Effect of ageing in the electrolyte and water on porous anodic films on zirconium

The present study demonstrates the significant influence of ageing in the formation electrolyte on the morphology and composition of anodic films grown on zirconium in 0.35 M ammonium fluoride in glycerol. Ageing after anodizing, by immersion in the electrolyte for 1 h, is shown to promote a transition from a porous to a nanotubular morphology, due to the dissolution of the fluoride-rich intratubular material in which the nanotubes are embedded. The morphological change is accompanied by a significant loss of zirconium and fluorine from the film. In contrast, ageing in deionized water has little influence on the films.     

Dielectric function of palladium capped zirconium thin films as a function of absorbed hydrogen

We report measurements of the optical transmission, between 240 and 1040 nm, and electrical resistivity of polycrystalline zirconium thin films as they absorb hydrogen. Both are measured as H₂ pressure is increased up to 880 mbar, at room temperature. Films, 20–22 nm thick, are deposited on fused quartz substrates by e-beam evaporation at 5.3 × 10^?7 mbar base pressure and covered with a 8.0 nm Pd over-coat. The morphology of the films is studied by means of AFM images. The complex refractive indices of Zr and Pd are extracted numerically from the transmission spectra by using a spectral projected gradient method for different hydrogen pressures. The corresponding dielectric functions for various Zr hydrogen concentrations are described with the parametric Drude-Lorentz and Brendel-Bormann (DL & BB) models. The Acceptance-Probability-Controlled Simulated Annealing approach is applied to calculate the parameters of the DL & BB model. This allows us to describe the effect of increasing hydrogen absorption on these parameters and in derived quantities, like the relaxation time and the effective mass of conduction electrons, the electrical resistance, the Fermi energy, and the electronic density of states at the Fermi level.     

锆对铸造3104铝合金微观结构和力学性能的影响

研究了不同锆添加量的3104合金的析出组织和力学性能。结果表明,随着Zr含量的增加,合金晶粒尺寸减小,当Zr质量分数大于或等于0.25%时,合金晶粒最小(20μm)。同时,晶粒形状由羽毛状变为等轴状。此外,Zr还可以通过形成Si相和其它金属间化合物来改善合金中Si和Mn元素的分布。维氏硬度分析表明,Zr的加入会降低Al-Mn-Fe 3104合金的硬度。此外,根据拉伸试验结果,当Zr质量分数不高于0.25%时,随着Zr含量的增加,合金的抗拉伸强度和延伸率都有所提高。适当的Zr含量可以起到钉扎位错和阻碍滑移的作用,提高合金的强度和韧性。     

Biomechanical three-dimensional finite element analysis of monolithic zirconia crown with different cement thickness

Limited information is available on the optimal cement thickness of monolithic zirconia crowns. This study was designed to evaluate the stress distribution in the posterior monolithic zirconia crowns with different cement thicknesses under masticatory force and maximum bite force using three-dimensional finite element analysis. The prepared and unprepared mandibular right first molar models were scanned and exported to the computer-aided design system. Solid models of monolithic zirconia crowns, which were cemented on prepared teeth were generated. Four models were fabricated applying different cement thicknesses (100 µm, 200 µm, 400 µm, and 600 µm). The solid models were imported into the finite element analysis software and meshed into tetrahedral elements. Four three-dimensional finite element models were simulated under masticatory force and maximum bite force: vertical (axial), angular (45°) and horizontal loads of 280 N at 5 points; vertical load of 700 N at 8 points were loaded, respectively. The stress distribution varied with the different cement thicknesses and directions of applied loads. The monolithic zirconia crowns with cement thicknesses exceeding 200 µm had wider distributions of peak maximum principal stress under the same loading conditions. Monolithic zirconia crowns have more stress concentrations on the occlusal surfaces, while the cement layers have more stress concentrations on the cervical areas. Thicker cement layers were associated with more concentrated stresses on the buccal and lingual cervical areas. The test results show that the cement thickness plays an essential role in the success of monolithic zirconia restorations in terms of reducing cement wash-out. Cement thickness of 100 µm is recommended for monolithic zirconia crowns.     

含Nb或Cu的锆合金显微组织研究

用TEM和SEM研究了含少量Nb或Cu的锆合金显微组织,结果表明:在微量Fe元素存在时,添加适量Cu元素,会促使低于α-Zr中平衡固溶含量的Nb元素与Fe元素优先析出形成沉淀相,致使α-Zr中Nb的固溶含量更低;若增加Cu元素含量,因Zr_2Cu第二相富集消耗了Fe元素,反而使Nb元素不易析出。在Zr-1.0Cu合金中加入适量的Nb元素,可以细化Zr_2Cu第二相的尺寸,促使其分布更加均匀。在大量Nb稳定的β-Zr第二相存在的情况下,添加的少量Cu会富集在β-Zr第二相中,而不会以Zr_2Cu第二相形式析出。     

Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis

The purpose of this study was to evaluate the effects of coping designs on the stress distributions in posterior zirconia crowns by non-linear three-dimensional finite element analysis. Three-dimensional finite element models of a mandibular right first molar with layers of veneering porcelain, zirconia coping, cement, and abutment tooth were designed by computer software (HyperWorks 10.0). Ten zirconia crowns with different designs were produced according to various shoulder positions and heights. The shoulders (1-mm width) exhibited incremental height increases of 1 mm, 2 mm, and 3 mm on the buccal, lingual, and proximal sides, respectively. An axial compressive dynamic load simulating the progressive load was applied until a stainless steel ball model (7 mm in diameter) deepened the veneer surface to 0.7 mm in depth. Loads were placed on the inner inclines of the mesiobuccal, distobuccal, and mesiolingual cusps. Residual maximum principal stresses (MPSs) at the veneer and coping under progressive loading were determined for each zirconia crown. Reinforcements with the shoulders on the buccal, lingual, and proximal axial walls resulted in lower MPSs in the veneering porcelain but higher MPSs in the zirconia coping. As the shoulder height increased, the tensile stresses decreased, while the compressive stresses increased in the veneering porcelains. It can be concluded that the shoulder height and position in the zirconia coping will affect the MPSs of the crown. Our findings conclusively reveal the critical role of the shoulder design of the coping in preventing veneer fracture on posterior zirconia restorations by reducing tensile stresses in veneering porcelain.     

Densification,microstructure, elastic and mechanical properties of reactive hot-pressed ZrB2–ZrC–Zr cermets

In this study, cermets composed of zirconium diboride and zirconium carbide with intergranular zirconium were sintered by reactive hot-pressing. Relative density exceeding 97% was obtained for the sintered cermets having four distinct compositions varying in concentration of excess Zr. Their densification behaviour was examined by monitoring displacement during sintering. The microstructure was characterized by scanning electron microscopy and X-ray diffraction, and the elastic and mechanical properties were evaluated at room temperature. The effects of Zr concentration on the densification and mechanical properties were assessed. The ZrB₂ and ZrC micron-grains coarsened with increasing amount of Zr starting material. In addition, the cermets exhibited high flexural strength (546–890 MPa) and fracture toughness (6.63–10.24 MPa m^1/2), which simultaneously increased with increasing Zr concentration. However, the elastic moduli and hardness (11–18 GPa) decreased with increasing Zr. The shear modulus and Young's modulus were in the range of 150–190 GPa and 360–440 GPa, respectively.     

Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites

ZrB₂ powder was coated with 5% ZrOC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO₂ retention, improving ZrB₂ densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB₂ composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO₂ content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB₂-based composite systems.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

ZrB_2－刚玉莫来石质复合材料显微结构及力学性能的研究

通过对不同刚玉－莫来石含量的ＺｒＢ２材料力学性能及显微结构的研究，探讨了刚玉莫来石对ＺｒＢ２强度和韧性的影响，同时也分析了强化和韧化机制。