Solid-state sintering of core-shell ceramic powders fabricated by particle atomic layer deposition

The properties of technical ceramics are highly dependent on their microstructure, which evolves during sintering. Sintering is the process by which ceramic parts are subjected to high temperatures to activate chemical diffusion and the consumption of porosity. During the initial stage of sintering, interparticle necks between neighboring particles form and subsequently increase in size, consuming porosity as the particle centers move closer together. To experimentally determine how this process depends on particle surface composition, particle atomic layer deposition (ALD) was used to deposit a thin film of amorphous aluminum oxide (Al₂O₃) onto yttria-stabilized tetragonal zirconia (3YSZ) particles, producing core-shell structured powders. The uniformity of the Al₂O₃ film was confirmed with transmission electron microscopy and energy dispersive spectroscopy. Scanning electron microscopy was used to observe microstructural evolution during sintering, and the dihedral angles of Al₂O₃ and 3YSZ grains were measured to determine the ratio of interfacial energies between the 3YSZ|3YSZ, 3YSZ|Al₂O₃, and Al₂O₃|Al₂O₃ interfaces. Analysis of the densification kinetics revealed that the initial stage of densification is dependent on the material at the surface of the particles (ie, the Al₂O₃ film) and is controlled by the diffusion of Al³⁺ cations through Al₂O₃. Once the Al₂O₃ film has coalesced, the sintering behavior is controlled by the densification of the core material (3YSZ). Thus, core-shell powders fabricated by particle ALD sinter by a two-step process where the kinetics are dependent on the material present at interparticle contacts.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Effect of Alumina on the Structure and Mechanical Properties of Spark Plasma Sintered Boron Carbide

Uniform densification of relatively thick (~7 mm) consolidated boron carbide plates at relatively low temperatures (e.g. 1800°C) and low facture toughness are two of the primary challenges for further development of boron carbide applications. This work reports that these two challenges can be overcome simultaneously by adding 5 wt% alumina as a sintering aid. Nearly fully dense (97%), fine grained boron carbide (B₄C) samples were produced using spark plasma sintering at 1700°C and above in the B₄C‐5 wt% Al₂O₃ system. The alumina and boron carbide matrix reacted to form an Al₅O₆BO₃ (a mullite‐like phase) during sintering. The Al₅O₆BO₃ phase facilitated uniform densification via liquid phase sintering. This secondary phase is dispersed throughout the intergranular pores, providing obstacles for crack propagation and resulting in tougher boron carbide ceramics.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

Microstructure and improved mechanical properties of Al2O3/Ba-β-Al2O3/ZrO2 composites with YSZ addition

Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were fabricated by solid-state reaction sintering of Al₂O₃, BaZrO₃, and yttria stabilized zirconia (YSZ) powders. The effects of YSZ addition on microstructure and mechanical properties have been investigated. The incorporation of YSZ promoted the densification of the composites and formation of tetragonal ZrO₂ phase. The microstructure of the composites was characterized by elongated Ba-β-Al₂O₃ phase and equiaxed ZrO₂ particles including added YSZ and reaction-formed ZrO₂. The Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites with YSZ addition exhibited improved fracture toughness, as a result of multiple toughening effects including crack deflection, crack bridging, crack branching, and martensitic transformation of ZrO₂ formed by the reactions between Al₂O₃ and BaZrO₃. Moreover, owing to the grain refinement of Al₂O₃ matrix, dispersion strengthening of the added YSZ particles, and an increase in density of the composites, the Vickers hardness and flexural strength of Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were dramatically enhanced in comparison with the composites without YSZ addition.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Effect of waste-derived MA spinel on sintering and stabilization behavior of partially stabilized double phase zirconia

Cubic-stabilized zirconia ceramic composites have been synthesized by conventional sintering, starting from commercial m-ZrO₂, Y₂O₃, and waste-derived magnesium aluminate spinel (MA) powders. In this work, the effect of sintering temperature and MA content on stabilization and densification properties of YSZ have been duly considered. MA-free YSZ0 composite sintered at 1600°C-1700°C revealed m- and t-ZrO₂ dual-phase structure where its m-ZrO₂ was partially stabilized upon temperature rising into tetragonal phase by Y³⁺ diffusion inside zirconia structure. YSZ10-50 composites containing 10-50 wt% MA demonstrated dissimilar behavior where their m-ZrO₂ was transformed and stabilized into a cubic form by diffusion of Y³⁺, Mg⁺², and Al⁺³ inside zirconia lattice. Furthermore, densification of YSZ10-50 powder mixtures by conventional sintering at 1600°C for 2 hours resulted in fully dense compacts with micrometer-sized grains. The outcomes indicate that MA has a significant effect on m-ZrO₂ stabilization into the cubic phase structure at room temperature. In this respect, this study offers huge potentials for developing fully stabilized c-ZrO₂ ceramics that could be possibly used as industrial ceramics for structural applications of harsh chemical and thermal environmental conditions.     

In-situ fabrication of Al2O3–SiC nanocomposites using B2O3 as sintering aid

Al₂O₃–SiC nanocomposites with 5 and 10 vol% SiC have been in-situ fabricated by sol-gel method followed by carbothermal reduction of alumina–silica gel using B₂O₃ as sintering aid. Green bodies were formed by cold isostatic pressing of calcined gel, which was prepared by an aqueous sol-containing aluminum chloride, TEOS, sucrose and boric acid. Pressureless sintering was carried out in Ar–12%H₂ atmosphere at 1700 °C. Addition of B₂O₃ (1 or 3 wt%) was an effective densification aid in the Al₂O₃–5 vol% SiC composites, while the densification of Al₂O₃–10 vol% SiC composites was not affected by adding B₂O₃. The composite material containing 5 vol% SiC doped with 3 wt% B₂O₃ reached 98.7% of full density. Nano-sized β-SiC particles were formed in-situ by means of a reaction between mullite and carbon at 1600 °C. Scanning electron microscopy revealed that the spherical in-situ synthesized SiC nanoparticles were well distributed through the composite and located predominantly to the interior of alumina matrix grains.     

Sintering and conductivity of Sc-doped CaZrO3 with Fe2O3 as a sintering aid

This paper reports the effect of 0.1–0.5 wt% Fe₂O₃ addition on sintering and electrical properties of CaZr_0.95Sc_0.05O_3-δ ceramics synthesized by combustion method. Addition of the sintering aid was shown to enhance ceramic densification and grain coarsening at a reduced sintering temperature and a shorter holding time (1430 °C, 2 h). Effect of the sintering aid on electrical conductivity of the ceramics was investigated using impedance spectroscopy. The highest total conductivity was achieved for the composition with 0.5 wt% Fe₂O₃; it was about an order of magnitude higher than that of the composition without Fe₂O₃. The effect of Fe₂O₃ addition on the conductivity of the grain interior and grain boundaries has been discussed. It was concluded that ceramic densification, grain coarsening and formation of small amounts of calcium ferrite at the grain boundaries upon Fe₂O₃ addition were responsible for the conductivity enhancement.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

Sintering and grain growth behaviour of magnesium aluminate spinel: Effect of lithium hydroxide addition

Lithium hydroxide, LiOH, in the amounts ranging from 0.1 to 1.2 wt% has been used as a sintering aid to improve the densification of MgAl₂O₄. The addition of 0.3 wt% LiOH promotes densification and limits grain growth. The activation energy of sintering, calculated using master sintering curve approach, decreases from 790 ± 20 kJ.mol^?1 to 510 ± 20 kJ.mol^?1 with the addition of 0.3 wt% of LiOH. In addition, MgAl₂O₄ was also mixed with 10 wt% of LiOH to amplify the formation of reaction products. High-temperature XRD results showed that secondary phases (MgO and LiAlO₂) are produced above 1040 °C. The secondary phases start to disappear at T > 1200 °C, and MgAl₂O₄ is produced. While adding small amounts of LiOH, up to ca. 0.3 wt%, is beneficial for densification and suppressing grain growth, there exists a critical concentration of Li⁺ that is accounted for by the preferential incorporation of lithium ions into MgAl₂O₄ crystal lattice.     

Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

Effect of La2O3 addition and sintering mode on the mechanical properties and microstructural evolution on an 8YSZ ceramic alloy

In this research, the influence of La₂O₃ addition on the microstructure, phase stability and mechanical properties of 8?mol% yttria stabilized zirconia (8YSZ) was studied. 8YSZ with La₂O₃ (9, 12 and 15?wt%) ceramics were fabricated by microwave and conventional sintering at 1400?°C/ 20?min and 1400?°C/ 5?h, respectively. Irrespective of the sintering technique, the relative sintered density was found to decrease with increasing amount of La₂O₃. The grain growth of 8YSZ was enhanced significantly by the addition of La₂O₃. The XRD results demonstrated that addition of La₂O₃ up to 15?wt% did not disrupt the cubic 8YSZ phase regardless of sintering technique; additionally evolution of pyrochlore phase, La₂Zr₂O₇ was observed in all sintered specimens. Vickers hardness of 8YSZ ceramic compacts were also found to decrease with increasing amount of La₂O₃.     

AlN with high strength and high thermal conductivity based on an MCAS-Y2O3-YSZ multi-additive system

The additive composition of an AlN ceramic substrate material was optimized to achieve high strength and thermal conductivity. MgO-CaO-Al₂O₃-SiO₂ (MCAS) glass and Y₂O₃ were used as basic additives for improved sintering properties and thermal conductivity, thereby allowing for AlN to be sintered at a relatively low temperature of 1600 °C without pressurization. Yttria-stabilized zirconia (YSZ) was added (0–3 wt%) to further improve the strength of the AlN ceramic. YSZ and Y₂O₃ reacted with AlN to produce ZrN, Y₄Al₂O₉, and Y₃Al₅O₁₂ secondary phases. The formation of these yttrium aluminate phases improved the thermal conductivity by removing oxygen impurities, while ZrN formed at the AlN grain boundaries provided resistance to grain boundary fractures for improved strength. Overall, the AlN ceramic with 1 wt% MCAS, 3 wt% Y₂O₃, and 1 wt% YSZ exhibited excellent thermal and mechanical properties, including a thermal conductivity of 109 W/mK and flexural strength of 608 MPa.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Hydrolysis protection and sintering of aluminum nitride powders with yttria nanofilms

Aluminum nitride (AlN) is a promising material for electronic substrates and heat sinks. However, AlN powders react with water that adversely affects final part properties and necessitates processing in organic solvents, increasing the cost of AlN parts. Small quantities of yttrium oxide (Y₂O₃) are commonly added to AlN particles to enable liquid phase sintering. To mitigate the reaction of AlN particles with water, particle atomic layer deposition (ALD) was used to coat AlN powders with conformal films of Y₂O₃ prior to densification and powder processing. When AlN particles were coated with 6 nm thick films of amorphous Y₂O₃, the hydrolysis reaction was significantly suppressed over 48 h, demonstrating that Y₂O₃ nanofilms on AlN powders act as a barrier coating in an aqueous solution. AlN powders with Y₂O₃ addition by particle ALD sintered to high relative densities (≥90% theoretical) after sintering at 1800°C for 50 min.     

Alumina Doped Ni/YSZ Anode Materials for Solid Oxide Fuel Cells

The Al₂O₃–Ni–YSZ (Y₂O₃ stabilised ZrO₂) anode materials with 0–6 wt% Al₂O₃ were prepared by tape casting method after being ball‐milled for 48 h. The influence of Al₂O₃ content on flexural strength, electrical conductivity, open porosity, relative density and thermal expansion coefficient (TEC) of Al₂O₃–Ni–YSZ anode was investigated. The introduction of Al₂O₃ significantly enhances the flexural strength of Al₂O₃–Ni–YSZ anode. The flexural strengths of 430 and 299 MPa are achieved for the specimen containing 0.25 wt% Al₂O₃ before and after reduction, respectively, while the flexural strengths are 201 and 237 MPa for the Ni–YSZ samples. The density decreases with increasing Al₂O₃ content and the open porosity increases correspondingly, after being sintered at 1350 °C for 4 h. The electrical conductivity at ambient temperature does not fall off when Al₂O₃ content is less than 1 wt%, but decreases rapidly when the content is above 3 wt% due to the formation of NiAl₂O₄. A maximum electrical conductivity of 1418 S cm^–1 is obtained in the sample containing 0.5 wt% Al₂O₃. The TEC of the samples decreases with the introduction of Al₂O₃ in the temperature range of 20–850 °C.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.