Addition of carbon nanotubes during the preparation of zirconia nanoparticles: influence on structure and phase composition

Zirconia powder with an average particle size of 10 nm was prepared by adding carbon nanotubes (CNTs) in the hydrolytic process of ZrO(NO₃)₂. The formation of zirconia nanoparticles was attributed to the addition of CNTs, which affected the zirconia precursor structure. The relation between the stability of cubic (c) and tetragonal (t) phase zirconia at room temperature, and the zirconia particle size and addition of carbon was studied. It was revealed that zirconia particles with nanometer size had took the form of t-phase and that the addition of carbon tended to favor m-phase ZrO₂ to c-phase ZrO₂ transformation and c-phase ZrO₂ stabilization at low temperature.     

Highly translucent and strong ZrO2-SiO2 nanocrystalline glass ceramic prepared by sol-gel method and spark plasma sintering with fine 3D microstructure for dental restoration

Balance of better mechanical strength and good translucency for dental restorative materials is always a challenge. A translucent glass ceramic/ceramic with improved mechanical properties or a strong glass ceramic/ceramic with good translucency would therefore be interesting for dental application. Nanocrystalline glass ceramics (NCGC) attract a lot attention because of their superior optical and mechanical properties. This study aims to obtain ZrO₂-SiO₂ nanocrystalline glass-ceramic that possesses high mechanical strength as well as excellent translucency by controlling the content, size, and connection of nanocrystalline ZrO₂ in a ZrO₂-SiO₂ glass-ceramic material. Toward this end, well-homogenized nano-powders with three different compositions, 45%ZrO₂-55%SiO₂ (molar ratio, 45Zr), 55%ZrO₂-45%SiO₂ (55Zr), and 65%ZrO₂-35%SiO₂ (65Zr)_, were synthesized, followed by a fast sintering process. Highly-translucent nanocrystalline glass ceramics composed of tetragonal ZrO₂ were obtained. Samples with high zirconia content showed that the structure of the skeleton was predominately built by nano-sized ellipsoidal ZrO₂ particles bonded by grain boundaries, with amorphous SiO₂ filling the voids between the ZrO₂ particles. The achieved flexural strength measured by piston-on-three-ball test was as high as 1014 MPa. To our knowledge, this is one of the highest flexural strength values of glass ceramics ever reported, which is higher than transparent zirconia and alumina ceramics. The 3D structure of nanocrystalline zirconia in silica matrix did enhance the flexural strength of the NCGC. The results of this study suggest that the new ZrO₂-SiO₂ NCGC has great potential of using as dental restoration.     

The characteristic and osteogenic effect of a nanoporous coating of zirconia implant

Zirconia ceramics have become increasingly used as dental implant material which is mainly due to their “tooth-like” color, superior biocompatibility, and excellent mechanical properties. However, the bio-inertness of the zirconia surface limits its use for a wider range of clinical applications. To enhance the surface bioactivity, a combination of magnetron sputtering, and anodization has in the present study been used to deposit a homogeneous nano-porous coating (ZrNP) on the zirconia surface (forming ZrO₂/ZrNP with a ZrNP pore diameter of 20–30 nm, and a ZrNP layer thickness of ～2 μm). EDS and XRD analysis showed that the distribution of the surface chemical components, as well as the crystalline structure of the zirconia (ZrO₂), remained unchanged. On the other hand, the hydrophilicity of the ZrO₂/ZrNP surface became significantly improved (as compared with a control group). In addition, the bond strength of the coating was decreased as a result of the anodization treatment. Moreover, both the Zr and ZrNP nanolayers exhibited improved proliferation and osteogenic properties in the in vitro cell experiment. Also, the ZrNP nanolayer was found to promote the biocompatibility and osteogenic ability to the largest extent. The ZrNP nanolayer did also provide numerous adhesion sites for MC3T3-E1 whereby the cell morphology could be enhanced. Additionally, the nano porous ZrNP layer improved the new bone formation around the zirconia implant and did robustly enhance the push out bond strength, thereby indicating the best osteointegration ability. In conclusion, a nano porous zirconium layer has been found to be very promising for the improvement of the bioactivity of the zirconia implant material.     

Effect of in-situ oxidation on the nanomechanical evolution of Fe-base coating with ceramic particles produced by internal rotating plasma spraying

In this work, we investigated FeCrMo coatings with 35 wt% ceramic, which were produced on a cylinder surface by an internal rotating plasma spraying and oxidized in air. The ceramic particles consisted of 80 wt% alumina and 20 wt% zirconia, which can improve the mechanical properties of the coating. For microstructural characterization, scanning electron microscopy was combined with energy dispersive X-ray analysis and electron probe micro analysis, and the nanomechanical properties were measured by a nanoindentation tester. The structure of Fe-base coating matrix consists of Al₂O₃, ZrO₂ and a small amount of mixed Fe–Cr oxides, and the ceramic particles exhibit a uniform distribution. During oxidation, a mixed oxide layer containing Fe₂O₃ and Fe₃O₄ forms on the surface of the Fe-base coating, and the thickness of the oxide layer increases with increasing oxidation temperature. Due to the in-situ oxidation, the nanohardness and Young's modulus of the Fe-base coating decreased with increasing oxidation temperature. The nanohardness evolution of Fe-base coatings oxidized at different temperatures is discussed.     

Solvent effect on synthesis of zirconia support for tungstated zirconia catalysts

Solvothermal reaction of zirconium n-butoxide (ZNB) in different solvent media, such as 1,3-pentanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol resulted in the formation of zirconium dioxide (ZrO₂) nanostructure. Then, the 15%W/ZrO₂ (WZ) catalysts using different zirconia supports were prepared by impregnation method. The effects of solvent on preparation of zirconia on the catalytic performance of WZ catalysts in esterification of acetic acid and methanol at 60 °C were investigated. The experimental results showed that ZrO₂ particles prepared in 1,4-butanediol (ZrO₂-BG) have a spherical shape, while in other glycols the samples were irregularly-shaped particles. The reaction results of esterification illustrated that the W/ZrO₂-BG catalysts had high surface acidity and showed high acetic acid conversion. The W/ZrO₂-PeG catalysts (ZrO₂ particles prepared in 1,5-pentanediol, PeG) exhibited the lowest surface acidity among other samples due to strong interaction of proton species and the zirconia supports as proven by TGA. One of the possible reasons can be attributed to different amounts of carbon residue on the surface of catalysts.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

Nonisothermal crystallization kinetics of flame‐sprayed polyamide 1010/nano‐ZrO2 composite coatings

Polyamide1010 (PA1010) and its composite with nanometer‐sized zirconia (PA1010/nano‐ZrO₂) coatings were deposited using a flame spray process. The kinetics of nonisothermal crystallization of PA1010/nano‐ZrO₂ composite coatings was investigated by differential scanning calorimetry (DSC) at various cooling rates. Several different analysis methods were used to describe the process of nonisothermal crystallization. The results showed that the modified Avrami equation and Mo's treatment could describe the nonisothermal crystallization of the composite coatings very well. The nano‐ZrO₂ particles have a remarkable heterogeneous nucleation effect in the PA1010 matrix. The values of halftime and Z_c showed that the crystallization rate increased with increasing cooling rates for both PA1010 and PA1010/nano‐ZrO₂ composite coating, but the crystallization rate of PA1010/nano‐ZrO₂ composite coating was faster than that of PA1010 at given cooling rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Design and preparation of composite coatings with increased reflectivity under high-energy continuous wave laser ablation

High-energy continuous wave (CW) laser ablation can cause severe damage to structural materials in an extremely short time, which generates considerable concern in terms of material safety. For the purpose of reducing or even eliminating such laser-induced damage, a novel composite coating consisting of a boron-modified phenolic formaldehyde resin incorporating ZrC and SiC has been designed and prepared. The experimental results reveal that ZrC and SiC are rapidly oxidized to ZrO₂ and SiO₂ respectively, leading to the formation of a white ceramic layer consisting of ZrO₂ particles and melted SiO₂. After ablation at 1000 W/cm² for 50 s, elemental analysis indicates that no Si can be found in the central ablation zone because of gasification. A relatively compact ZrO₂ layer is formed through the sintering of adjacent ZrO₂ particles, which effectively improves the reflectivity of the coating from 7.3% (before ablation) to 63.5% (after ablation). The high reflectivity greatly reduces the absorption of laser energy. In addition, no obvious ablation defects are observed in the composite coating. The excellent anti-laser ablation performance of the coating makes it a promising system for protecting a material against the effects of long-term CW laser ablation.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Effect of high energy ball milling on strengthening of Cu-ZrO2 nanocomposites

In this paper, copper matrix nanocomposites reinforced by 5 and 10?wt% ZrO₂ particles were produced by mechanical milling technique at different milling times. The produced nanocomposite powders were investigated by X-ray diffraction technique and transmission electron microscopy. The effect of high energy ball milling on the morphology, microstructure and microhardness of the produced composites has been investigated. After that cold compaction was applied to the prepared powders under a pressure of 700?MPa and sintered at 950?°C for 2?h in hydrogen atmosphere. The results showed that increasing milling time improves microhardness of the prepared nanocomposites. The microhardness of Cu-10%ZrO₂ after 20?h milling is 3.76 times larger than pure Cu. This improvement is attributed firstly to the presence of ZrO₂ nanoparticles in addition to the improvement coming from the grain refinement and crystallite size reduction occurred due to mechanical alloying. So, in spite of the crystallite size of Cu-10%ZrO₂ nanocomposite is reduced to 10.75?nm compared to 105.5?nm for pure Cu, the presence of ZrO₂ nanoparticles plays a major role on mechanical properties improvement.     

Heat treatment of nanostructured thermal barrier coating

A nanostructured thermal barrier coating has been prepared by air plasma spraying using YSZ (8 wt% Y₂O₃ partially stabilized zirconia) nano powder. The effect of annealing on the nanostructured zirconia coating has been investigated. The grain size of the nanostructured zirconia coating increased with increasing annealing time and temperature. Grains grew with preferential direction and into a columnar structure. The growth activation energy in the nanocrystalline grains is very low, which comes from existence of micro-pores in the coating and the grain-rotation-induced grain coalescence (GRIGC) mechanism.     

Synthesis of the ZrO2–SiO2 microspheres as a mesoporous candidate material

A series of mesoporous ZrO₂?CSiO₂ microspheres with different amounts of silica were synthesized by a polymerization-induced colloid aggregation process, using zirconyl chloride and commercial SiO₂ colloids as the raw materials. The microspheres were characterized by scanning electron microscopy (SEM), N₂ adsorption?Cdesorption isotherms, X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). According to the SEM results, the ZrO₂ and ZrO₂?CSiO₂ microspheres had spherical morphologies and the sizes of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios. The XRD spectrum of the pure ZrO₂ microspheres contained characteristic peaks for a monoclinic crystalline zirconia structure. The XRD spectrum of the ZrO₂?CSiO₂ microspheres showed a tetragonal crystalline structure. The specific surface areas of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios, and the pore volumes also increased. The average pore size of the ZrO₂?CSiO₂ microspheres was 3?C5?nm. The FT-IR spectrum of the ZrO₂?CSiO₂ microspheres confirmed the formation of Zr-O-Si bonds. The Zr-O-Si bonds make the metastable tetragonal zirconia stable at room temperature.     

Characterization of zirconia modified porous stainless steel supports for Pd membranes

A thin well-bonded zirconia film was formed on a porous stainless steel support by a sol-gel dip-coating method followed by firing in hydrogen atmosphere. The zirconia layer has a cubic phase structure. The surface composition of the film was analyzed by X-ray photoelectron spectroscopy. A comparison between H₂-fired and air-fired samples indicated that the surface/interface composition of the sample fired in air was rich in Fe₂O₃, whereas that of the fired in H₂ contained more metastable cubic zirconia phase and spinel type iron chromate. Binding energy shifts of Zr and Fe in the mixed oxides, together with the presence of iron and chromium oxides suggests that interfacial reaction between the coating and the substrate occurred, which is favorable for a good adhesion of the layers. The average pore size of the support modified by multilayer zirconia coatings declined drastically from original 2 μm to the coated top-layer 0.4 μm. The ZrO₂ film showed a hydrogen selectivity of 2.7 for H₂/N₂, and a hydrogen flux of 1.159 × 10^?3 mol/m².s at room temperature and with a pressure difference of 101 kPa across the film.     

Modified thick thermal barrier coatings: microstructural characterization

Thick thermal barrier coatings were modified with laser glazing and phosphate based sealing treatments. Surface porosity of the sealed coatings decreased significantly in all cases. Structural analysis showed a strong preferred crystal orientation of the t′ZrO₂ phase in direction [002] in laser-glazed 25CeO₂–2.5Y₂O₃–ZrO₂ coating. In laser-glazed 22MgO–ZrO₂ coating the major phase was rhombohedral Mg₂Zr₅O₁₂. In phosphate sealed 8Y₂O₃–ZrO₂ coating the strengthening mechanism was identified as adhesive binding without chemical bonding. Coating microstructures were determined by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy and optical microscopy. Coatings were also characterized by X-ray diffraction, microhardness and porosity.     

Modified hydrothermal treatment route for high-yield preparation of nanosized ZrO2

Nanosized ZrO₂ particles are applied in high-performance thermal barrier coatings and catalyst carriers. To synthesize nanosized zirconia, precipitation from aqueous solutions followed by hydrothermal treatment is widely conducted. In this work, a modified hydrothermal treatment route is described for high-yield fabrication of well-dispersible nanosized t–ZrO₂. Zirconium oxychloride and sodium hydroxide were used as the precursor and precipitant, respectively. N, N-bis(2-hydroxyethyl) glycine (bicine) was used as surface stabilizer to inhibit the early agglomeration of nuclei, and ultrasonication was used to enhance the dispersion of ZrO₂ nanocrystals. The hydrothermal treatment was optimized for reaction temperature, time, fill fraction, and solid content. The synthesized zirconia was characterized using X-ray diffraction, dynamic light scattering, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The yield of zirconia increased to 134 g/L after hydrothermal. Tetragonal ZrO₂ obtained with hydrothermal treatment at 200 °C for 8 h at a fill fraction of 80% has a good dispersibility, with an average particle size of 20 nm and a narrow size distribution.     

Alkali resistance,microstructural and mechanical performance of zirconia-coated basalt fibers

Sol–gel approach was developed to apply zirconia coatings on basalt fibers. Dense or porous ZrO₂ coatings were obtained, depending on the process parameters. The alkali resistance of uncoated and ZrO₂-coated basalt fibers in alkali solution was studied. The morphology, elemental and phase composition of etched fibers as a function of etching time were studied by means of a set of analytical methods. A scheme of etching of as-received and coated basalt fiber was proposed. Zirconia coating slows down the corrosion of basalt fiber in alkali solution. The denser zirconia coating slows down the corrosion to a higher extent than the porous coating. The uncoated and coated basalt fibers were tested in mini composites with cement matrix. It was shown that the surface of the coated fiber is affected by the alkaline medium of the cement matrix to a smaller extent than the surface of as-received basalt fiber.     

Vapor-Phase Dehydration of 1,3-butanediol over CeO2–ZrO2 Catalysts

The dehydration of 1,3-butanediol was investigated over CeO₂–ZrO₂ catalysts prepared by impregnation at temperatures of 325–375 °C. Pure CeO₂ selectively catalyzed the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol, while pure ZrO₂, which was less active than pure CeO₂, catalyzed the dehydration to 3-buten-1-ol. In the CeO₂/ZrO₂ catalyst in which CeO₂ was supported on zirconia, the presence of a small amount of CeO₂ suppressed the formation of 3-buten-1-ol and induced the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol and the subsequent dehydrogenation of 3-buten-2-ol to form 3-buten-2-one and butanone. The activity would be related to the redox features of CeO₂. The monoclinic phase of zirconia support decreased while the cubic CeO₂ phase increased as CeO₂ content was increased. In contrast, in the ZrO₂/CeO₂ catalyst in which ZrO₂ was supported on cubic CeO₂, only the cubic CeO₂ phase was observed and ZrO₂ species appeared in the form of a solid solution of CeO₂–ZrO₂ with fluorite structure. Regardless of zirconia loading, ZrO₂ species did not affect the catalytic activity of ZrO₂/CeO₂, which was controlled by CeO₂ species.     

Influences of evaluation methods and testing load on microhardness and Young's modulus of ZTA and ATZ ceramics

In this work, microindentation tests under different conditions were performed to determine the influence of indentation load on Young's modulus and microhardness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA).     

Mechanical properties of HAp–ZrO2 composites

Hydroxyapatite is a well-known and valuable implant material with bioactive properties. Full utilisation of the unique properties of hydroxyapatite ceramics is, however, possible only after its proper reinforcement, i.e., by preparation of composites. In the present work zirconia reinforced hydroxyapatite composites were obtained by hot pressing method. The reinforcing phase in the form of ZrO₂ particles was selected due to the satisfactory biocompatibility of ZrO₂ and also because of its exceptional mechanical properties.Our investigations were aimed at assessing the influence of varying ZrO₂ on the phase composition and mechanical properties of HAp–ZrO₂ composites. In order to produce dense sinters, we used three types of initial zirconia powders which differed in morphology and contents of the tetragonal and monoclinic phases. We studied the influence of these oxides on thermal stability of hydroxyapatite matrix as well as on the phase composition and mechanical properties of the composite materials produced.     

Long-time ablation protection of carbon/carbon composites with different-La2O3-content modified ZrC coating

Long time oxidation protection at ultra-high temperatures or ablation protection has been a choke point for C/C composites. In this study, long time ablation protection of different-La₂O₃-content (5–30 vol.%) modified ZrC coating for SiC-coated carbon/carbon (C/C) composites was investigated. Results showed that ZrC coating with 15 vol.% La₂O₃ had good ablation resistance and could protect C/C composites for at least 700?s at 2160 °C. A high-thermal-stability and low-oxygen-diffusivity oxide scale containing m-ZrO₂ particles and molten phases with La_0.1Zr_0.9O_1.95 and La₂Zr₂O₇ was formed during ablation, offering the ablation protection. La could erode grain boundaries of ZrO₂ to refine ZrO₂ by short-circuit diffusion and m-ZrO₂ particles were retained due to less bulk diffusion than grain-boundary diffusion of La into ZrO₂. The erosion resulted in the formation of molten phases containing fine nano-ZrO₂, which served as viscous binder among m-ZrO₂ particles and crack sealer for the oxide scale.