analyses of slip cast mesoporous submicron alumina

Mesoporous submicron α−Al2O3 green compacts were fabricated using slip casting of ultrafine alumina powders. The pre-sintered samples were sintered in air atmosphere at 1300°C, 1350°C, 1400°C, and 1500°C to obtain a variety of grain morphologies namely submicron equiaxed and micro rod structures. The resulting grain diameters lie between ∼ 270nm and ∼ 1590nm and total porosity fraction between 0.05% and 13%. The room temperature flexural strength (σ) evaluations and fractography analyses of sintered alumina samples were performed. It was observed that the total porosity fraction dictates the flexural strength as compared to grain diameter. Further, it was found that the flexural strength exhibited a decreasing trend for an increase in the total porosity fraction, and proved to be a better effective parameter than open porosity fraction. The fractography analyses suggest that samples sintered at 1300°C and 1350°C predominantly underwent intergranular fracture, while those sintered at 1400°C and 1500°C underwent a mixture of intergranular and transgranular fracture.

Effect of rate-controlled sintering on characteristics of hot isostatically pressed transparent Yb-doped Lu2O3 ceramics

Ytterbium-doped transparent lutetium oxide (Yb:Lu2O3) ceramics were fabricated from laser ablated nanoparticles via vacuum pre-sintering followed by hot isostatic pressing (HIPing). Nano-sized powder was uniaxially pressed and pre-sintered at 1550 °C using a conventional regime composed of a slow ramp (3 °C/min) with subsequent holding (CRH), and a rate-controlled sintering (RCS) regime consisting of a progressive reduction in densification rates. Although the pre-sintered samples approached identical relative density of ∼97%, the average grain size was decreased more than 30% by application of RCS regime. Pre-sintered Yb:Lu2O3 compacts were then HIPed to achieve full density and transparency. The optical attenuation coefficient of the ceramic pre-sintered by CRH regime was found to exceed that of the ceramic fabricated using RCS method by a factor of 2.1 at the wavelength of 1080 nm.

Theoretical and phenomenological analogies between flash sintering and dielectric breakdown in α-alumina

α-alumina pre-sintered samples were subjected to flash sintering at 1200 °C under differing electric field strengths. The analysis of the relation between the incubation time and pre-sintering temperature clearly shows that the presence of pores and surfaces within the sample plays a central role in field-assisted sintering behavior of the material. The observed behavior is accounted for by the strong non-linear electrical conductivity shown by porous alumina at high field strength. The observed non-ohmic conductivity can also be related to the “pre-breakdown” behavior previously described by Frenkel. Literature results suggest that the field involved in flash sintering of alumina is comparable with the dielectric strength at high temperature. We can also state that dielectric breakdown and flash sintering in alumina are associated with similar physical phenomena.

Luminescent rare-earth-doped transparent alumina ceramics

Present study aims at development and characterization of transparent polycrystalline alumina ceramics doped with optically active additives exhibiting light induced emission properties. Tested materials were prepared using wet shaping technique (slip casting) followed by pressure-less sintering and hot isostatic pressing (HIP). Three different rare-earth dopants (Nd, Eu, Er-0.2 at.% with respect to Al2O3) were introduced into alumina in order to obtain induced light emission properties. The samples were pre-sintered using two different regimes: single step – SSP and two-step pre-sintering – TSP. Final microstructure of HIP-ed samples pre-sintered by TSS was similar to that of the SSP pre-sintered samples; any differences in the average grain size were within the range of standard deviation of the measurement. The real in-line transmittance (RIT) ranged between 8 and 34%, depending on the type of dopant and the pre-sintering regime. The highest RIT values were obtained for Er-doped alumina, the lowest for Nd-doped alumina. The RIT was correlated with the size of the alumina matrix grains and decreased with increasing average grain size of the material. Despite the similarity of microstructure and density, significantly higher RIT was determined in the samples pre-sintered with the use of the two stage regime, irrespective of the dopants used. The photoluminescence spectra of rare-earth-doped samples both in visible and NIR spectral region showed typical absorption/emission/excitation bands due to the presence of optically active Nd3+, Eu3+ or Er3+ ions in the host matrix.

Influence of hot isostatic pressing on ZrO2–CaO dental ceramics properties

Different hot isostatic pressing conditions were used to obtain zirconia ceramics, in order to assess the influence of HIP on phase transformation, compressive strength, Young’s modulus and density. First, CaO stabilized zirconia powder was synthesized through sol–gel method, using zirconium propoxide, calcium isopropoxide and 2-metoxiethanol as precursors, then HIP treatment was applied to obtain final dense ceramics. Ceramics were morphologically and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Density measurements, compressive strength and Young’s modulus tests were also performed in order to evaluate the effect of HIP treatment. The zirconia powders heat treated at 500°C for 2h showed a pure cubic phase with average particle dimension about 70nm. The samples that were hot isostatic pressed presented a mixture of monoclinic–tetragonal or monoclinic–cubic phases, while for pre-sintered samples, cubic zirconia was the single crystalline form. Final dense ceramics were obtained after HIP treatment, with relative density values higher than 94%. ZrO2–CaO ceramics presented high compressive strength, with values in the range of 500–708.9MPa and elastic behavior with Young’s modulus between 1739MPa and 4372MPa. Finally zirconia ceramics were tested for biocompatibility allowing the normal development of MG63 cells in vitro.

Enhanced magnetization and electric polarization in Bi2Fe4O9 ceramics by magnetic field pre-sintering

A series of polycrystalline Bi2Fe4O9 ceramics are synthesized by a modified Pechini method, which are sintered at different temperatures from 700 to 900°C after nonmagnetic field and magnetic field pre-sintering processes. X-ray diffraction patterns reveal that all samples are single phase. Compared with the nonmagnetic field pre-sintered samples, enhanced magnetization and electric polarization are found in the magnetic field pre-sintered ones. With sintering temperature increasing, induced weak ferromagnetic property is depressed after magnetic field pre-sintering. The results indicate that it is possible to tune the magnetic and electric properties of Bi2Fe4O9 by magnetic field pre-sintering.

Machined Surface Quality of Pre-Sintered Hardenable PM Steel

The benefit of pre-sintered machining is to avoid machining difficulty of sintered parts especially hardenable PM steels. Pre-sintering treatments of green PM part at temperatures lower than the normal sintering temperature of 1120 °C result in green strength improvement high enough for machining. In this study, the influences of various pre-sintering temperatures and several machining conditions on machined surface quality of pre-sintered PM samples were investigated. The pre-sintered samples were machined by a turning process using a carbide cutting insert with varied cutting speeds at a fixed feed rate and depth of cut without cutting lubricant. Chromium alloyed PM steel (Astaloy® CrM) powder samples with (0.5 wt. %C) and without graphite (0 wt. % C) additions mixed with 1 wt. % of zinc stearate were prepared as green parts by cold compaction in a cylindrical die with diameter of 30 mm. Green density was about 7.00 g/cm3 and height of each sample was controlled by hydraulic pressure and powder weight of 80 g/sample. The green samples were treated by pre-sintering treatment before machining testing. Surface quality of each machined sample was evaluated by average surface roughness and surface texture by SEM analysis and the appearance of outlet edge breakout. The experimental results revealed that the pre-sintered samples with graphite addition showed better surface quality in terms of surface roughness and surface texture and small outlet edge breakout appearance. Moreover, at high pre-sintering temperatures of 900 and 1,000°C, the samples showed similar average surface roughness under the same turning conditions. The obtained surface textures were better than those of the samples pre-sintered at 700 and 800°C. The outlet edge breakout could not be found in the graphite-added samples pre-sintering at 1,000°C.

Effects of pre-sintering and annealing on the optical transmittance of Zr-doped Y2O3 transparent ceramics fabricated by vacuum sintering conjugated with post-hot-isostatic pressing

In this work, Zr-doped Y2O3 transparent ceramics were fabricated by vacuum pre-sintering at various temperatures ranging from 1650 to 1800°C combined with a post-hot-isostatic pressing (HIP) treatment. The pre-sintered samples all show black opaqueness mainly due to the formation of oxygen vacancies, which can be removed by post-annealing in air. The HIP treatment can also eliminate the blackness as well as residual pores, giving rise to optical transparency. The in-line transmittance of the samples after HIP was found to depend strongly on the microstructure of the as-sintered samples. The optimal microstructure of these types of samples before the HIP treatment should be fine grained with only intergranular pores, which can easily be removed by HIP. Annealing before HIP was shown to be necessary to enhance the transmittance of the samples. The effects of the pre-sintering heating rate on the optical transmittance of the HIP-treated samples were also studied.

Improvement of magnetic properties of Cr3+ substituted SrFe12O19

M type strontium hexaferrites SrFe12 − xCrxO19 (x = 0.0–0.6) were prepared by the ceramic process. The structure, morphology and magnetic properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and B–H hysteresis curve measurements. The XRD patterns show the formation of the M-type hexaferrite phase in the pre-sintered samples with non-magnetic phase, α-Fe2O3. The SEM results indicate that the Cr substitution does not obviously affect the structure of strontium ferrite. The results of magnetic mensuration, especially for the SrFe11.8Cr0.2O19 samples, reveal that rather high magnetic properties can be obtained in the sintering temperature range of 1135–1165°C.