As-sintered Ceramics Research Articles

3D printing PMN-PT textured ceramics for transducer applications

3D printing stereolithography (SL) technique were successfully applied in the preparation of <001>-textured 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-29PT) ceramics in this work. The as-sintered textured ceramics show obvious grain orientation control effects, and the relative density reaches 93% of the theoretical density. The obtained ceramics demonstrate typical dielectric/ferroelectric/piezoelectric properties, enabling the device applications. The ultrasonic transducer with the center frequency of 6.7 MHz and the −6 dB bandwidth of 25% are designed and prepared based on the obtained textured ceramic. This work signifies the great potential of 3D printing SL technology on the fabrication of piezoelectric ceramic elements for advanced applications.

A Novel Preparation Method of (Ti,Zr,Nb,Mo,W)B2-SiC Composite Ceramic Based on Reactive Sintering of Pre-Alloyed Metals

High-entropy diboride-based (MeB2-based) ceramics are promising high-temperature structural materials because of their excellent mechanical properties, high-temperature stability, and oxidation resistance. In order to achieve low-temperature sintering of the high-entropy ceramics, a novel preparation method of high-entropy (Ti,Zr,Nb,Mo,W)B2-SiC ceramics based on reactive sintering of pre-alloyed solid-solution metals and nonmetals of Si, C, B4C was conducted in the current work. Mechanical alloying behavior of the mixed metal powders, as well as the phase composition, microstructure, mechanical properties, and oxidation behavior of the as-sintered MeB2-SiC ceramic were investigated. The XRD, SEM, and EPMA results indicated that the primary MeB2 solid-solution and SiC phases could be successfully formed during reactive sintering at a relatively low temperature of 1650 °C. The as-sintered MeB2-SiC ceramics had a high relative density of 97.8% and high mechanical properties (hardness of 19.74 ± 0.8 GPa, flexure strength of 533 ± 38 MPa, and fracture toughness of 6.01 ± 0.77 MPa·m1/2). Combining the oxidation behavior and microstructure evolution of the oxidation layer, a continuous and relatively dense MeOx-SiO2 oxidation layer was gradually formed and covered on the external surface, leading to decelerating oxidation behavior after an oxidation exposure time of 10 min.

Investigating the effect of solid loading on microstructure, mechanical properties, and translucency of highly translucent zirconia ceramics prepared via stereolithography-based additive manufacturing

A study was conducted to explore the potential of 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) ceramics for dental restorations, using stereolithography (SLA) printing technique. Four different solid loadings were established in the ceramic paste systems to study their effects on microstructure, mechanical properties, and translucency. The study examined the rheological behavior and photopolymerization performance of the ceramic pastes with varying solid loadings. Results showed that, an increase in powder concentration resulted in a decrease in cure depth (Cd) and penetration depth (Dp). A narrower pore size distribution was observed in the green bodies with a high solid loading, facilitating the achievement of final densification. The green and sintered densities were highest at 52 vol%, with values of 3.46 ± 0.01 g/cm3 and 6.01 ± 0.02 g/cm3, respectively. Additionally, all of the green strengths exceeded 30 MPa, with a maximum of 35.09 ± 2.02 MPa obtained at 44 vol%. The maximum flexural strength and minimum contrast ratio (CR) value of 746 ± 75 MPa and 0.40 ± 0.01 were achieved at 52 vol% after sintering. No significant differences were observed in the phase composition and hardness of the as-sintered ceramics. Though significant differences were observed in photopolymerization performance, four materials showed similar structural reliability considering Weibull modulus and characteristic strength.

Titanium nitrides as novel reactants for in-situ synthesis of TiB2-based composites with improved properties

A novel in-situ reactive approach based on the reactions among TiN, aluminum and boron has been developed to synthesize TiB 2 -based composites including TiB 2 - h BN (TB) and TiB 2 - h BN-AlN (TBA). Fully dense ceramics with fine-grained microstructure were successfully obtained via spark plasma sintering at 1850 °C/60 MPa/5 min. Microstructure analysis suggests h BN flakes were homogenously distributed in the TiB 2 matrix. For AlN, however, they were elongated into plate-like grains during sintering, in which lots of defects in terms of stacking faults and twinning structures were observed. The mechanical and thermophysical properties of as-sintered ceramics were comprehensively investigated and compared. Incorporating AlN significantly improved the flexure strength, hardness, fracture toughness and thermal conductivity of TiB 2 - h BN ceramics. The electrical conductivity of TB (3.06 × 10 6 S/m) is larger than that of TBA (2.35 × 10 6 S/m) at room temperature, but the value (6 × 10 5 S/m) was lower than that of TBA (6.9 × 10 5 S/m) at 1173 K. Based on the measurement of electrical and thermal conductivity, electron and phonon contributions to thermal conductivities of TB and TBA were calculated and their temperature dependences were illustrated.

Effects of Zr4+-doping on the properties of (Lu, Gd)2O3: Eu transparent ceramics: Insight from the photoluminescent spectra in as-sintered and annealed state

We show the effects of the Zr4+-doping on the microstructural, optical, and luminescent properties of (Lu, Gd)2O3: Eu transparent ceramics in its as-sintered state and annealed state. Zr4+-dopant greatly enhanced the transparency of (Lu, Gd)2O3: Eu ceramic, giving Lu0.42Gd0.5Zr0.03Eu0.05O1.5+δ ceramic with a transmittance of up to 80% at 600 nm. It is found that Zr4+-dopant dominates the luminescent properties of Eu ions in both as-sintered and annealed ceramics. Energy transfer from Eu2+ to Eu3+ was found by photoluminescence spectra, and the ratio of Eu2+/(Eu2++Eu3+) was affected by Zr4+ content in as-sintered ceramics. Moreover, the oxygen vacancy concentration increased with increasing Zr4+ content in as-sintered ceramics. The annealing process enhanced the intensity of all the excitation bands of 4f→4f transitions and the intensity of charge transfer band (CTB) decreased as the Zr4+ content increased in the annealed ceramics. This work highlights that Zr4+ doping is a potential way to improve the transmittance and red emission intensity of 4f→4f transition of Eu3+ in the (Lu, Gd)2O3: Eu ceramic for extended applications.

Processing and properties of reactively densified TiB2-AlN-hBN conductive ceramics with tunable compositions

Reactive sintering has been widely utilized to densify ceramics. Although the technology has numerous advantages, the composition of as-sintered ceramics is hard to be adjusted unless additional products are added. In this work, a novel solid-state route based on the reactions among TiN, Al and B were proposed to consolidate TiB2-AlN-hBN ceramics (TAB) with tunable compositions (hBN ≤ 42 vol%, AlN ≤ 45 vol%). Dense TAB with high relative density, refined grain size and homogeneous microstructure were obtained via spark plasma sintering at 1800 °C and 60 MPa. It was found that exothermic reactions between the reactants could be ignited during heating, and the order of temperature at which the combustion process occurred was just opposite to the sequence of adiabatic temperature for individual reactions. Effects of hBN and AlN amounts on the densification, microstructure, mechanical properties, machinability and electrical resistivity of as-sintered ceramics were comprehensively investigated. Compared to AlN, hBN content played a more obvious role on those properties. As hBN contents increased from 0 to 42 vol%, the flexural strength, fracture toughness, Vickers hardness and modulus of TAB continuously decreased. TAB with hBN amounts ≥ 13 vol% exhibited better machinability with surface roughness lower than 2.9 µm after machining. Nevertheless, their electrical resistivity values at room temperature fluctuated in a narrow range between 43 μΩ·cm and 83 μΩ·cm, irrelevant with the hBN amounts.

Fast grain growth phenomenon in high-entropy ceramics: A case study in rare-earth hexaaluminates

It is generally reported that the grain growth in high-entropy ceramics at high temperatures is relatively slower than that in the corresponding single-component ceramics owing to the so-called sluggish diffusion effect. In this study, we report a fast grain growth phenomenon in the high-entropy ceramics (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)MgAl₁₁O₁₉ (HEMA) prepared by a conventional solid-state reaction method. The results demonstrate that the grain sizes of the as-sintered HEMA ceramics are larger than those of the corresponding five single-component ceramics prepared by the same pressureless sintering process, and the grain growth rate of HEMA ceramics is obviously higher than those of the five single-component ceramics during the subsequent heat treatment. Such fast grain growth phenomenon indicates that the sluggish diffusion effect cannot dominate the grain growth behavior of the current high-entropy ceramics. The X-ray photoelectron spectroscopy (XPS) analysis reveals that there are more oxygen vacancies (O_V) in the high-entropy ceramics than those in the single-component ceramics owing to the variable valance states of Eu ion. The high-temperature electrical conductivities of the HEMA ceramics support this analysis. It is considered that the high concentration of O_V and its high mobility in HEMA ceramics contribute to the accelerated migration and diffusion of cations and consequently increase the grain growth rate. Based on this study, it is believed that multiple intrinsic factors for the high-entropy ceramic system will simultaneously determine the grain growth behavior at high temperatures.

B4C-based hard and tough ceramics densified via spark plasma sintering using a novel Mg2Si sintering aid

Full-dense B 4 C-based ceramics with excellent mechanical properties were fabricated using spark plasma sintering with Mg 2 Si as a sintering aid at a low temperature of 1675 °C while applying a uniaxial pressure of 50 MPa. The effect of Mg 2 Si addition on the densification behaviours, mechanical properties and microstructure of as-sintered ceramics were investigated. Not only did the formation of ultra-fine grained SiC using the in-situ reaction effectively inhibit the growth of B 4 C grains, but it also contributed to the strength and toughness of the resultant ceramics. Additionally, microalloying Mg imparted more metal bonding characteristics to the B 4 C matrix, thereby improving their ductility. The results indicate that the composite containing 7 wt% Mg 2 Si had excellent mechanical properties, including a light weight of 2.54 g/cm 3 , Vickers hardness of 34.3 GPa, fracture toughness of 5.09 MPa m 1/2 and flexural strength of 574 MPa.

Significantly toughened PZT-based piezoelectric ceramics simultaneously keeping excellent electrical properties via incorporating trace amount of yttria-partially-stabilized zirconia

Piezoelectric ceramics are prone to cracking during functioning due to the repetitively applied high stress. Hence, toughened ceramics with robust microstructures are highly desired for resisting the potential cracks. Toughening of Lead Zirconium Titanate (PZT) based piezoelectric ceramics with high d33·g33 for ignition applications is investigated in this work. Yttria-partially-stabilized zirconia (YPSZ) particles were incorporated into the PZT-based piezoelectric ceramics via conventional solid state reaction. Measurements of mechanical properties of as-sintered ceramics indicate that incorporation of an appropriate amount of YPSZ significantly improves the fracture toughness of PZT-based ceramics by 38.7%, meanwhile the piezoelectric coefficient still remains at a high level with a decrease of less than 7%. When doping 0.5 wt% YPSZ, the optimal comprehensive properties can be obtained as follows: fracture toughness KIC = 1.41 MPa·m1/2, Vickers hardness HV = 4.60 GPa, piezoelectric coefficient d33 = 462 pC/N, planar electromechanical coupling factor kp = 60.1%, and dielectric constant ε33T/ε0 = 1750. This successful toughening mechanism paves a way in developing functional ceramics simultaneously possessing superior fracture toughness and electrical properties via doping an appropriate content of YPSZ submicron particles.

Influence of zirconia on the sintering behaviour and mechanical properties of reaction-sintered mullite-based composite ceramics

ABSTRACTHigh-performance mullite-based composite ceramics were prepared successfully using natural kaolin and alumina as raw materials and ZrO2 as an additive. The influence of sintering temperature and ZrO2 content on the sintering behaviour and mechanical properties of zirconia-toughened mullite ceramics was studied systematically. With increasing sintering temperature from 1450°C to 1560°C, the primary phases of as-sintered composite ceramics were mullite and corundum with a small amount of ZrO2, and the bulk density of the composite ceramics increased from 2.29 to 2.72 g cm–3. Furthermore, the ZrO2 phase transition promoted transgranular fracture, and ZrO2 grains were pinned at the grain boundaries, thereby enhancing the mechanical strength of the composite ceramics. Moreover, the AZS12 sample, with 12 wt.% ZrO2 and sintered at 1560°C, had the greatest flexural strength and fracture toughness of 91.6 MPa and 2.47 MPa m–1/2, respectively. Adding ZrO2 to the composite ceramics increased their flexural strength by ~37.6%.

Microstructural refinement and mechanical response of Al2O3–ZrO2 eutectics fabricated by a novel pulse discharge plasma assisted melting method

In the present work, microstructural refinement and mechanical response of Al2O3–ZrO2 eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al2O3–ZrO2 eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al2O3–ZrO2 eutectics is investigated.

As-Sintered Manganese-Stabilized Zirconia Ceramics with Excellent Electrical Conductivity

As promising electrolyte materials in intermediate-temperature solid oxide fuel cells (IT-SOFCs), Sc-stabilized ZrO2 (ScSZ) and Y-stabilized ZrO2 (YSZ) electrolytes continue to be plagued by high cost and low intermediate conductivity. To mitigate these problems, Mn has been chosen as a new stabilization element for the synthesis of Mn-stabilized ZrO2 ceramics (MnSZ) through solid state sintering. Microstructures and electrical properties of micron-crystalline Zr1−xMnxO2−δ (x = 0.15, 0.20 and 0.25) ceramics electrolytes for IT-SOFCs have been systematically evaluated. Within the applied doping content, Mn2+ ions can enter the ZrO2 crystal lattice, leading to the formation of single cubic phase samples. Electrical conductivity measurements in the temperature range between 400 °C and 1000 °C show a sharp increase in conductivity due to Mn doping. The highest conductivity is obtained from the Zr0.75Mn0.25O2−δ samples, being 0.0144 S/cm at 600 °C and 0.182 S/cm at 1000 °C. The electrical conductivity at 600 °C is twice higher than that of the YSZ and two orders of magnitude higher than that of the ScSZ. These properties can fulfill the conductivity requirement (∼1 × 10−2 S/cm) for the electrolyte. Therefore, based on this study, we propose that Mn stabilized ZrO2 is a promising candidate as a solid electrolyte for IT-SOFCs.

Controlling light-induced dielectric response of Sr/Ni-modified (K0.5Na0.5)NbO3 ceramics by narrow bandgap method

The change of dielectric constant at UV-VIS light irradiation was named as photo dielectric response (PDR). High PDR discovered in lead-free semiconducting ferroelectric ceramics will help to develop new types of photoelectric devices. The (1-x)(K0.5Na0.5)NbO3-xSr(Ni0.5Nb0.5)O3-δ (KNN-xSNN) new system was successfully synthesized by solid-phase sintering. The phase structure of KNN-xSNN ceramics gradually evolves from the orthogonal phase to the cubic phase. The size of grains of as-sintered ceramics is reduced to the sub-micron level of 150–180 nm. The bandgap is tuned to ∼1.17 eV. High dielectric tunability in UV-VIS region can be achieved, which shows the tunability of ∼452% at 80 Hz and ∼18.1% at 100 kHz under 365 nm light, and ∼133.2% under 440 nm light in the KNN-0.04SNN ceramic. Meanwhile, the reproducible capacitive switching response under the light off and on state can be observed, which shows a capacitive ratio of about 6. These results indicate that this system has considerable advantages in the development of contactless devices under UV-VIS region.

Microstructural Evolution and High-Performance Giant Dielectric Properties of Lu3+/Nb5+ Co-Doped TiO2 Ceramics.

Giant dielectric (GD) oxides exhibiting extremely large dielectric permittivities (ε’ > 104) have been extensively studied because of their potential for use in passive electronic devices. However, the unacceptable loss tangents (tanδ) and temperature instability with respect to ε’ continue to be a significant hindrance to their development. In this study, a novel GD oxide, exhibiting an extremely large ε’ value of approximately 7.55 × 104 and an extremely low tanδ value of approximately 0.007 at 103 Hz, has been reported. These remarkable properties were attributed to the synthesis of a Lu3+/Nb5+ co-doped TiO2 (LuNTO) ceramic containing an appropriate co-dopant concentration. Furthermore, the variation in the ε’ values between the temperatures of −60 °C and 210 °C did not exceed ±15% of the reference value obtained at 25 °C. The effects of the grains, grain boundaries, and second phase particles on the dielectric properties were evaluated to determine the dielectric properties exhibited by LuNTO ceramics. A highly dense microstructure was obtained in the as-sintered ceramics. The existence of a LuNbTiO6 microwave-dielectric phase was confirmed when the co-dopant concentration was increased to 1%, thereby affecting the dielectric behavior of the LuNTO ceramics. The excellent dielectric properties exhibited by the LuNTO ceramics were attributed to their inhomogeneous microstructure. The microstructure was composed of semiconducting grains, consisting of Ti3+ ions formed by Nb5+ dopant ions, alongside ultra-high-resistance grain boundaries. The effects of the semiconducting grains, insulating grain boundaries (GBs), and secondary microwave phase particles on the dielectric relaxations are explained based on their interfacial polarizations. The results suggest that a significant enhancement of the GB properties is the key toward improvement of the GD properties, while the presence of second phase particles may not always be effective.

Synthesis and high thermal stability of Mn doped Y2/3Cu3Ti4O12 negative temperature coefficient ceramic

In this paper, high thermal stability of Mn doped Y2/3Cu3Ti4O12 negative temperature coefficient (NTC) ceramics have been successfully fabricated by the combination of Pechini powder synthesis and solid-state reaction sintering. The substitution of Mn for Ti plays a critical role in controlling the phase structure, microstructure, electrical properties, and stability. The structure of as-sintered ceramics is a single Y2/3Cu3Ti4O12 phase at x ​≤ ​0.5, while the CuO phase begins to appear at x ​≥ ​0.7. X-ray photoelectron spectroscopy further confirms the change of cation concentration, one of the critical factors for electrical conductivity in Y2/3Cu3Ti4-xMnxO12 ceramics. In the temperature of −50–250 ​°C, the resistivity all decreases with increasing temperature and Mn content, the aging coefficient of all samples is within 0.4%, and the minimum can reach 0.2%. We strongly propose that the Mn substitution and Pechini method are highly desirable for the fabrication of high stability NTC thermistors for medi-temperature region application.

New high temperature NTC thermistors based on BaSm2Ti4O12 ceramics

In our research it is firstly found that BaSm2Ti4O12 (BSTO) ceramics only show negative temperature coefficient (NTC) effect in wide temperature range. The effects of sintering temperature on the microstructure and electrical properties of BSTO have been investigated. The relative density of as-sintered ceramics increases at first and then decreases with increasing sintering temperature. In the temperature of 400-850℃, all samples show a linear relationship between the natural logarithm of resistivity and the reciprocal of temperature, indicating the NTC characteristics. The electrical resistivity decreases with increasing sintering temperatures. However, the activation energy Ea400/850℃ and material constant B400/850℃ decrease first and then increase, which are attributed to the variation of oxygen vacancy. The value of resistivity ρ400℃, Ea400/850℃ and B400/850℃ is in the range of 4.8×107–1.43×108Ω•cm, 1.33–1.41 eV and 15300–16400 K, respectively. This material has broad prospects in the field of high-sensitivity high-temperature thermistors research.

Stabilization of 6H-Hexagonal SrMnO3 Polymorph by Al2O3 insertion

We demonstrate the structural evolution of polymorphic phases in Al2O3-inserted SrMnO3 ceramics synthesized by solid state reaction. While the 4H-hexagonal phase is predominant in pure SrMnO3 ceramics, a small amount of 6H-hexagonal polymorph is identified in addition to the primary 4H-hexagonal SrMnO3 and the secondary hexagonal SrAl2O4 phases in the as-sintered ceramics, evidenced by x-ray diffraction and subsequent Rietveld refinement analyses. The existence of the 6H-hexagonal SrMnO3 phase is corroborated using Raman spectroscopy. The chemical compositions and electronic structures of the Al2O3-inserted SrMnO3 compounds are also examined using energy dispersive spectroscopy and x-ray photoelectron spectroscopy, respectively. The first-principles calculations reveal that there is no clear difference between the total energies of 4H- and 6H-hexagonal polymorphs regardless of the presence/absence of Sr and oxygen vacancies. Possible origins are discussed with the estimation of actual strain based on the refined lattice parameter of 6H SrMnO3.

Variation of leakage current conduction mechanism by heat treatment in Bi-based lead-free piezoelectric ceramics

The leakage current mechanism in the as-sintered and quenched 0.75BiFeO3–0.25BaTiO3 (0.75BF–0.25BT) ceramics is evaluated by the space-charge-limited current (SCLC), Poole–Frenkel (P–F) emission, Schottky (ST) emission, and Fowler–Nordheim (F–N) tunneling mechanism. The discrepancy observed in the optical dielectric constant of BF and BT between the reported value and the values calculated from the P–F and ST emission plots suggests that the P–F emission and ST emission mechanisms do not contribute to the leakage current behavior of the 0.75BF–0.25BT ceramics. Subsequently, F–N tunneling is observed under a high electric field in the as-sintered 0.75BF–0.25BT ceramics, whereas the direct tunneling effect is exhibited throughout the measured electric field in the quenched ceramics. The SCLC mechanism is dominant in both the as-sintered and quenched 0.75BF–0.25BT ceramics. A change from Ohmic conduction to trap-filled-limit conduction is observed with an increase in the applied electric field in the as-sintered ceramics, whereas the quenched ceramics only revealed Ohmic conduction over the entire range of the measured electric field. The different behaviors exhibited in the SCLC mechanism between the as-sintered and the quenched ceramics imply that the conduction mechanism can be controlled by heat treatment.

Effects of TiN content on the properties of hot pressed TiB2–SiC ceramics

This research intended to study the impacts of different contents of the TiN additive on the mechanical properties and microstructural features of the TiB2–SiC-based composites. Three different samples of TiB2-15 vol% SiC- x vol% TiN (x = 0, 3.5, and 7) were produced by hot-pressing at 2000 °C under 35 MPa for 120 min. Thanks to advancement of some reactions among the TiB2 surface oxides and the SiC reinforcement, two in-situ phases of TiC and SiO2 were produced during the sintering. Nevertheless, the TiN incorporation resulted in generating another in-situ compound (TiC0.3N0.7) in the relevant as-sintered ceramics. Moreover, introducing TiN significantly refined the microstructure of the composites, leading to higher mechanical characteristics. Finally, the highest flexural strength (781 MPa) and Vickers hardness (27.1 GPa) values were attained for the sample introduced by 7 vol% TiN.

Electrical properties of SiC-AlN ceramics pressureless sintered under N2 atmosphere

SiC-AlN ceramics were pressureless sintered at 2150 °C under N2 atmosphere, while B4C and C were added to promote the sintering process. The microstructure, electrical and mechanical properties of the as-sintered ceramics prepared by different AlN addition are discussed. More AlN addition tends to lead a smaller grain size. As AlN contents go up, the varistor voltage of the samples increases sharply from 17.304v·mm−1 to more than 100 v·mm−1 because of the improvement of grain boundary resistance. Correspondingly, the reasons for the increase of grain boundary resistance are: (1) the improvement of carrier compensation increases the Schottky barrier height and width; (2) more grain boundaries increase the number of Schottky barriers. In addition, the electrical properties of the same components sintered under N2 and Ar have been compared, that the varistor voltage of the same component SiC-AlN ceramics sintered under N2 are much higher than those under Ar (about 2 orders of magnitude). With the increase of AlN contents, the mechanical properties remain unchanged except the little reduction of Vickers hardness.