Intergranular amorphous films formed by DC electric field in pure zirconia

It is difficult to obtain pure ZrO₂ sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO₂ can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO₂ flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.     

Ceria‐doped scandia‐stabilized zirconia (10Sc2O3·1CeO2·89ZrO2) as electrolyte for SOFCs: Sintering and ionic conductivity of thin,flat sheets

In this study, thin, flat sheets, about 90 μm thick, were prepared from ceria‐doped scandia‐stabilized zirconia with molecular formula 10Sc₂O₃·1CeO₂·89 ZrO₂ (10Sc1CeSZ) by tape casting and subsequent sintering at different thermal cycles. A sintering thermal cycle was selected that yielded defect‐free flat sheets, with practically negligible porosity (between 0.35% and 0.10%) and average grain diameters ranging from 1.32 to 6.30 μm. Ionic conductivity at 600°C was as high as 21 mS/cm. Ionic conductivity increased with average grain diameters up to 2.7 μm. At higher average grain diameters, conductivity remained practically constant.     

The improving thick film properties of Er2O3 doping fcc-ZrO2 type solid electrolyte for SOFC applications

In this study, the binary system of (ZrO₂)_1-x(Er₂O₃)_x was investigated in the doping range of x; 0.02 ≤ x ≤ 0.12 by the Pechini method. According to X-ray diffraction (XRD) measurement results, Er₂O₃ doping face-centered cubic (fcc) ZrO₂-based solid solution was stabilized in the doping range of 0.08 ≤ x ≤ 0.12 at 1200°C for 12 hours. Thick films of fcc-ZrO₂ type powders were produced using ethyl cellulose organic binder mixture and spin-coating method. The crystallographic, microstructural, and electrical conductivity properties of the thick films were characterized via XRD, SEM, and a.c. impedance measurements, respectively. 8-ESZ ((ZrO₂)_1-x(Er₂O₃)_x, x = 0.08) thick film electrolyte showed the highest electrical conduction level which is 2.51 × 10⁻² ohm⁻¹ cm⁻¹ at 850°C under 150 mL min⁻¹ O₂ volumetric flow rate. All thick film properties of fcc-ESZ materials were optimized and improved experimentally for using as a solid electrolyte component in solid oxide fuel cell (SOFC) systems. A pre-treatment of 8-ESZ and the cathode-supported type electrochemical cell were primarily fabricated. The power density measurements of 40-LNF (LaNi_1-xFe_xO₃, x = 0.4) Cathode|Cathode Active (50:50 wt % 40-LNF:8-ESZ)| 8-ESZ Electrolyte|Anode Active (60:40 wt % NiO:8-ESZ)|NiO Anode Electrode cell stack suggest that the produced electrolytes had the usefully properties for SOFC applications.     

The structures and dielectric properties of Ba0.80Sr0.20TiO3/Pb0.82La0.18TiO3 composite thick films with addition of PbO–B2O3 glass

Sol–gel derived Ba_0.80Sr_0.20TiO₃ (BST) and Pb_0.82La_0.18TiO₃ (PLT) powders and a low-melting PbO–B₂O₃ glass powder were mixed to prepare paste. The composite thick films (∼40 μm) were fabricated by screen-printing the paste onto the Al₂O₃ substrates with screen-printed silver bottom electrode and then sintered at the low temperature 650–800 °C, respectively. X-ray diffraction (XRD), transmission microscope (TEM), scanning electron microscope (SEM) and an impedance analyzer were used to analyze the structures, microstructures and dielectric properties of the powders and the composite thick films. The results show that the composite thick films containing sol–gel derived Ba_0.80Sr_0.20TiO₃ and Pb_0.82La_0.18TiO₃ perovskite phases have been fabricated by using the PbO–B₂O₃ glass as a sintering aid. Compared to conventional sintering at ≥1200 °C, high densification of the composite thick films is achieved at temperature as low as 800 °C by the “wetting” and “infiltration” of the liquid phase on the particles. The homogenization of the BST and PLT perovskite phase in the composite thick films is evitable by controlling the sintering temperature and time. The formation of the small amount of pyrochlore phase in composite thick films sintered at 800 °C is resulted from both the volatilization of PbO and the interaction between the PLT and PbO–B₂O₃ glass. The relative dielectric properties of the composite thick films exhibit good temperature-stable behavior, and the variation of the relative dielectric constant is less than 10% in the temperature range 0–300 °C.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

Microwave enhanced sintering of tape-cast ferroelectric films

Porous Pb(Zr_xTi_1−x)O₃(PZT) thick films that had been prepared by tape casting were densified by microwave energy. The microwave absorption effect is substantially correlated with the film thickness. In microwave-processed PZT thick films, rapid particle necking causes densification with no grain growth nearly in a short treatment time of 20 min at 820 °C. The same porous PZT thick films are difficult to densify in a conventional process. A 30-μm-thick PZT thick film has a pure perovskite structure. Self-supporting PZT thick films with a crack-free and uniform microstructure formed in a microwave process have larger coercive field than conventionally processed bulk PZT. The polarization, 14 μC/cm², of PZT thick films in a microwave process exceeds that, 7 μC/cm², of PZT bulk formed in a conventional process.     

Shrinkage rate control during a flash state by current-ramping for 3 mol% Y2O3-doped ZrO2 polycrystals

3 mol% Y₂O₃-doped ZrO₂ green compacts with rectangular shapes were sintered by maintaining the shrinkage rates at constant values under alternating electric fields by ramping the electric current during flash states. Green compacts were furnace-heated under 40-100 V_rms/cm until current hits an initial current limit of 100 mA_rms. After then, current-ramping was started to keep the shrinkage rates constant by increasing the limit current value using a programmable power supply operating in a current control mode. Highly densified 3 mol% Y₂O₃-doped ZrO₂ polycrystals with a density of 6.05 g/cm³ as a bulk density and a grain size of about 0.4 μm were obtained at a furnace temperature of about 930℃, 50 V_rms/cm with 1000 Hz and shrinkage rate of about 120 μm/min (0.8%/min against initial lengths of green compacts). The Vickers hardness and indentation fracture toughness of the compact exhibit similar values to those obtained from thermally sintered compacts.     

Equimolar YO1.5 and TaO2.5 co-doped ZrO2 as a potential CMAS-resistant material for thermal barrier coatings

Calcium-magnesium-alumino-silicates (CMAS) corrosion in thermal barrier coatings (TBCs) is becoming more serious with increasing operation temperature of turbine engines. Here, we report an equimolar YO_1.5 and TaO_2.5 co-doped ZrO₂ (Zr_0.66Y_0.17Ta_0.17O₂, ZYTO) as a potential CMAS-resistant material for TBCs, which shows a significantly enhanced CMAS resistance than the conventional 17 mol% YO_1.5-stabilized ZrO₂ (17YSZ). After exposure at 1300°C for 100 hours, the CMAS infiltration depth in ZYTO bulk is ~80 μm (for a 20 mg/cm² CMAS deposition), in contrast to ~700 μm in 17YSZ bulk (50 hours). Compositional and morphological analyses on the CMAS reaction zone reveal that the excellent CMAS resistance of ZYTO originates from the uniform corrosion through grain and grain boundary, along with densification of the reaction layer. The high CMAS infiltration rate of 17YSZ is attributed to the severe dissolution and infiltration through grain boundary. The reaction mechanisms of CMAS with ZYTO and 17YSZ bulks are discussed and a strategy of enhancing the CMAS resistance is proposed for ZrO₂-based TBC materials.     

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Ferroelectric ZrO2 ultrathin films on silicon for metal-ferroelectric-semiconductor capacitors and transistors

Enhanced ferroelectric properties of nanoscale ZrO₂ thin films by an HfO₂ seed layer are demonstrated in metal-ferroelectric-semiconductor (Si) capacitors and transistors prepared with a low thermal budget of 400 °C. The seeding effect of the HfO₂ layer leads to the enhancement of crystallization into the orthorhombic phase and the increase of remnant polarization of the sub-10 nm ZrO₂/HfO₂ bilayer structure. The ferroelectric field-effect transistor with the ZrO₂/HfO₂ bilayer gate stack reveals a large memory window of ~1.2 V and a steep subthreshold swing below 60 mV/decade. As compared with the Hf_0.5Zr_0.5O₂ thin film, superior ferroelectric properties of the ZrO₂/HfO₂ bilayer structure show great potential for ferroelectric memory devices fabricated on Si substrates.     

Gd2Zr2O7 ceramics synthesized by solid-state reactive sintering: Effects of starting powders with different-scale particle sizes

In this work, the effects of starting oxide powders with different-scale particle sizes on the synthesis of gadolinium zirconate pyrochlore (Gd₂Zr₂O₇, GZO) and its physical properties were studied. Micron Gd₂O₃ (μG), micron ZrO₂ (μZ), nano Gd₂O₃ (nG), and nano ZrO₂ (nZ) powders were used. GZO ceramics were prepared by employing solid-state reactive sintering at 1300 °C, 1400 °C, 1500 °C and 1600 °C with mixed powders of different sizes (μGμZ, μGnZ, nGμZ and nGnZ). X-ray diffraction and Raman analyses of the ceramics revealed that nG has a more significant impact on the crystallization process than nZ. All ceramics synthesized with different sized oxide powders crystallized into pyrochlore phases except for those synthesized with μGnZ mixed powders, which resulted in a fluorite phase. The results indicated that decreasing the particle size of only ZrO₂ to synthesize pyrochlore-phase Gd₂Zr₂O₇ with high crystallinity may not be effective. Samples obtained at 1500 °C were further analyzed. Scanning electron microscopy results revealed that all four ceramics have a non-homogeneous grain size and that the average grain size ranges from 5.40 to 8.30 μm. In addition, the density and Vickers hardness measurements showed that the use of nanopowders significantly improves the mechanical properties.     

Microstructure evolution and electromechanical properties of (K,Na) NbO3-based thick films

This study investigated an unconventional method of electrophoretic deposition (EPD) for the processing of environmentally benign (K_0.5Na_0.5)_0.99Sr_0.005NbO₃ (KNNSr) thick films on Pt/alumina substrate. EPD allows rapid, economical, and low-waste processing of thick films and thus offers an integration advantage for electronics manufacturing. To understand the functional response of the KNNSr thick films, the effect of the sintering temperature and atmosphere on their structure, microstructure, and electromechanical properties was investigated. KNNSr thick films densify in constrained conditions in a very narrow temperature range only a few 10°C below the melting temperature of 1140°C. Up to 1100°C the relative density increases to 80%, upon further heating to 1110°C we observed only the grain growth and pore coalescence. The densification is not affected significantly by the atmosphere. The local domain structure of 25-33 μm thick KNNSr films was similar, while the dielectric and electromechanical properties increased with the increasing sintering temperature. KNNSr thick film sintered at 1100°C has a thickness-coupling factor k_t of 0.4, comparable to that of bulk. The results reveal that the EPD enables the economic processing of high-performance thick films on complex-shape substrates that are difficult to fabricate using conventional thick-film methods.     

Effect of Ga2O3 on Sintering and Phase Stability of Sc2O3‐Doped Tetragonal Zirconia Prepared via Aqueous Solution Route

The effects of the presence of Ga₂O₃ on low‐temperature sintering and the phase stability of 4, 5, and 6 mol% Sc₂O₃‐doped tetragonal zirconia ceramics (4ScSZ, 5ScSZ, and 6ScSZ, respectively) were investigated. A series of zirconia sintered bodies with compositions (ZrO₂)_0.99?x(Sc₂O₃)_x(Ga₂O₃)_0.01, x = 0.04, 0.05, and 0.06 was fabricated by sintering at 1000°C to 1500°C for 1 h using fine powders that were prepared via the combination of homogeneous precipitation method and hydrolysis technique using monoclinic zirconia sols synthesized through the forced hydrolysis of an aqueous solution of zirconium oxychloride at 100°C for 168 h. The presence of 1 mol% Ga₂O₃ was effective in reducing sintering temperature necessary to fabricate dense bodies and enabled to obtain dense sintered bodies via sintering at 1100°C for 1 h. The phase stability, that is, low‐temperature degradation behavior of the resultant zirconia ceramics was determined under hydrothermal condition. The zirconia ceramics codoped with 1 mol% Ga₂O₃ and 6 mol% Sc₂O₃ (1Ga6ScZ) fabricated via sintering at 1300°C for 1 h showed high phase stability without the appearance of monoclinic zirconia phase, that is the tetragonal‐to‐monoclinic phase transformation was not observed in the 1Ga6ScZ after treatment under hydrothermal condition at 150°C for 30 h.     

Lead zirconium titanate thin films prepared by thermal annealing of multilayer structures composed of PbO,ZrO2 and TiO2

Lead zirconium titanate [Pb(Zr_xTi_1?x)O₃ or PZT] thin films were prepared by the thermal annealing of multilayer films composed of binary oxide layers of PbO, ZrO₂ and TiO₂. The binary oxides were deposited by metal organic chemical vapor deposition. An interdiffusion reaction for perovskite PZT thin films was initiated at approximately 550 °C and nearly completed at 750 °C for 1 h under O₂ annealing atmosphere. The composition of Pb/Zr/Ti in perovskite PZT could be controlled by the thickness ratio of PbO/ZrO₂/TiO₂ where the contribution of each binary oxide at the same thickness was 1:0.55:0.94. The electrical properties of PZT (Zr/Ti = 40/60, 300 nm) prepared on a Pt-coated substrate included a dielectric constant ?_r of 475, a coercive field E_c of 320 kV/cm, and remnant polarization P_r of 11 μC/cm² at an applied voltage of 18 V.     

Giant dielectric constant and tunable phase-transition characteristics of ZnF2/BaF2-modified BaTiO3 thick films

High performance capacitors have been investigated to meet higher integration density with optimized charge-storing capability. Here, we introduce nonconventional thick film dielectric compositions based on 95BaTiO₃–xZnF₂–(2−x)BaF₂–3glass (x = 0.5, 1.0, and 1.5) where the relative content of ZnF₂/BaF₂ is critical in controlling dielectric behavior. The thick films were prepared on Cu foils by regular screen-printing and then firing at 950°C in inert atmosphere. As an optimal example, thick film composition modified with 1.0ZnF₂/1.0BaF₂ exhibited a dielectric constant of ~1903 and a dielectric loss of ~0.04 at 1 MHz with dispersive dielectric relaxation behavior, which are far better than any reported corresponding values so far. Particularly, it was very interesting to observe that Curie temperature was tunable from −19 to +34°C, depending on the relative content of fluoride additives only within the 2 mol% range. Dependency of the relative contents of the fluorides is primarily investigated with regard to microstructure and dielectric properties.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Phosphor‐in‐glass thick film formation with low sintering temperature phosphosilicate glass for robust white LED

Phosphor‐in‐glass (PiG) thick film was fabricated on a borosilicate glass substrate using a conventional screen printing method and employing phosphosilicate glass to allow low‐temperature sintering. The vehicle content and sintering temperature were optimized to form a thick film with a thickness of ~50 μm. Commercial yellow (Y₃Al₅O₁₂:Ce³⁺) and red (CaAlSiN₃:Eu²⁺) phosphors were successfully incorporated within the glass matrix and then sintered at 550°C. Color‐tunable white LEDs were achieved using the PiG thick films as a color converter by varying the glass to phosphor (GtP) ratio. The high luminous efficacy of up to ~120 lm/W and high color rendering index of up to 89 in combination with the thermal quenching property prove the practical feasibility of the PiG thick films for high‐power/high‐brightness LED applications.     

Microstructure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of Sc‐Modified BiFeO3–BaTiO3 Multiferroic Ceramics with MnO2 Addition

0.725BiFe_1?xSc_xO₃–0.275BaTiO₃ + y mol% MnO₂ multiferroic ceramics were fabricated by a conventional ceramic technique and the effects of Sc doping and sintering temperature on microstructure, multiferroic, and piezoelectric properties of the ceramics were studied. The ceramics can be well sintered at the wide low sintering temperature range 930°C–990°C and possess a pure perovskite structure. The ceramics with x/y = 0.01–0.02/1.0 sintered at 960°C possess high resistivity (~2 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 19.1–20.4 μm/cm²), good piezoelectric properties (d₃₃ = 127–128 pC/N, k_p = 36.6%–36.9%), and very high Curie temperature (618°C–636°C). The increase in sintering temperature improves the densification, electric insulation, ferroelectric, and piezoelectric properties of the ceramics. A small amount of Sc doping (x ≤ 0.04) and the increase in the sintering temperature significantly enhance the ferromagnetic properties of the ceramics. Improved ferromagnetism with remnant magnetization M_r of 0.059 and 0.10 emu/g and coercive field H_c of 2.51 and 2.76 kOe are obtained in the ceramics with x/y = 0.04/1.0 (sintered at 960°C) and 0.02/1.0 (sintered at 1050°C), respectively. Because of the high T_C (636°C), the ceramic with x/y = 0.02/1.0 shows good temperature stability of piezoelectric properties. Our results also show that the addition of MnO₂ is essential to obtain the ceramics with good electrical properties and electric insulation.     

Granule spray process for fabrication of adherent,low thermal conductivity ceramic coatings

This study proposes an effective strategy to fabricate ceramic thick films with controllable porosity but strong mechanical adhesion. The technique uses pre-heat-treated granules for a dry spray coating process, granule spray in vacuum. For proof-of-concept, Y₂O₃ spherical granules were prepared and subsequently pre-heat-treated at various temperatures to control their strength. During film deposition, hard granules (i.e., pre-heated at 1400 °C) caused damage to the substrate and pre-deposited film, owing to a strong hammering effect, which lead to limited film growth (1–2 μm). Contrarily, soft granules (i.e., pre-heated below 800 °C) produced a powder compact with low adhesion, resulting in delamination of some parts. This is attributed to the reduction in kinetic energy caused by the elastic cushioning effect during deposition. Regarding the granules with appropriate strength (i.e., pre-heated at 1000 °C), relatively porous (relative density: 74%) but mechanically well-adhering (adhesion strength: 41 MPa) thick films (~60 μm) were successfully coated on Al substrates. These films exhibited four times higher adhesion strength than those prepared with Y₂O₃ coatings using atmospheric plasma spraying. The films showed low thermal conductivities (~0.76 W/m·K at R.T.), suggesting potential application of our approach in the field of thermal insulation.     

Correlation between sintering conditions and water contact angles for Ti–O thick films screen printed on an alumina substrate

TiO₂, TiO_2−x and Ti₃O₅ thick-film structures on corundum Al₂O₃ substrates were prepared using screen-printing technology. The screen-printed deposits were sintered up to 1500 °C in oxidising and reducing atmospheres to vary the Ti⁴⁺/Ti³⁺ ratio and consequently water contact angle. The structure of the thick films was studied with an X-ray powder diffractometer (XRD). The microstructural characteristics and the chemical composition were checked with a scanning electron microscope, equipped with an energy-dispersive spectrometer (EDS). The Ti–O films, up to 55 μm thick, exhibited excellent adhesion to the substrate and had uniform grain- and pore-size distributions. Ti₃O₅ and Al₂O₃ were found to be compatible phases up to 1500 °C in a reducing atmosphere. However, rutile-type TiO₂ and Al₂O₃ are not compatible compounds at temperatures up to 1400 °C, in either oxidising or reducing atmospheres. TiO₂ and TiO_2−x form two types of reaction products with Al₂O₃. These reaction products were found to have various Ti/Al ratios.