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.     

Hydrothermal Degradation Behavior of Y‐TZP Ceramics Sintered by Nonconventional Microwave Technology

Different factors such as the characteristics of starting powders, their processing, the sintering technique and the final sintering temperature were assessed with the goal to improve the low‐temperature degradation (LTD) resistance of 3Y‐TZP materials without compromising on the mechanical properties. The degradation of hydrothermally treated specimens was studied by AFM, nanoindentation technique, micro‐Raman spectroscopy, and electron microscopy. 3Y‐TZP previously prepared in laboratory by colloidal processing, and sintered by microwave method at low temperature (1200°C) led to excellent mechanical and LTD resistance, as compared to dental restorations based on Y‐TZP commercial material. In the former, the presence of m‐phase was almost nonexistent even after 200 h of exposure to LTD conditions and the initial mechanical properties were maintained, giving 16 and 250 GPa mean values for hardness and Young's modulus, respectively. The influence of the fast‐technology by microwave heating is presented with a nonconventional sintering method to fabricate 3Y‐TZP ceramics for dental application with very high resistance against LTD and optimized mechanical properties.     

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.     

Low‐temperature aging of baddeleyite‐based Ca‐TZP ceramics

The aging kinetics during low‐temperature aging of calcia‐stabilized tetragonal zirconia polycrystal (Ca‐TZP) ceramics prepared by high‐energy milling of natural zirconia mineral (baddeleyite) was studied by X‐ray diffraction under hydrothermal treatment conditions. Aging kinetics was investigated for ceramics with different contents of calcia. It was found that the kinetics may be well‐described within Johnson‐Mehl‐Avrami‐Kolmogorov model. Model parameters were determined by data fitting procedure. Change in exponential factor within Johnson‐Mehl‐Avrami‐Kolmogorov model with time is shown. Analytical model to describe aging kinetics is proposed. The transformation nucleation rate, initial diameter, and depth of the transformed areas and their growth rates are estimated. Degradation of hardness and fracture toughness is also reported for Ca‐TZP after low‐temperature aging for different contents of the stabilizer.     

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.     

Low‐Temperature Phase Transition in AgNbO3

AgNbO₃ is a weak ferroelectric with antiferroelectricity due to Ag displacements at room temperature. A dielectric anomaly at 250 K, which has not been observed previously, reveals a transition between the weak ferroelectric phase (M1 phase) at the higher temperature and a new ferroelectric phase (M0 phase) at the lower temperature in AgNbO₃. This transition was further verified by the pyroelectric current and differential scanning calorimetry measurements. The spontaneous polarization value is found to be much larger in the M0 phase than that of the M1 phase. A well‐defined saturating ferroelectric hysteresis loop can also be observed at 77 K, showing a remnant polarization value of 2.4 μC/cm² and a coercive field of 25 kV/cm. All the above results indicate that the larger polarization of the M0 phase mainly comes from the alignment of the antiferroelectric displacements of the Ag atoms.     

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.     

Design and Fabrication of Transparent and Gas‐Tight Optical Windows in Low‐Temperature Co‐Fired Ceramics

Low‐temperature co‐fired ceramics (LTCC) enable the fabrication of microfluidic elements such as channels and embedded cavities in electrical devices. Hence, LTCC facilitate the realization of complex and integrated microfluidic devices. Examples can be applied in many areas like reaction chambers for synthesis of chemical compounds. However, for many applications it is necessary to have an optically transparent interface to the surroundings. The integration of optical windows in LTCC opens up a wide field of new and innovative applications such as the observation of chemiluminescent reactions. These chemical reactions emit electromagnetic radiation and thus offer a method for noninvasive detection. Thin glasses (≤500 μm) were bonded by thermocompression onto a LTCC substrate. As the bonding agent, a glass frit paste was used. Borosilicate glasses, fused silica as well as silicon were successfully bonded onto LTCC. To join materials with a large coefficient of thermal expansion mismatch (i.e., fused silica and LTCC), it is necessary to limit the heat input to the bond interface. Therefore, a heating structure was integrated into the LTCC substrate beneath the bond interface. This bonding process provides a gas‐tight optical port with a high bond strength.     

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.     

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.     

Low‐Dielectric Constant and Low‐Temperature Curable Polyimide/POSS Nanocomposites

With the rapid development of ultra large scale integrated circuits, low stress, low thermal expansion, low dielectric constant, and low temperature curable (<250 °C) polyimides (PIs) with excellent mechanical, thermal properties are required. Unfortunately, high curing temperatures above 300 °C and limited dielectric property still remain to be solved. Herein, a new type of aminopropyl isobutyl polysilsesquioxane (POSS) with single vertex activity is introduced by in situ polymerization resulting in the PI‐POSS nanocomposites which exhibit a low dielectric constant (κ ≤ 2.6). Furthermore, low‐temperature curing at 200 °C (99.4% imidization) under the catalysis of quinoline is also achieved. The as‐prepared PI‐POSS nanocomposites also show excellent mechanical properties of which the tensile strength can reach up to 148 MPa and the elongation at break achieves 98%. Moreover, the temperature of weight loss 5% is as high as 550 °C and the glass transition temperature can also reach 349 °C. The as‐prepared PI‐POSS nanocomposites prove excellent electrical performance and mechanical properties, showing a huge market prospect of 5G chip packaging and millimeter wave antenna in the future.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

Low‐Temperature Sintering and Piezoelectric Properties of CuO‐Added KNbO3 Ceramics

Dielectric and piezoelectric properties of CuO‐added KNbO₃ (KN) ceramics were investigated. The CuO reacted with the Nb₂O₅, formed a CuO–Nb₂O₅‐related liquid phase during the sintering, and assisted the densification of the KN ceramics at low temperatures. Moreover, some of the Cu²⁺ ions replaced the Nb⁵⁺ ions in the matrix and behaved as a hardener. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The 1.0 mol% CuO‐added KN ceramic sintered at 960°C for 1.0 h, which showed a maximum relative density, exhibited a high phase angle of 86.9°, P_r of 14.8 μC/cm², and E_c of 1.8 kV/mm. This specimen also exhibited good dielectric and piezoelectric properties: ε^T₃₃/ε_o of 364, d₃₃ of 122 pC/N, k_p of 0.29, and Q_m of 611.     

Localized Temperature Stability in Low‐Temperature Cofired Ceramics

Low‐temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. For LTCC modules targeting radio and microwave frequency (RF and MW) applications, a low or near 0 ppm/°C temperature coefficient of resonant frequency (τ_f) ensures temperature stability of embedded resonator and filter functions. The base dielectrics of most commercial LTCC systems have a τ_f in the range ?50 to ?80 ppm/°C. This study explored a method to achieve a zero τ_f on stripline (SL) resonators by locally cofiring, in a multilayer LTCC structure, compensating dielectrics (CD) with an opposite τ_f to that of the host dielectric. The formulation, synthesis, dielectric properties, and microstructure of SrTiO₃ (STO)‐based low‐fire τ_f CD are presented. Chemical interactions and physical compatibility between the compensating and the host LTCC dielectrics are investigated for cofireability. The dependence of τ_f compensation on the wt% of STO, the printed thickness, and the location of the CD in multilayer LTCC are discussed. The most effective τ_f compensation is achieved by integrating CD next to the resonator lines, and can be explained by the concentration of electromagnetic energy via total internal reflection of electromagnetic waves inside the CD layer.     

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.     

Low‐Temperature Magnetic and Dielectric Anomalies in Rare‐Earth‐Substituted BiFeO3 Ceramics

In this work, we report on the magnetic and dielectric anomalies observed in dense Bi_1–xRE_xFeO₃ ceramics (RE = Dy, Tb; 0 ≤ x ≤ 0.3) at cryogenic temperatures. For compositions with a high content of rare‐earth ions, thermomagnetic experiments revealed a distinct anomaly in the magnetization curves at temperatures below 200 K. The temperature of the magnetic anomaly along with a thermal hysteresis was found to be dependent on the rare‐earth concentration and magnetic field strength. Low‐temperature dielectric measurements showed an anomalous relaxor‐like behavior of the relative permittivity and dielectric loss in highly doped ceramic samples. The anomalies in low‐temperature magnetization and dielectric response are suggested to result from the presence of GdFeO₃‐like orthoferrite phase and/or bismuth rare‐earth‐mixed iron garnet impurities.     

Low‐Temperature Preparation of β‐Si3N4 Porous Ceramics with a Small Amount of Li2O–Y2O3

Porous β‐Si₃N₄ ceramics are sintered at 1600°C in N₂ and postheat treated at 1500°C under vacuum using Li₂O and Y₂O₃ as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li₂O. The addition of Y₂O₃ is advantageous for the formation of high aspect ratio β‐Si₃N₄ grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β‐Si₃N₄ porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li₂O and 0.33 wt% Y₂O₃, and it is suitable for high‐temperature applications.     

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.     

Low‐Temperature Control of Twins and Abnormal Grain Growth in BaTiO3

The microstructure of polycrystalline barium titanate (BaTiO₃) thin films processed with a liquid‐phase can be controlled by the crystallographic orientation of the underlying sapphire substrate. During postdeposition crystallization, the tendency for {111} twin nucleation, which drives subsequent abnormal grain growth, depends upon the specific sapphire facet. Specifically, tilting away from the close‐packed c‐plane modifies the orientation, morphology, and relative amount of an interfacial BaAl₂O₄ second phase. These factors control the density of twin formation, and thus overall grain size of the crystallized BaTiO₃. As the substrate orientation transitions from c‐plane, to r‐plane, to a‐plane, the twin density is reduced, the average grain size decreases systematically from 270 to 130 nm, and the grain structure becomes overall more homogeneous. This twinning mechanism and abnormal grain growth occur by 900°C, several hundred degrees lower than reported previously.