Rapid microwave sintering of alumina ceramics with an addition of carbon nanotubes

Compacted alumina samples with an addition of 1.0 and 2.5 wt % carbon nanotubes (CNT) have been microwave sintered at heating rates 50 and 100°С/min to a maximum temperature of 1550–1600°С with zero hold time. The densification kinetics has been studied using optical dilatometry. No noticeable influence of CNT on the development of thermal instability during rapid microwave sintering of alumina has been detected. The relative densities of the samples containing 1.0 and 2.5 wt % CNT sintered at a maximum temperature of 1550 °C and zero hold time were 93.8 and 87.5%, respectively. Microwave sintering with a repeated development of thermal instability has resulted in an increase in the final density to 95.0%.     

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm⁻².     

Rapid transformation from Cs-geopolymers to Cs-defined ceramics by microwave sintering

Cs-defined ceramics are considered to be ideal solidification forms for ¹³⁷Cs with high solubility, high volatility, and high mobility in Cs-containing nuclear wastes. In this study, a process consisting of low-temperature solidification and subsequent rapid microwave sintering was proposed to treat the simulated nuclide ¹³⁷Cs. Cs-geopolymer (CsGP) precursors were first prepared to form solidified forms containing simulated ¹³⁷Cs under ambient conditions. Afterwards, a rapid transformation from CsGP to Cs-defined ceramics was realized by microwave sintering below 1100 °C for 30 min, which could effectively avoid the Cs volatility while Cs entered into Cs-defined ceramic lattices under conventionally long-term and high-temperature sintering. As-prepared Cs-defined ceramics exhibited superior chemical durability, and the normalized Cs⁺ leaching rate was as low as 3.06 × 10⁻⁴ g/(m^2.d).     

Effect of microwave sintering on the properties of copper oxide doped Y-TZP ceramics

The effect of various amounts of copper oxide (CuO) up to 1?wt% on the densification behaviour and mechanical properties of 3?mol% yttria-tetragonal zirconia polycrystal (Y-TZP) were studied by using microwave (MW) sintering method. The MW sintering was performed at temperatures between 1100?°C and 1400?°C, with a heating rate of 30?°C/min. and holding time of 5?min. The beneficial effect of MW in enhancing densification was also compared for the undoped and 0.2?wt% CuO-doped Y-TZP when subjected to conventional sintering (CS) method. The results showed that significant enhancement in the relative density and Vickers hardness were observed for the undoped Y-TZP when MW-sintered between 1100?°C and 1250?°C. It was revealed that the 0.2?wt% CuO-doped Y-TZP and MW sintered at 1250–1300?°C could attain ≥?99.8% of theoretical density, Vickers hardness of about 14.4?GPa, fracture toughness of 7.8 MPam^1/2 and exhibited fine equiaxed tetragonal grain size of below 0.25?µm. In contrast, the addition of 1?wt% CuO was detrimental and the samples exhibited about 50% monoclinic phase upon sintering coupled with poor bulk density and mechanical properties. The study also revealed that the addition of 0.2?wt% CuO and subjected to conventional sintering produced similar densification as that obtained for microwave sintering, thus indicating that the dopant played a more significant role than the sintering method.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Inversion boundary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO₂/Bi₂O₃ ratio. The selection of a composition with an appropriate TiO₂/Bi₂O₃ ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi₂O₃ is important as the grain growth is slowed down by a larger amount of Bi₂O₃-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO₂ which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn₂TiO₄ spinel phase only affects grain growth in compositions with a TiO₂/Bi₂O₃ ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb₂O₃ trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb₂O₃/Bi₂O₃ ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb₂O₃/Bi₂O₃ ratios.     

Tin oxide-based ceramics of high density obtained by pressureless sintering

Dense tin oxide-based ceramic semiconductors have a high potential as electrodes for aluminum production, glass industry, sputtering targets for transparent conducting thin films, varistors, and thermoelectrics due to their specific electrical properties, high corrosion resistance and ability to withstand high temperatures. The application of these ceramics is still limited because of the necessity to reach density of 99% of TD or greater and high electrical conductivity, and because of the manufacturing difficulties to produce components of different shapes and sizes with low cost. The paper reviews the results of the works conducted towards obtaining tin oxide-based ceramics with density up to 99.5+% of TD through low-temperature pressureless sintering. The selected sintering aids/dopants and firing conditions promoted both liquid phase sintering and electrical conductivity. The uniform microcrystalline structure is obtained.     

Improved sintering behavior and microwave dielectric properties of SnO2-based ceramics and their LTCC materials with ultrawide temperature stability

Microwave dielectric ceramics with simple composition, a low permittivity (ε_r), high quality factor (Q × f) and temperature stability, specifically in the ultrawide temperature range, are vital for millimetre-wave communication. Hence, in this study, the improvements in sintering behavior and microwave dielectric properties of the SnO₂ ceramic with a porous microstructure were investigated. The relative density of the Sn_1-xTi_xO₂ ceramic (65.1%) was improved to 98.8%, and the optimal sintering temperature of Sn_1-xTi_xO₂ ceramics reduced from 1525 °C to 1325 °C when Sn⁴⁺ was substituted with Ti⁴⁺. Furthermore, the ε_r of Sn_1-xTi_xO₂ (0 ≤ x ≤ 1.0) ceramics increased gradually with the rise in x, which can be ascribed to the increase in ionic polarisability and rattling effects of (Sn_1-xTi_x)⁴⁺. The intrinsic dielectric loss was mainly controlled by r_c (Sn/Ti–O), and the negative τ_f of the SnO₂ ceramic was optimised to near zero (x = 0.1) by the Ti⁴⁺ substitution for Sn⁴⁺. This study also explored the ideal microwave dielectric properties (ε_r = 13.7, Q × f = 40,700 GHz at 9.9 GHz, and τ_f = ?7.2 ppm/°C) of the Sn_0.9Ti_0.1O₂ ceramic. Its optimal sintering temperature was decreased to 950 °C when the sintering aids (ZnO–B₂O₃ glass and LiF) were introduced. The Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic also exhibited excellent microwave dielectric properties (ε_r = 12.8, Q × f = 23,000 GHz at 10.5 GHz, and τ_f = ?17.1 ppm/°C). At the ultrawide temperature range (?150 °C to +125 °C), the τ_ε of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic was +13.3 ppm/°C, indicating excellent temperature stability. The good chemical compatibility of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic and the Ag electrode demonstrates their potential application for millimetre-wave communication.     

Flash microwave sintering of zirconia by multiple susceptors cascade strategy

In the field of flash sintering, microwave energy represents an interesting way to densify ceramics complex shapes, thanks to a contactless volumetric heating. Attaining a fast and homogeneous heating is a critical parameter and hybrid heating, using silicon carbide susceptors, is generally used. In this study, an original multiple susceptors cascade strategy is developed, using both SiC and 3D-printed ZrO₂ susceptors. This novel configuration follows perfectly the flash heating scheme, even for high heating rates up to 1000 K.min^-1 and leads to a high stability of the “flash” hybrid heating. Flash microwave sintering produced dense (97 % relative density) microstructures within 45 s. Based on comprehensive multiphysics simulations of the overall process, in-situ dilatometry measurements, kinetics method analysis and microstructural characterizations, this work highlights the sintering behavior of zirconia and the temperature distribution during flash microwave sintering.     

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi₂O₃, Mn₂O₃, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2?5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi₂O₃-Mn₂O₃-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.     

Pressure-assisted microwave sintering: A rapid process to sinter submicron sized grained MgAl2O4 transparent ceramics

This work focuses on the development of an original process based on a 2.45 GHz single-mode microwave cavity equipped with a uniaxial press, to sinter transparent spinel MgAl₂O₄ ceramic in air. The samples were conventionally pre-sintered to a density of 90% TD before microwave sintering to the final stage of densification. The influence of thermomechanical cycle on the material properties was investigated. Transmittance, grain size distribution, hardness and fracture toughness of the samples were measured and correlated to the microstructure. This new sintering process has allowed obtaining transparent samples with sub micrometric grain size and high mechanical properties, with relatively short times and low temperature. These first results can be compared to some obtained by SPS or HIP. The technical input of this method is that all the process is here conducted in air atmosphere.     

Multiscale defect responses in understanding degradation in zinc oxide varistor ceramics

Dielectric responses of multiscale defects, including electron trapping behaviours of intrinsic point defects in grains, heterogeneous interface of depletion/intergranular layers, and dynamic responses of interface states at grain boundaries, were investigated in ZnO varistor ceramics via the DC ageing process. The dielectric relaxation peaks corresponding to zinc interstitials and oxygen vacancies increased, revealing that the donor densities gradually increase with ageing time. Dynamic response of the interface states, which were observed to undergo significant dc conductance, using traditional dielectric spectroscopy, is unveiled based on an optimised (∂ε'/∂lnω)/ε′ spectroscopy. The energy level of interface states monotonously decreased while the relaxation peak value increased, corresponding to decreased interface states with degradation time. Collectively, the degradation of Schottky barrier during DC ageing process is discussed based on multiscale defect responses with the help of the (∂ε'/∂lnω)/ε′ spectroscopy.     

Effect of MgO/La2O3 additives on sintering behavior and microwave dielectric properties of (Sr,Ca)TiO3-(Sm,Nd)AlO3 ceramics

The effect of MgO/La₂O₃ additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃−0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT-3SNA) ceramics prepared via conventional solid-state route were systematically investigated. MgO/La₂O₃ as additives showed no obvious influence on the phase composition of the 7SCT-3SNA ceramics and all the samples exhibited pure perovskite structures. The presence of MgO/La₂O₃ additives effectively reduced the sintering temperature of 7SCT-3SNA ceramics due to the formation of a liquid phase at a relatively low temperature during sintering progress. The 0.5 wt% MgO doped 7SCT-3SNA sample with 0.5 wt% of La₂O₃, sintered at 1320 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 45.57, a Q×f value of 46205 GHz (at 5.5 GHz), and τ_f value of −0.32 ppm/°C, which showed dense and uniform microstructure as well as well-developed grain growth.     

Microstructure and electrical properties of rare earth doped ZnO-based varistor ceramics

ZnO-based varistor ceramics doped with Nd₂O₃ and Y₂O₃ have been prepared by the conventional ceramics method. The phase composition, microstructure and electrical properties of the ceramics have been investigated by XRD, SEM and a V–I source/measure unit. The XRD and EDS analyses show the presence of ZnO, Bi₂O₃, Zn₇Sb₂O₁₂, Y₂O₃, Nd-rich phase and Y-containing Bi-rich phase. The electrical properties analyzed show that the nonlinear coefficient of the varistor ceramics is in the range of 4.4–70.2, the threshold voltage is in the range of 247.1–1288.8 V/mm, and the leakage current is in the range of 1.51–214.6 μA/cm². The 0.25 mol% Nd₂O₃ added varistor ceramics with 0.10 mol%Y₂O₃ sintered at 1050 °C exhibits excellent electrical properties with the high threshold voltage of 556.4 V/mm, the nonlinear coefficient of 61 and the leakage current of 1.55 μA/cm². The results illustrate that doping Nd₂O₃ and Y₂O₃ in ZnO-based varistor ceramics may be a very promising route for the production of the higher threshold voltage and the nonlinear coefficient of ZnO-based varistor ceramics.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Low-voltage ZnO varistor fabricated by the solution-coating method

A novel solution nano-coating technique, by coating ZnO powder with a mixed solution of dopants, has been developed to produce high performance low-voltage ZnO varistors. The sintering temperature in the present route is about 50 °C lower than that in the conventional oxide mixing route. The microstructure and electrical characteristics were examined by XRD, SEM and dc power supply and the results showed that the specimens prepared by the solution-coating route have bigger grain sizes, more evenly distributed intergranular phases, higher densities and nonlinearity coefficients, lower breakdown fields and leakage currents than those from the conventional oxide mixing route. The improved current–voltage properties are attributed to the excellent performance of the nano-composite ZnO powder and the advantages of the solution nano-coating technique.     

Mechanical modelling of microwave sintering and experimental validation on an alumina powder

Microwave sintering (MW) allows fast heating (≤30 min) and densification of ceramic materials, like alumina Al₂O₃. In order to predict the final material properties (density, size and grain size) the mechanical SOVS (Skorohold Olevsky Viscous Sintering) model is adapted and validated for conventional sintering of alumina. The model is implemented on ABAQUS with UMAT subroutine. Secondly, the SOVS model is modified for the microwave sintering by adapting the shear viscosity Arrhenius type law. Pre-exponential and exponential coefficients are modified for MW sintering. The calculated relative densities are compared to experimental results from conventional and microwave sintering and the relative difference remains under 3%. The coefficients identified for the MW sintering reveal a decrease in the shear viscosity by around 10 and an increase by up to 50 times in the grain boundaries diffusion coefficient.