Low-temperature sintering of magnesium aluminate spinel doped with manganese: Thermodynamic and kinetic aspects

This work describes the role of manganese (Mn) as a sintering aid for magnesium aluminate (MgAl₂O₄) nanoparticles. Mn-doped MgAl₂O₄ nanoparticles, synthesized by coprecipitation method, showed increased surface area when contrasted to undoped MgAl₂O₄. Fast firing of compacted-doped nanoparticles achieved high degree of densification at temperatures as low as 1100°C with very moderate grain growth, resulting in average sizes at the nanoscale (~60 nm). Differential scanning calorimetry was used to quantify the exothermic heat effects of sintering, which combined with quantitative microstructural evolution analysis enabled calculation of both surface and grain boundary energies. The results revealed that Mn effectively reduces the surface and grain boundary energies which led to dihedral angle broadening and consequently increased sintering stress. Experimental data also revealed a concomitant decrease in the activation energy of sintering with Mn doping which dropped from 644 kJ/mol for undoped MgAl₂O₄ to 285 kJ/mol, informing Mn acts as a sintering aid in a thermokinetic manner.     

Uniform and fine Mg-γ-AlON powders prepared from MgAl2O4: A promising precursor material for highly-transparent Mg-γ-AlON ceramics

High-purity and sinterability Mg-γ-AlON (Mg_0.1Al_1.53O_1.89N_0.27) powders were synthesized by gas pressure sintering (GPS) of mixed powders of commercial Al₂O₃ and AlN, and lab-made MgAl₂O₄. The Mg-γ-AlON powders exhibited a uniform particle morphology and a small particle size of d₅₀ = 3.4 μm, owing to the use of MgAl₂O₄ as the Mg source. Highly-transparent Mg-γ-AlON ceramics were fabricated using the synthesized Mg-γ-AlON powders by spark plasma sintering (SPS) at 1800 °C for 5 min under an axial pressure of 80 MPa, followed by hot isostatic pressing (HIP) at 1800 °C for 2 h under a nitrogen gas pressure of 190 MPa. The ceramics showed a high in-line transmittance of ～ 80.5% at 450 nm, ascribed to the high sinterability of the MgAl₂O₄ raw powder that leads to a pore-free and fully densified microstructure. This indicates that MgAl₂O₄ as sintering additive is superior over MgO and MgF₂ in the fabrication of Mg-γ-AlON transparent ceramic.     

Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant

YAG precursors were co-precipitated from a mixed solution of aluminum and yttrium nitrates using ammonia water and ammonium hydrogen carbonate as precipitants, respectively. Phase evolution of the precursors during calcination and sinterability of the resultant YAG powders were compared between the two methods. The use of ammonia water produced a hydroxide precursor with an approximate composition of Al(OH)₃·0.3[Y₂(OH)₅(NO₃)·3H₂O] which transformed to pure YAG at about 1000°C via YAlO₃ phase. Severe agglomeration caused poor sinterability of the resultant YAG powders. The use of ammonium hydrogen carbonate produced a carbonate precursor with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.8H₂O. The precursor directly converted to pure YAG at about 900°C. The precursor was loosely agglomerated and the resultant YAG powders showed good dispersity and excellent sinterability. For the same calcination temperature of 1100°C, YAG powders from the hydroxide precursor and the carbonate precursor densified to ∼81.2 and ∼99.8% of the theoretical, respectively, by vacuum sintering at 1500°C for 2 h.     

A two-step heating strategy for low-temperature fabrication of high sinterability and fine MgAlON powder with MgAl2O4 as Mg source

Based on the reaction sequence during synthesis of MgAlON powder by solid-state reaction, a two-step heating strategy is proposed to low-temperature fabricate fine MgAlON powder of high sinterability by using MgAl₂O₄ as Mg source, respectively together with AlON and Al₂O₃+AlN. By introduction of an additional dwelling at 1550 °C to the first heating step, more α-Al₂O₃ dissolve into the solid solution at this temperature. By this way, overlarge particles of Al₂O₃ by agglomeration could be avoided in the next heating step to enable fast full reaction at a lower temperature. By dwelling 30 min at 1550 °C followed by 60 min at 1700 °C, single phase MgAlON powders were successfully prepared by solid-state reaction of all the two batches. The fine MgAlON powder synthesized by MAS+Al₂O₃+AlN batch exhibited high sinterability as the MgAlON ceramics pressureless sintered by this powder at 1880 °C without dwelling showed a transmittance up to 68.3%. The phase assemblage and morphology evolution of the mixture during solid-state reaction were monitored, which verified the effectiveness of the proposed two-step heating strategy. The low synthesis temperature of the two-step heating scheme benefits to prepare pure MgAlON powder with small particle size.     

Enhanced ionic conductivity of Sm,Gd-doped ceria induced by modification of powder synthesis procedure

Nanostructured Ce_0.85Gd_0.05Sm_0.10O_2?δ powders have been obtained by a classical and a modified Pechini method. The textural parameters of the synthesized powders were evaluated using N₂ adsorption–desorption. X-ray diffraction (XRD) showed that the powders were single phase with fluorite-type structure. The dilatometry measurements evidenced a strong influence of the synthesis procedure of Gd, Sm-co-doped ceria on its sintering behavior. A decrease in powder sintering temperature down to 1200 °C was obtained when Triton X-100 was added into the synthesis reaction mixture. The AC impedance spectroscopy of the sintered pellets was also performed in the 200–800 °C temperature range, in air. The sample prepared using the non-ionic surfactant exhibited higher ionic conductivities and lower activation energies than the sample synthesized by classical Pechini method over the entire investigated temperature range.     

Effect of Sm and Gd dopants on structural characteristics and ionic conductivity of ceria

Sm- and Gd-doped ceria electrolytes Ce_0.9Gd_0.1O_1.95 (GDC) and Ce_0.9Sm_0.1O_1.95 (SDC) were prepared by using the Pechini method. The microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry/differential thermal analysis (TG/DTA) and Fourier Transform Infrared Spectroscopy (FTIR). The TG/DTA and XRD results indicated that a single-phase fluorite structure formed at a relatively low calcination temperature, 400 °C. The XRD patterns of the samples revealed that the crystallization of the SDC powders was superior than that of the GDC powders at 400 °C. The sintering behavior and ionic conductivity of the GDC and SDC pellets were also investigated. The sintering results showed that the SDC samples were found to have higher sinterability than the GDC samples at a relatively low sintering temperature, 1300 °C, a significantly lower temperature than 1650 °C, which is required for ceria solid electrolytes prepared by solid state techniques. The impedance spectroscopy results revealed that SDC has a higher ionic conductivity compared to GDC.     

Low temperature sintering of PZT powders coated with Pb5Ge3O11 by sol–Gel method

Low temperature sintering of PZT powders was investigated using Pb₅Ge₃O₁₁(PGO) as a sintering aid. PZT powders with 150 nm particle size were coated with PGO which was prepared from precursor solutions of Ge(OiPr)₄ and Pb(NO₃)₂ by sol–gel method. 1 wt% PGO-added PZT powders were densified at 750°C for 2 h to sintered bodies with the relative density of approximately 95%. An addition of PGO improved the sinterability of PZT powders with a reduction of sintering temperature by about 300°C. Dielectric and piezoelectric properties of PGO-added PZT ceramics sintered at ≦950°C were superior to those without PGO additives. However, a higher sintering temperature above 1000°C deteriorated the dielectric and piezoelectric properties of PGO-added PZT ceramics. This may be attributed to the change of microstructure involving the formation of solid solution between PZT and PGO. The 1 wt% PGO-added PZT bodies sintered at 750°C exhibited an electromechanical coupling factor, Kp, of about 56%.     

Lowering of sintering temperature and microwave dielectric properties of BaTi4O9 ceramics prepared by the polymeric precursor method

Low temperature sintering and microwave dielectric properties of barium polytitanate (BaO–4TiO₂) ceramics prepared by means of polymeric precursor route based on the Pechini process were investigated. Pure and fine BaTi₄O₉ powders with particle sizes of 100–200 nm were derived by thermal decomposition of amorphous gel precursor (above 750 °C). They formed single orthorhombic BaTi₄O₉ phase and showed fine and well-dispersed by XRD and SEM observation. The high sintering ability of the prepared powders enabled the fabrication of dielectric ceramics at low sintering temperatures (1200–1300 °C). The well-sintered BaTi₄O₉ ceramics with high relative densities (95%) were found to show excellent microwave dielectric properties compared to those prepared by conventional method at the same sintering temperature.     

Co-precipitation synthesis route to yttrium aluminum garnet (YAG) transparent ceramics

Yttrium aluminum garnet (YAG) precursor was synthesized via a coprecipitation method with aluminum nitrate and yttrium nitrate as raw materials, using ammonium hydrogen carbonate (AHC) as the precipitant. Fine and low-agglomerated YAG powder was obtained by calcining the precursor at 1200 °C. The primary crystallites were measured to be ～120 nm in size and weakly agglomerated to a particle size of ～500 nm, indicating a high degree of sinterability. With 0.5 wt% tetraethyl orthosilicate (TEOS) and 0.1 wt% magnesia as sintering aids, transparent YAG ceramics were fabricated by vacuum sintering at 1730–1790 °C for various hours. The influences of sintering temperature and holding time on the microstructure and transmittance of YAG ceramics were discussed.     

The role of the synthesis route to obtain densified TiO2-doped alumina ceramics

Alumina specimens doped with 1 wt.% of titanium oxide were successfully prepared by three different synthesis routes: Pechini method, coprecipitation and sol–gel processes. This paper describes the phase sequence in each synthesis process and its effect on the final particle size and shape, as well as, on the microstructure of the calcined powders and the sintering behaviour. The intermediate phases to obtain α-alumina were κ-Al₂O₃, θ-Al₂O₃ and γ-Al₂O₃for the Pechini, coprecipitation and sol–gel processes, respectively, as could be detected by FT-IR and XRD. Secondly, the calcined powders were isopressed and sintered at 1625 °C for 4 h. Density measurements, and microstructure were investigated by Archimedes method and TEM/SEM, respectively. The sintering behaviour of the materials is discussed on the basis of the characteristic of the metastable phases obtained by each route. Coprecipitation yielded rounded particles with the smallest size. Sol–gel process produced larger grains with vermicular shapes and Pechini method led to hexagonal corundum crystals.     

Effect of heat treatment conditions on the growth of MgAl2O4 nanoparticles obtained by sol-gel method

MgAl₂O₄ nanoparticles (NPs) were prepared by sol–gel method using aluminium nitrate, magnesium nitrate and citric acid as starting materials, phenolic formaldehyde resin and carbon black as additives. Growth of MgAl₂O₄ NPs in different heat treatment conditions (temperature, atmosphere, carbon additives and in Al₂O₃-C system) was investigated. MgAl₂O₄ NPs were formed at 600 °C in air atmosphere with serious agglomeration of nanoparticles having diameter of approximate 30 nm. The size of MgAl₂O₄ NPs increased greatly from 40 to 50 nm to several hundreds of nanometres as the temperature was raised from 800 °C to 1400 °C. Partial sintering of NPs was observed upon heating at temperatures higher than 1200 °C in air. In reducing atmosphere, the size of MgAl₂O₄ NPs (about 30–50 nm) changed slightly with increasing temperature. This was attributed to the dispersion of carbon inclusions in the MgAl₂O₄ grain boundaries, inducing a steric hindrance effect and inhibiting the growth of particles. MgAl₂O₄ NPs (30–50 nm) in the Al₂O₃-C system were in-situ formed at high temperatures with the use of dried precursor gels. MgAl₂O₄ NPs can contribute to improving the thermal shock resistance of Al₂O₃-C materials.     

A benzoate coprecipitation route for synthesizing nanocrystalline GDC powder with lowered sintering temperature

Electrolyte powders with low sintering temperature and high-ionic conductivity can considerably facilitate the fabrication and performance of solid oxide fuel cells (SOFCs). Gadolinia-doped ceria (GDC) is a promising electrolyte for developing intermediate- and low-temperature (IT and LT) SOFCs. However, the conventional sintering temperature for GDC is usually above 1200 °C unless additives are used. In this work, a nanocrystalline powder of GDC, (10 mol% Gd dopant, Gd_0.1Ce_0.9O_1.95) with low-sintering temperature has been synthesized using ammonium benzoate as a novel, environmentally friendly and cost-effective precursor/precipitant. The synthesized benzoate powders (termed washed- and non-washed samples) were calcined at a relatively low temperature of 500 °C for 6 h. Physicochemical characteristics were determined using thermal analysis (TG/DTA), Raman spectroscopy, FT-IR, SEM/EDX, XRD, nitrogen absorptiometry, and dilatometry. Dilatometry showed that the newly synthesized GDC samples (washed and non-washed routes) start to shrink at temperatures of 500 and 600 °C (respectively), reaching their maximum sintering rate at 650 and 750 °C. Sintering of pelletized electrolyte substrates at the sintering onset temperature for commercial GDC powder (950 °C) for 6 h, showed densification of washed- and non-washed samples, obtaining 97.48 and 98.43% respectively, relative to theoretical density. The electrochemical impedance spectroscopy (EIS) analysis for the electrolyte pellets sintered at 950 °C showed a total electrical conductivity of 3.83 × 10^?2 and 5.90 × 10^?2 S cm^?1 (under air atmosphere at 750 °C) for washed- and non-washed samples, respectively. This is the first report of a GDC synthesis, where a considerable improvement in sinterability and electrical conductivity of the product GDC is observed at 950 °C without additives addition.     

Synthesis and densification of lanthanum silicate apatite electrolyte for intermediate temperature solid oxide fuel cell via co-precipitation method

Lanthanum silicate apatite (LSA, La_9.33+xSi₆O_26+1.5x, x = 0–0.67) has been widely investigated as a promising electrolyte material for intermediate temperature solid oxide fuel cell (SOFC). In this work, a facile and low-cost co-precipitation method is used to synthesize LSA precursor powders. The well dispersed nanopowders (ca. 70 nm) with pure hexagonal LSA phase are obtained by calcining the precursor at 900 °C. Impurity of La₂SiO₅, caused by the different precipitation productivities of La(NO₃)₃ and TEOS, can be eliminated through lowering the La/Si ratio in the starting mixtures. The dispersant (PEG200) plays a crucial role in co-precipitation processes, which can effectively mitigate the agglomeration and therefore significantly improve the sinterability of the nanoparticles. Dense LSA ceramic with relative density of 98% is obtained after sintering at 1550 °C, which exhibits a conductivity of 0.13 mS cm⁻¹ at 500 °C.     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Synthesis and Investigation of Solid Solutions Based on Indium Oxide in In<Subscript>2</Subscript>O<Subscript>3</Subscript>–MeO<Subscript>2</Subscript> (Me = Zr,Sn, Ti) Systems

Solid solutions in In₂O₃–MeO₂ (Me = Zr, Sn, Ti) systems based on indium oxide are synthesized by the coprecipitation method. It is found that the ultrasound treatment of the coprecipitation products reduces the degree of agglomeration of the initial particles by a factor of 3 and initiates the crystallization process of the precipitates. Nanocrystal (5–8 nm) precursor powders are obtained at 400°C. The optimal regime for sintering powders based on In₂O₃ to form a ceramic with a dense microstructure is chosen. The influence of the temperature, alloying additives, and the partial pressure of oxygen on the specific conductivity of indium oxide solid solutions is studied.     

Synthesis and characterization of nano MgAl2O4 spinel by the co-precipitated method

Magnesium aluminate (MgAl₂O₄) spinel powders were prepared by co-precipitation of stoichiometric amounts of magnesium and aluminum chlorides at 80 °C. Some sintering aids such as ZnO and MnO₂ were added in the form of chlorides during the precipitation to study their effect on densification. The co-precipitated materials were a mixture of Mg–Al double hydroxide with the presence of few amounts of gibbsite and brucite. After heat-treatment of the precipitated powders up to 1000 °C, a crystalline spinel powder was obtained. The presence of 0.5, 1, 2 and 3 wt.% of ZnO or MnO₂ as sintering aids increased sinterability after firing up to 1550 °C. The highest density was obtained for the samples containing 2 wt.% ZnO or 3 wt.% MnO₂ which reached about >94 and 96% theoretical density (TD), respectively. The mechanical properties increased by adding ZnO or MnO₂, an exception being the sample containing 0.5 wt.% of ZnO for which relatively smaller value were obtained.     

Processing of dense high-entropy boride ceramics

Dense (Hf_0.2,Zr_0.2,Ti_0.2,Ta_0.2,Nb_0.2)B₂ high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B₄C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B₂ with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.     

Fabrication and properties of Co:MgAl2O4 transparent ceramics for a saturable absorber from coprecipitated nanopowder

The 0.05 at.% Co:MgAl₂O₄ precursor was synthesized by the coprecipitation method from a mixed solution of magnesium, aluminum, and cobalt nitrates using ammonium carbonate as the precipitant. 0.05 at.% Co:MgAl₂O₄ transparent ceramics were successfully obtained via vacuum sintering and hot isostatic pressing (HIP) of 0.05 at.% Co:MgAl₂O₄ nanopowder calcined at 1100°C for 4 hours. The properties of powder and ceramics were comprehensively investigated. X-ray diffraction (XRD) results showed that Co:MgAl₂O₄ nanopowder had a pure spinel phase. Also, the in-line transmittances of the HIP posttreated Co:MgAl₂O₄ ceramics with the thickness of 1.2 mm were 82% at 400 nm and 84.7% at 900 nm. The average grain sizes of 0.05 at.% Co:MgAl₂O₄ ceramics before and after the HIP posttreatment were 11 and 28 μm, respectively. The calculated ground state absorption cross section of 0.05 at.% Co:MgAl₂O₄ ceramics was 2.9 × 10⁻¹⁹ cm², indicating that this ceramics is a promising material applied as a saturable absorber for passive laser Q-switches in the 1.3-1.7 μm domain.     

Elaboration of CaLa2S4 transparent ceramics from novel precursor powders route

A cost-effective processing route for the production of calcium lanthanum sulfide CaLa₂S₄ (CLS) via a novel fast fabrication of precursor powders is reported. The sinterability of the newly developed powders was investigated by use of Hot-Pressing and pressureless sintering. Complementary techniques (XRD, SEM-EDS, chemical analysis, SSA, FT-IR spectroscopy) were employed to correlate the sintering processes and parameters to the microstructural/compositional developments and optical performances of the densified ceramics. Dense (>99.8% theor.) and homogeneous CLS ceramics were produced by pressureless sintering at 1250 °C for 12 h in H₂S followed by hot-pressing at 1000 °C for 6 h in a powder bed to prevent sulfur loss. Transparency free of impurity absorption has been achieved in the LWIR region (optical transmission of 51% in the 12–14 µm range).     

Modified Pechini Synthesis of Proton‐Conducting Ba(Ce,Ti)O3 and Comparative Studies of the Effects of Acceptors on its Structure,Stability, Sinterability,and Conductivity

The phase structure, chemical stability, sinterability, and electrical performance of the proton‐conducting Ba(Ce,Ti)O₃ solid solution with a series of acceptors M (M = In, Y, Sm) synthesized by a modified Pechini method were systematically investigated. The substitution of cerium with titanium was proved as an effective way to improve the stability of BaCeO₃. Especially for the BaCe_0.95Ti_0.05O_3?δ sample doped with In, no change in the phase was found even after treatment in the atmosphere containing both CO₂ and H₂O at 700°C for 10 h. Thanks to the highly sinteractive powders with particle size of ~100 nm, dense ceramics were easily acquired. Moreover, compared with the undoped BaCe_0.95Ti_0.05O₃ sample, In, Y, and Sm dopants further improved the sinterability of the solid solution. In particular, In played a role of sintering aid and led to the largest linear shrinkage of the ceramics. As to the electrical performance, the transport properties of the samples under various atmospheres were analyzed and compared. The impedance tests demonstrated the best electrical performance of the Y‐doped samples.