Enhanced Hydrothermal Resistance of Y‐TZP Ceramics Through Colloidal Processing

Two commercial zirconia powders with 3 mol% of yttria (TZ3YE and TZ3YS, labeled as ZE and ZS, respectively) supplied by Tosoh (Japan) were used for this study. Maximum colloidal stability for ZE was achieved by dispersing the powders in a mixture of water/ethanol of 90:10 (wt/wt) using a sonication probe. The rheological behavior of the suspensions was optimized in terms of solids content ranging from 20 to 33 vol% and sonication time (0–6 min), the best results being obtained after 2 min. ZS samples were prepared to a solids loading of 30 vol% in water dispersing with 2 min‐sonication. Samples obtained by slip casting in plaster molds were used for dynamic sintering studies, and fully dense and nanostructured specimens were obtained at temperatures of 1300°C–1350°C (ZE samples) and 1400°C per 2 h (ZS samples). The Hardness (H) and Young's Modulus (E) properties of the specimens were studied by nanoindentation technique giving 17 and 250 GPa mean values for H and E, respectively. The specimens were then forced to a low‐temperature degradation (LTD) treatment at 130°C for 240 h in steps of 60 h. Raman spectroscopy and nanoindentation results of hydrothermally treated samples showed the absence of transformation from tetragonal to monoclinic phase until 180 h whereas the mechanical properties maintained constant even at the sample surface. After 240 h of LTD, the monoclinic phase was detected on all specimens by Raman peaks centered at 180, 191, and 383 cm^?1. The nanoindentation study revealed an important loss of mechanical features reaching 10 and 175 GPa for H and E, respectively. In the case of the ZS specimens, no monoclinic phase is detected after 240 h of LTD treatment and no decay of E or H is detected. The free defect microstructure reached for the ZS specimen revealed a higher hydrothermal resistance so that it is concluded that the excellent behavior against thermal degradation is possible due to the large uniformity obtained by colloidal processing rather than the particle size of the starting powders.     

Preparation of Transparent 3Y‐TZP Nanoceramics with No Low‐Temperature Degradation

The progresses of the relative density, average grain size (GS), and maximum pore size entering into the final sintering stage are investigated in 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP) compacts in the range of 87%~99% theoretical density. It is found that during conventional pressureless sintering, the pores in the compact enlarged dramatically, which appears to be the major obstacle for preparing fully dense nanoceramics. Transparent 3Y‐TZP nanoceramics with an average GS of 87 nm were prepared by exploiting microstructural refinement on nanoscale. The yields a maximal optical transmittance of 67% and shows no noticeable low‐temperature degradation after 100 h aging at 134°C under a hydrothermal pressure of two bars.     

Rapid Grain Growth in 3Y‐TZP Nanoceramics by Pressure‐Assisted and Pressure‐Less SPS

Pressure‐less spark plasma sintering (SPS) is a new approach during which rapid densification of ceramic nanopowder green bodies is accompanied by rapid grain growth. Although the origin of this phenomenon has not yet been fully understood significant, difference in grain growth between pressure‐less and pressure‐assisted SPS was expected. In this work 3Y‐TZP nanopowder with average particle size of 12 nm was consolidated using two‐step approach: (1) at an intermediate temperature (600°C to 1000°C) SPS warm pressing followed by (2) high temperature (1400°C to 1600°C) pressure‐less SPS. The standard one step pressure‐assisted SPS experiments were quoted as references. Rapid grain growth was observed during both pressure‐less and standard SPS. The samples prepared by both approaches at the same sintering temperature (1400°C–1600°C) achieved identical grain size and grain size distribution, if large pores were eliminated in early stage by SPS warm pressing. The electric current, electromagnetic field, and mechanical pressure is proven to have a negligible direct influence on grain growth in 3Y‐TZP ceramics at temperatures above 1000°C under standard SPS conditions.     

Fretting fatigue wear behavior of Y‐TZP dental ceramics processed by non‐conventional microwave sintering

The fretting wear behavior of self‐mated Y‐TZP dental materials obtained by nonconventional microwave and conventional sintering has been investigated. Two 3Y‐TZP materials, a widely utilized commercial dental ceramic (LAVA) and a lab‐prepared 3Y‐TZP powder based equivalent have been assessed. Relative density and mechanical properties as well as the grain size variations upon sintering have been evaluated. After exposure to selected gross slip regime fretting wear conditions, the wear tracks have been characterized allowing the measurement of the coefficient of friction, track profiles, and pit features. The results indicate that microwave sintering results in a similar fretting wear behavior as observed for conventional‐sintered 3Y‐TZP, as the measured volumetric wear loss is of a comparable order of magnitude. Regarding the influence of the grain size, the analysis revealed that a large grain size (>300 nm) results in an increased wear volume and that a higher resistance to fretting wear is constrained to a mid‐range particle size (100–250 nm). Since the fracture toughness of all investigated ceramic grades was comparable, the influence of the fracture toughness on fretting could not be assessed. Abrasive grooving, delamination, and microcracking have been identified as major wear mechanisms inside the wear tracks for both conventional‐ and microwave‐sintered 3Y‐TZP. In general, microwave sintering can provide 3Y‐TZP dental materials with a comparable fretting wear resistance as that observed for conventional sintering using lower dwell sintering temperatures and a shorter processing time.     

Lattice and Grain‐Boundary Diffusion of Al in Tetragonal Yttria‐Stabilized Zirconia Polycrystalline Ceramics (3Y‐TZP) Analyzed Using SIMS

Aluminum oxide was deposited on the surface of 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP). The samples were annealed at temperatures from 1523 to 1773 K. Diffusion profiles of Al in the form of mean concentration vs. depth in B‐type kinetic region were investigated by secondary ion mass spectroscopy. The experimental results for the lattice diffusion (D_B) and grain boundary diffusion (D_GB) are as follows: and where δ is the grain‐boundary width and s is the segregation factor.     

3Y‐TZP In‐Depth Phase Transformation by Raman Spectroscopy: A Comparison of Three Methods

Zirconia phase transformation is usually studied on surface. For in‐depth study, three methods were proposed using Raman microspectroscopy quantitative evaluation: direct measurement on sample cross section, confocal Raman spectroscopy (CRS), and progressive pinhole aperture enlargement (PPAE). The aim of this study was to compare transformation profiles obtained with these three methods on the same sample. Three 3Y‐TZP samples were aged, respectively, for 25, 72, and 90 h in artificial saliva. Transformation profiles were determined with cross‐sectional measurement, CRS and PPAE. A transformation profile simulation model based on PPAE measurements is proposed, using the convolution of the excitation intensity profile and the Beer–Lambert law (optical properties of zirconia). The simulation model was validated with the determination of 3Y‐TZP transformation factor, T = 1.15 μm^?1, identical for the three aging durations. Both cross section and PPAE measured similar in‐depth transformation decrease, but with a 10 μm‐shift: transformed zirconia layer is more important in cross‐sectional protocol (36 μm with PPAE and 46 μm with cross‐sectional after 90 h aging). Complementary measurements on a 10 h aged sample, where transformation is initiated by Low‐Temperature Degradation, showed that sample preparation and polishing, necessary in the cross‐sectional method, were responsible for the higher transformation. PPAE method enables noninvasive in‐depth measurements with limited optical and mechanical biases.     

Rheological Characterization of Aqueous 3Y‐TZP Inks Optimized for Direct Thermal Ink‐Jet Printing of Ceramic Components

Aqueous 3Y‐TZP inks with solid contents of 22 and 27 vol% were used for fabricating three‐dimensional ceramic components by the direct ink‐jet printing process (DIP). The DIP fabrication was realized using a thermal ink‐jet (TIJ) printing system. Despite the different physical properties of the inks, both inks were successfully ejected and deposited. To define the optimum window of the ink properties required for a stable printing operation, both ceramic inks as well as a typical TIJ ink were characterized in terms of particle size distribution, zeta potential, viscosity, surface tension, and the inverse Ohnesorge number (Oh^?1). Moreover, single drops of all inks were deposited and analyzed by scanning electron microscopy (SEM) to examine the form and integrity of the ejected drops. Demonstration objects (a base with curved channels and a sample molar tooth) were DIP fabricated using both of the ceramic inks. These objects show the potentials of the DIP process for ceramics manufacturing particularly by using TIJ printing systems.     

Hydrothermal Corrosion and Strength Degradation of Aluminum Nitride Ceramics

The hydrothermal corrosion and strength degradation of aluminum nitride (AlN) ceramics were investigated. The weight gain in AlN ceramics after corrosion occurred because of the formation of boehmite. The reaction kinetics of AlN with water were diffusion controlled through the boehmite product layer. At 180°C, immersion in water caused no strength degradation, and water vapor caused a 20% strength degradation. At 300°C, immersion in water caused a 20% strength degradation, and water vapor caused a 30% strength degradation.     

Mechanical Behavior of Lightweight Ceramics Based on Sintered Hollow Spheres

A simple micromechanical model, describing the Young's modulus, fracture toughness, and density of a lightweight ceramic structure produced by the sintering of hollow spheres and based on shell theory, was developed. The approach combined an analysis of the elastic deformation using shell theory and a simple analysis of the densification in terms of the micro-structural parameters. The results were compared with experimental data for materials produced by the sintering of hollow glass spheres and gave reasonable to good agreement, especially in the prediction of the nonlinear variation of Young's modulus and fracture toughness with density. The model should provide a useful basis for rationalizing the relation between mechanical properties and fabrication procedure for such materials.     

Low‐Temperature Sintered NTC Thermistor Ceramics for Thick‐Film Temperature Sensors

Negative temperature coefficient (NTC) thermistor thick films were fabricated by screen printing on alumina substrates and firing at 900°C. Spinel‐type NiMn₂O₄ exhibits limited stability in air between 730 and 970°C only and interacts with the Bi₂O₃ additive. The Zn–Co‐substituted spinel Zn_0.75Ni_0.5Co_0.5Mn_1.25O₄ with 3 wt% additive shows complete densification at 900°C; no interaction between spinel and additive was observed. Alternatively, a Cu–Zn–Co‐substituted Cu_0.37Zn_0.52Ni_0.44Co_0.44Mn_1.23O₄ spinel with excellent sintering characteristics even without sintering additive was investigated. The thermistor films display a sheet resistance of about 300 kΩ/□ and B = 3300 K. The firing behavior, microstructure formation, and electrical properties of NTC thick films are reported.     

Microstructures and Properties of 3Y‐TZP/Ba0.5Sr0.5Fe12O19 Composites Prepared by Two Methods

Two composites of Ba_0.5Sr_0.5Fe₁₂O₁₉ grains dispersed in a 3 mol% yttria‐doped zirconia (3Y‐TZP) matrix are prepared by two methods. The mechanical and electromagnetic properties of the both composites are investigated. The maximum values of the bending strength and the fracture toughness are 786 MPa and 8.6 MPa m^1/2, respectively. The maximum value of the magnetization is 17.2 emu/g. In addition, the distribution of the Ba_0.5Sr_0.5Fe₁₂O₁₉ phase is described using the box dimension of the fractal theory. The impedance spectra of the both composites are fitted using the equivalent circuit model with a constant phase element (CPE). The obtained p value of the CPE can indirectly reflect the morphology of the Ba_0.5Sr_0.5Fe₁₂O₁₉ phase in both composites according to the universal relaxation law.     

Frequency‐Dependent Qf Value of Microwave Dielectric Ceramics

The microwave dielectric properties of some typical low‐loss dielectric ceramics (CaNdAlO₄, Ba(Mg_1/3Nb_2/3)O₃, Ba(Zn_1/3Nb_2/3)O₃, Ba₄Sm_9.33Ti₁₈O₅₄, Ca_0.71Nd_0.26TiO₃ and SrTiO₃) were evaluated at different frequencies by using high‐order TE_0np resonant modes of resonant cavity method. The dielectric constant shows little dependence on frequency for these microwave dielectric ceramics, while the Qf value increases with frequency in the measurement frequency range of 3–14 GHz. The results indicate that the Qf value as a frequency‐independent constant at microwave frequencies is not valid for these microwave dielectric ceramics, while the constant Qf value has been accepted as a common recognition for a long period. The frequency dependence of Qf value is attributed to the defects‐induced extrinsic dielectric loss, which cannot be avoided in actual microwave dielectric ceramics.     

微波条件下植物纤维素降解工艺条件的优化

在微波加热条件下,采用HCl/H<,2>O<,2>催化剂对植物纤维素进行降解.通过中心组合实验优化的适宜降解条件为:催化剂体积分数5.0%、反应温度100℃、反应时间2.0 h,在此条件下,纯植物纤维素的降解率为70.6%,较常规加热降解率提高了32.01%.SEM分析微渡加热和常规加热下降解产物的结构特征,发现微波作...     

Degradation of Spark Plasma Sintered Yttria Stabilized Zirconia (YSZ) and CeO2‐YSZ Ceramics in Supercritical Water

Zirconia ceramics are often considered in the nuclear industry for high temperature applications due to their radiation resistance, chemical inertness, high thermal stability and low thermal conductivity. However, wide application of zirconia ceramics is hindered by their degradation behavior in aqueous media. In this research, moderately dense 8 mol% Yttria Stabilized Zirconia (YSZ) as well as YSZ based ceramics containing 5, 10 and 15 mol% CeO₂ were fabricated via Spark Plasma Sintering (SPS). Sintered ceramics were subjected to static degradation testing in supercritical water at 400°C and 31 MPa. The weight loss and microstructural changes of the ceramics associated with supercritical water exposure were studied, and the results suggest that the addition of CeO₂ significantly reduced the degradation rate and improved the bulk structural stability of the ceramic composite. This behavior was attributed to the enhanced stabilization of the zirconia lattice, preventing the evolution of the undesirable monoclinic‐ZrO₂ phase.     

Grain Boundary Curvatures Measurements in Annealed Yttria‐Stabilized Zirconia (3Y‐TZP) and Their Relation to Mean Grain Size

It was determined that the mean grain boundary radius of curvature in 3 mol% yttria‐stabilized zirconia isothermally annealed without and with a DC electric field  = 18 V/cm was uniquely proportional to the mean linear intercept grain size , the proportionality constant α = 3/2 being in accord with the Rios‐Fonseca stereological model.     

Characterization of Three-Dimensional Through-Thickness Transformation Zone in 9-mol%-Ce–TZP Ceramics

The through-thickness t–m transformation zone was characterized in two 9-mol%-Ce–TZP ceramics with 1.6- and 2.2-μm grain size. Short, double-cantilever-beam (sDCB) specimens were used to measure the crack-resistance curves ( R -curves). After the R -curve measurements were made, the sDCB specimens were cut through the thickness, and the t–m transformation zone was characterized. The stress-induced t–m transformation zone remained nearly the same shape and size through the thickness, but the amounts of the t–m transformation at the two free surfaces were much higher than inside the specimens. The results are discussed based on fracture mechanics.     

Porous,Sintered Glass‐Ceramics from Inorganic Polymers Based on Fayalite Slag

The present paper deals with the synthesis of porous, sintered glass‐ceramics obtained at temperatures below 1150°C, originating from inorganic polymers based on fayalite slag. Firing led to the evaporation of water, dehydroxylation, and oxidation of Fe²⁺ above 345°C. For heating >700°C, the Si–O stretching band shifted from the 1160 and 750 cm^?1 to the 1255 and 830 cm^?1 region, due to a structural reorganization of the amorphous phase, whereas Fe–O bands appeared at 550 cm^?1. The final microstructure consisted predominantly of an amorphous phase, hematite, and franklinite. The open porosity and compressive strength decreased and increased, respectively, as the firing temperature increased. The final values suggest properties comparable to that of structural lightweight concrete, still, the materials synthesized herein, are lighter, and made primarily from secondary resources.     

微波干燥技术在蜂窝陶瓷中的应用

蜂窝陶瓷挤压成形后的强度低、难干燥等问题．严重制约了蜂窝陶瓷的广泛应用。近年来,微波干燥技术已应用于蜂窝陶瓷上,可以解决其强度低、难干燥等问题。结果表明,微波干燥技术能降低成形后蜂窝陶瓷坯体约10％的水分。通过分析蜂窝陶瓷干燥的过程,提出了热风干燥与微波干燥相组合的方法,结合两者的优势,以达到优化节能的目的。     

Fracture Resistance Behavior of High‐Thermal‐Conductivity Silicon Nitride Ceramics

Silicon nitride ceramics were prepared from a high‐purity silicon powder doped with 2 mol% Y₂O₃ and 5 mol% MgO as sintering additives via a route of sintering of reaction‐bonded silicon nitride (SRBSN). The materials sintered at 1900°C for 3, 6, 12, and 24 h had thermal conductivities of 109, 125, 146, and 154 W/m/K, and four‐point bending strengths of 786, 676, 608, and 505 MPa, respectively. The fracture toughness values, determined by the single‐edge‐precracked‐beam (SEPB) method, were 8.4, 8.6, 9.7, and 10.7 MPa m^1/2 for the materials sintered for 3, 6, 12, and 24 h, respectively, which were similar to the results measured by the chevron‐notched‐beam (CNB) test method. The materials sintered for longer times (12 and 24 h) showed stronger R‐curve behaviors over longer range of crack extension, in comparison with the materials sintered for shorter times (3 and 6 h).     

Structure and Microwave Dielectric Behavior of A‐Site‐Doped Sr(1−1.5x)CexTiO3 Ceramics System

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr_(1?1.5x)Ce_xTiO₃ (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr_(1?1.5x)Ce_xTiO₃ ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti⁴⁺ ions was hastened by tetravalent ions (Ce⁴⁺). By contrast, this reduction was inhibited by trivalent ions (Ce³⁺). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (), and defect dipoles, namely, () and (). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (ε_r = 40), with the smallest reported value of temperature coefficient of resonant frequency (τ_f = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.