Solidification structure in pure zirconia liquid molten phase

Up to now few studies concerning the crystallographic features of zirconia materials (HZ) used as refractory material have been performed. The elaboration process through raw material fusion followed by a controlled cooling step lead to a material containing 94 wt.% of dendritic monoclinic zirconia. During cooling, ZrO₂ passes through three different structural phase transitions (SPTs), inducing formation of a complex microstructure made of different crystallographic domains called variants. This study focus on formation of these different monoclinic variants in a non-doped zirconia based material and the possibility to reconstruct initial cubic parent crystals formed at high temperature using optical microscopy and Electron Back Scattering Diffraction (EBSD). EBSD experiments allow to identify 24 monoclinic variants issued from tetragonal parent crystal and to highlight the contours of the parent cubic crystal. Then, the microstructure appearing during the first steps of the solidification of the material can be evidenced.     

Effect of zirconia particle size on the properties of alumina-spinel castables

The industrial application of alumina-spinel refractory castables has crucial requirements on the service performance. Thus, the effects of different sized desilicated zirconia particles on the castables microstructure, thermal-mechanical properties and high temperature elastic modulus have been investigated. The zirconia particle sizes were varied from 1000 µm to 2.5 µm (d₅₀). It was observed that the finer (below 88 µm) zirconia particles were beneficial to improve the cold modulus of rupture (CMOR) and the hot modulus of rupture (HMOR), but could not effectively enhance the thermal shock resistance. Fine zirconia particles can homogeneously disperse in the matrix and significantly promote the sintering process. Accompanied with the phase transformation of zirconia, both the high density of matrix cracks and the strong ceramic bonding (between the coarse grains and the matrix) were found in the refractory castables, which was responsible for an increase of CMOR. However, the binding characteristic could also give rise to the high stored elastic energy that was adverse to the thermal shock resistance, and the excessive amount of preexisting matrix cracks could induce more microdamage during the thermal shock. Additionally, it was proposed that the second-phase dispersion reinforcement and the highly ceramics bonding resulted in the superior HMOR when introducing fine ZrO₂ particles.     

Thermodynamic calculations of t to m martenstic transformation of ZrO2–CaO binary system

The phase transformation of different polymorphs in zirconia is very important for the processing and mechanical properties of zirconia ceramics. This paper adopts thermodynamic model which is suitable for ceramic system to calculate the Gibbs free energy change of tetragonal and monoclinic phases in ZrO₂–CaO binary system. The difference of the Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CaO as a function of composition and temperature, namely t → m phase transformation driving force, is thermodynamically calculated from the binary systems. Furthermore, in 8.0 mol% CaO–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, was obtained as 1270.3 K, and martensitic transformation starting temperature (M_s) for t → m transformation of this ceramic with a mean grain size of 2.0 mm was calculated as 805.9 K, which is good agreement with experiment one of 793 K with 12.9 K residual.     

Toughening mechanisms of ZTA ceramics at cryogenic temperature (77 K)

ZTA ceramics containing 20 wt% ZrO₂ were fabricated at different sintering temperatures (1450, 1500 and 1550 °C) by SPS and HP processes, respectively. The influence of sintering process on the mechanical properties of ZTA ceramics at 298 K and 77 K was investigated. It can be seen that the bending strength and fracture toughness of samples prepared by the two processes both improved at cryogenic temperature. The stress-induced martensitic transformation toughening mechanism was confirmed by the in-situ Raman technique. The tetragonal ZrO₂ would be even more easy to transform because of the residual stress generated when temperature decreased from 298 K to 77 K. Therefore, the transformation toughening effect would become stronger, result in the increase of mechanical properties.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Acoustic emission studies of low thermal expansion aluminum-titanate ceramics strengthened by compounding mullite

Mullite (Mu) with high strength was compounded into aluminum-titanate (AT) ceramics with low thermal expansion to increase their strength. For the AT–Mu system composites, thermal contraction and expansion and acoustic emission (AE) event count rate were measured during cooling using the AE technique and the characteristics of AT–Mu composites were evaluated. The expansion due to microcracking in the range of AE count peak temperatures to room temperature was obtained and the crack volume was estimated from the expansion by cracking. A linear relation with a very high correlation (r = 0.993) was observed between bending strength and crack volume at room temperature. From the linear plot, the bending strength at crack-free temperature in the best AT–Mu composite was shown to be 130 MP.     

Glasses for laser joining of zirconia ceramics

Laser joining of ZrO₂ ceramics using glasses and glass ceramics as sealing components requires optimized systems. The ternary systems SiO₂–BaO–B₂O₃ and BaO–SrO–SiO₂ were selected as a basis for development of suitable glass compositions for the laser joining process. Additives such as CaO, TiO₂, Al₂O₃, and MgO were used to control the crystallization processes and hence the thermal expansion coefficients during glass synthesis. The glass viscosity, the strength of the ceramic-glass-ceramic joint, and the joint tightness are other important glass properties which were optimized for the laser process. For glass G018-345, this yielded strengths of up to 225 MPa (Weibull modulus of m = 8.6) and He leak rates of up to 4.3 × 10⁻⁵ mbar l s⁻¹. Because of the varying viscosities obtained, the optimized glass systems could be used selectively in a temperature range of 700–900 °C.     

Investigation of optical,mechanical, and thermal properties of ZrO2-doped Y2O3 transparent ceramics fabricated by HIP

Highly transparent ZrO₂-doped Y₂O₃ ceramics were successfully synthesized using the hot isostatic pressing (HIP) process. The effects of the ZrO₂ content on the sintering behaviors, optical transmission spectra, Vickers hardness, grain size, size distribution, and Raman spectra were determined. The results indicated that decreased ZrO₂ content could promote increased transmittance, red-shifted infrared cutoff wavelength, increased thermal conductivity, decreased Vickers hardness, and increased lattice ordering. According to the optical transmission spectra, the optimized ZrO₂ content was 0.50 at%, at which point the ceramic exhibited a larger pre-sintering temperature range of 1650–1750 °C and the average grain size of 3.35 µm at 1750 °C. The grain size was significantly decreased at lower pre-sintering temperatures. Furthermore, a moderate Vickers hardness of 8.42 GPa and high thermal conductivity of 10.85 W/m K at room temperature were obtained for the optimized ceramic.     

Molten salt attack on t′ yttria-stabilised zirconia by dissolution and precipitation

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na₂SO₄–30 mol% NaVO₃ on ZrO₂–8 mol% YO_1.5 (8YSZ) at 950 °C was studied by two types of experiments. Sintered compacts were exposed to 25 mg cm^?2 salt dosage for up to 96 h. In the other set of experiments, 10–35 wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25 wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950 °C, followed by slow reactions to form YVO₄ and NaYV₂O₇ at 950 °C. The unreacted Y₂O₃ and monoclinic ZrO₂ precipitate out separately during rapid cooling (～300 °C/min). Slow cooling at ～3 °C/min leads to the formation of ZrOS apart from ZrO₂ and Y₂O₃.     

The ablation behavior of ZrB2/Cu composite irradiated by high-intensity continuous laser

Ultra high temperature ceramics (UHTCs) based composite ZrB₂/20 vol.% Cu was prepared by spark plasma sintering (SPS) at 1650 C° for 3 min. The ablation behavior of composite irradiated for 2–20 s by 20 MW/m² high-intensity continuous laser was investigated. The phase and microstructure evolution of ZrB₂/20 vol.% Cu during ablation was demonstrated by XRD and SEM, respectively. The results reveal that no macroscopic damage but only one ablated layer with 40 μm in thickness is observed even after being ablated for 20 s. It implies that ZrB₂/20 vol.% Cu composite exhibits good ablation resistance against high-intensity continuous laser. The continuous Cu in composite evaporates preferentially, which impacts on the following ablation behavior. The generated ZrO₂ at the spot center shows different forms such as closely packed nano-ZrO₂, micron ZrO₂ or melting ZrO₂ for different ablation time. The melting ZrO₂ is helpful to promote the ablation resistance of ZrB₂/20 vol.% Cu.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Phase formation during crystallization of a Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent – An X-ray diffraction and (S)TEM-study

Glass ceramics based on the system Li₂O/Al₂O₃/SiO₂ (LAS) often show a coefficient of thermal expansion close to zero. Although these glass-ceramics are of high economic importance, the fundamentals of the crystallization process are still not fully understood. In this paper, the effect of ZrO₂ addition as a sole nucleation agent on the crystallization of the LAS glass is described predominantly using transmission electron microscopy and X-ray diffraction. The composition of the studied green glass was close to that of the commercially available Robax™ glass (Schott AG), which, however, contained both, ZrO₂ and TiO₂ as nucleating agents. It was found that during thermal treatment, in a first step, already at temperatures around 10–20 K below the glass transition temperature, T_g, ZrO₂ nanocrystals with sizes in the range from 5 to 15 nm were precipitated. The next crystalline phase that forms during the crystallization process was LAS with a structure similar to the hexagonal high temperature phase of quartz. These crystals were much larger than the ZrO₂ crystals. If thermal treatment was carried out at higher temperatures, a dense network of LAS crystals was formed. Differently shaped crystals in samples with different thermal history were visualized, and an enrichment of Ba and Sb in the residual glass phase in the late stages of thermal treatment was found. Also, an enrichment of aluminum around the ZrO₂ crystals was evident, which is a hint at a preceding droplet phase separation from which the ZrO₂ crystals were precipitated. The crystallization is notably different from that of mixed ZrO₂/TiO₂ nucleating agents used in commercial lithium alumosilicate glass ceramics.     

Room-temperature UV-ozone assisted solution process for zirconium oxide films with high dielectric properties

A facile, low-cost, and room-temperature UV-ozone (UVO) assisted solution process was employed to prepare zirconium oxide (ZrO_x) films with high dielectric properties. ZrO_x films were deposited by a simple spin-coating of zirconium acetylacetonate (ZrAcAc) precursor in the environment-friendly solvent of ethanol. The smooth and amorphous ZrO_x films by UVO exhibit average visible transmittances over 90% and energy bandgap of 5.7 eV. Low leakage current of 6.0 × 10⁻⁸ A/cm² at 3 MV/cm and high dielectric constant of 13 (100 Hz) were achieved for ZrO_x dielectrics at the nearly room temperature. Moreover, a fully room-temperature solution-processed oxide thin films transistor (TFT) with UVO assisted ZrO_x dielectric films achieved acceptable performances, such as a low operating voltage of 3 V, high carrier mobility of 1.65 cm² V⁻¹ s⁻¹, and on/off current ratio about 10⁴–10⁵. Our work indicates that simple room-temperature UVO is highly potential for low-temperature, solution-processed and high-performance oxide films and devices.     

One-pot synthesis of monoclinic ZrO2 nanocrystals under subcritical hydrothermal conditions

This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO₂) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO₃)₂) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 °C for 24 h to obtain pure monoclinic-structured ZrO₂ nanocrystals. The crystallization temperature of the ZrO₂ phase was also greater than 200 °C. However, the products of reactions conducted at 200 °C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 °C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO₂ nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.     

Fabrication of structure-controlled hydroxyapatite/zirconia composite

The structure-controlled hydroxyapatite/zirconia (HAp/ZrO₂) composites were fabricated. At first, cylindrical hydroxyapatite (HAp) samples were prepared by the extrusion process, and then the extruded HAp cylindrical samples were coated with 3 mol% of Y₂O₃ partially stabilized ZrO₂ slurry, dried and aligned unidirectionally to form a composite bulk. The volume fraction of ZrO₂ in the HAp/ZrO₂ composite was estimated to be about 23 vol%. Bulk density and bending strength of the composites increased with sintering temperature. Fracture energy of HAp/ZrO₂ composite sintered at 1350 °C was approximately 1.6 times higher than that of monolithic HAp. Although the bending strength of HAp/ZrO₂ composite prepared in this study was relatively low, it exhibited high fracture energy than HAp monolithic and a non-brittle fracture behavior was obtained without using fiber as the reinforcement.     

Fabrication of well-crystallized mesoporous ZrO2 thin films via Pluronic P123 templated sol–gel route

Mesoporous zirconia (ZrO₂) thin films were prepared by dip-coating via Pluronic P123 templated sol–gel route. ZrOCl₂·8 H₂O was used as zirconium (Zr) precursors. Annealing of as-coated ZrO₂ thin films is important in order to stiffen the respective films and to remove the Pluronic P123. The mesoporous structure and crystallite size of ZrO₂ were characterized systematically by field-emission scanning electron microscope (FESEM), both low- and wide-angle X-ray diffraction, thermal analysis technique and Brunauer–Emmett–Teller method. At annealing temperature of 400 °C, amorphous ZrO₂ was transformed into tetragonal phase of ZrO₂ (t-ZrO₂). At 450 °C, t-ZrO₂ and monoclinic phase of ZrO₂ (m-ZrO₂) were obtained. By altering heating rate during annealing, volume fraction of t-ZrO₂ and m-ZrO₂ was changed. FESEM images showed that disordered mesostructures of ZrO₂ were formed after annealing. The surface area of mesoporous ZrO₂ obtained ranges from 54.33 to 93.39 m²/g.     

Preparation,microstructure and mechanical properties of ZrB2–ZrO2 ceramics

ZrB₂–ZrO₂ ceramics with ZrO₂ content varied from 15 to 30 vol.% were prepared by hot pressing. The content of ZrO₂ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂–ZrO₂ ceramics. ZrB₂–30 vol.% ZrO₂ provided the optimal combination of dense microstructure (2.6 μm, as the average grain size) and excellent properties, including the flexural strength of 803 MPa, and the hardness of 22.7 GPa tested under 9.8 N. The highest t-ZrO₂ transformability of 35.2 vol.% during fracture for ZrB₂–30 vol.% ZrO₂ brought the best toughness of 6.5 MPa m^1/2 compared with any other ceramic. In addition, the dependence of toughness on the test method as well as the hardness on the indentation load was also investigated.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Ball milling assisted hydrothermal synthesis of ZrO2 nanopowders

The formation of ZrO₂ nanopowders under various hydrothermal conditions such as temperature, time, autoclave rotation speed, heating rate and particularly assistance of ball milling during reaction was investigated. Full ZrO₂ formation (with monoclinic phase) from zirconium solution was completed at shorter times with increasing temperature such as after 4 h at 150 °C, 2 h at 175 °C and less than 2 h at 200 °C. Crystallite size increased from 2.9 to 4 nm with increasing reaction temperature from 125 °C to 200 °C, respectively. Ball milling assisted hydrothermal runs were performed to understand the effect of mechanical force on phase formation, crystallinity and particle size distribution. Monoclinic ZrO₂ was formed in both milled and non-milled runs when zirconium solution was used. Mean particle size for the 2 M solution was measured to be 94 nm for the milled and 117 nm for the non-milled powders. However, when amorphous aqueous zirconia gels (precipitated at pH 5.8) were used, tetragonal phase was also formed in addition to monoclinic phase. Mean particle size was measured to be 0.7 μm (d₉₀≅1.3 μm) for the milled and 7.9 μm (d₉₀≅13 μm) for the non-milled powders. Ball milling during hydrothermal reactions of both zirconium solution and aqueous zirconium gel resulted in smaller crystallite size and mean particle size and, at the same time, effectively controlled particle size distribution (or agglomeration) of nanopowders.