Rapid densification of SiC ceramic rollers by microwave sintering

Ultralarge silicon carbide (SiC) ceramic rollers are successfully prepared in a microwave multimode cavity at 2·45 GHz. The length of the rollers is 1220 mm with outer diameter of 62 mm and thickness of 7 mm. The optimised sintering temperature of microwave sintering is ～980°C, which is ～400°C lower than that of conventional sintering. Mullite was detected within microwave sintered samples, rather than conventional sintered ones, which preferentially appeared around SiC as well as inside the residual pores. This might be attributed to the unique features of microwave sintering of ‘hot spot effect’, plasma behaviour and high frequency electromagnetic fields.     

Fabrication of ceramic bioscaffolds from fly ash cenosphere by susceptor-assisted microwave sintering

Cenospheres (CS) are ceramic hollow microspheres and have been used to prepare composite foams for applications such as medical implants. However, its potential standalone application in the biomedical field is not fully explored. Herein, a susceptor-assisted microwave (SMW) sintering approach was used for producing CS foam scaffolds. Owing to the hybrid heating mechanism offered by the SMW process, sintering of the low-dielectric cenospheres was realized. We found that sintering was initiated at a lower temperature (1100 °C) compared to conventional heating (1250 °C) as reported in the literature, probably due to the lower activation energy required by SMW sintering. The physical and compositional properties of the sintered CS specimens were examined, and in vitro studies were performed. The as-fabricated CS foam possessed minimal effect on cell viability. Cells migrated and adhered well within the pores of the specimens, which indicates the potential of the CS as scaffold materials for cell engineering applications.     

Chromium-promoted preparation of forsterite refractory materials from ferronickel slag by microwave sintering

The chromium-promoted preparation of forsterite refractory materials from ferronickel slag was investigated by microwave sintering of the slag with the additions of sintered magnesia and 0–10 wt% chromium oxide (Cr₂O₃). The thermodynamic calculations revealed that the addition of Cr₂O₃ can promote the formations of spinel and liquid phase and maintain high content of forsterite below 1500 °C. The experimental results showed that there existed a stronger promoting effect of Cr₂O₃ additive on the properties of refractory materials in the microwave field than that in conventional sintering. It was attributed to the preferential formation and growth of spinel with stronger microwave absorption than other phases (e.g., enstatite), the existence of more forsterite, and the enhanced densification in association with the presence of more liquid phase at the same temperature. By microwave sintering of the mixture of ferronickel slag, 25 wt% sintered magnesia, and 4 wt% Cr₂O₃ at 1350 °C for 20 min, a superior refractory material with refractoriness of 1801 °C, thermal shock resistance of 6 times, bulk density of 2.97 g/cm³, apparent porosity of 1.4%, and compressive strength of 197 MPa was obtained. Compared with that prepared by conventional sintering at 1350 °C for 2 h, the refractoriness and thermal shock resistance were increased by 175 °C and 100%, respectively. The present study provided a novel method for preparing high-quality refractory materials from ferronickel slag and relevant industrial wastes.     

Effects of sintering temperature on structural and electromagnetic properties of MgCuZn ferrite prepared by microwave sintering

Nanocrystalline magnesium–copper–zinc (Mg_0.30Cu_0.20Zn_0.50Fe₂O₄) ferrites were prepared by microwave sintering technique. The effects of the sintering temperature on particle size and magnetic properties were investigated. In this article, optimum sintering temperature required for MgCuZn ferrite system for obtaining good electromagnetic properties, suitable for applications in low temperature co-fired ceramics (LTCC) chip components was studied. The grain size, initial permeability, dielectric constant and saturation magnetisations were found to increase, and dielectric loss was found to decrease with the increasing sintering temperature. Mg–Cu–Zn ferrites with a permeability of μ?=?1110 (at 1?MHz) were fully densified at the standard LTCC sintering temperature of 950°C.     

High density nano-grained Gd2Zr2O7 ceramic prepared by combined cold and microwave sintering

In this study, nano-grained Gd₂Zr₂O₇ (NGZO) ceramic with a high relative density was prepared by a novel cold sintering process (CSP) assisted by microwave sintering (CSP-MS). The CSP with water as the liquid phase at 280 °C yielded nano-grained ceramic with a relative density of 71.5%. NGZO with a high relative density of 97.1% and average grain size of 73 nm was obtained by subsequent microwave sintering of the cold-sintered sample at 1300 °C. Therefore, CSP-MS is feasible in preparing dense NGZO ceramics with small grain sizes at relatively low sintering temperatures.     

原料预合金化对微波无压烧结金刚石工具性能的影响

以Fe₆₀Cu₄₀预合金粉和单质粉为基础原料,利用微波无压烧结在850 ℃制备了FeCu/WC基金刚石复合材料刀头。通过探讨物料在微波场中的升温特性设计了微波无压烧结制度。采用扫描电子显微镜（SEM）、万能试验机、激光拉曼光谱等表征手段,对比研究了预合金粉和单质粉微波烧结样品的组织形貌等微结构信息,并获得了样品的硬度、致密度、横向断裂强度等力学性能。结果表明,与常规烧结方法相比,微波强化烧结可将温度降低至900 ℃以下,金刚石颗粒没有出现石墨化转变;FeCu/WC基配方在850 ℃微波烧结样品的洛氏硬度、相对致密度、横向断裂强度分别达到102.7 HRB、98.5%、1 035 MPa。并对微波烧结的强化机制和原料预合金化对于基体的增强机理进行了初步探讨,微波无压烧结对于预合金粉体金刚石刀头的制备表现出良好的应用前景。     

整体硬质合金刀具钝化的研制与应用

简要介绍了一种自制整体硬质合金刀具数控立式旋转钝化机的基本结构和主要特点。通过钝化试验结果分析表明,所研制设备的钝化效果达到进口设备的性能指标。     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.     

Fine grained Al2O3/SiC composite ceramic tool material prepared by two-step microwave sintering

Fine-grained Al₂O₃/SiC composite ceramic tool materials were synthesized by two-step microwave sintering. The effects of first-step sintering temperature (T1), content and particle size of SiC on the microstructure and mechanical properties were studied. It was found that the sample with higher content of SiC was achieved with finer grains, and the incorporation of SiC particles could bridge, branch and deflect the cracks, thus improving the fracture toughness. Higher T1 was required for the densification of the samples with higher content of SiC (>5?wt%). The sample containing 3?wt% SiC particles with the mean particle size of 100?nm, which was sintered at 1600?°C (T1) and 1100?°C (T2) for 5?min had the fine microstructure and optimal properties. Its relative density, grain size, Vickers hardness and fracture toughness obtained were 98.37%, 0.78?±?0.31?μm, 18.40?±?0.24?GPa and 4.97?±?0.30?MPa?m^1/2, respectively. Compared to the sample prepared by single-step microwave sintering, although near full densification can be achieved in both two methods, the grain size was reduced by 36% and the fracture toughness was improved by 28% in two-step microwave sintering.     

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).     

Comparison of the microwave and conventional sintering of Li2TiO3 ceramic pebbles

Microwave sintering was employed in the fabrication of Li₂TiO₃ ceramic pebbles using the powders synthesized via hydrothermal method. The as-prepared Li₂TiO₃ powders exhibited high reactivity with an average particle size as small as 40?nm. A comparative study between the microwave and conventional sintering behavior of Li₂TiO₃ pebbles was systematically investigated. The microstructure and density analyses showed that the presence of microwaves accelerated the densification and grain growth, thus decreasing the sintering temperature. Besides, an accelerated phase transformation from α-Li₂TiO₃ to β-Li₂TiO₃ was observed in microwave processing. The Li₂TiO₃ ceramic pebbles obtained by microwave sintering exhibited high density, good mechanical property and uniform microstructure, which might hold good potential as tritium breeding materials for blankets. The results showed that the microwave sintering was a promising process for the fabrication of Li₂TiO₃ pebbles.     

Preparation and properties of an Al2O3/Ti(C,N) micro-nano-composite ceramic tool material by microwave sintering

One kind of Al₂O₃/Ti(C,N) micro-nano-composite ceramic tool material with acceptable properties was prepared by microwave sintering. Effects of sintering temperature and holding time on densification, mechanical properties and microstructure were studied. The optimal relative density, fracture toughness and Vickers hardness were 98.4±0.30, 6.72±0.28 MPa m^1/2 and 18.42±0.59 GPa, respectively, which were obtained at 1550 °C for 10 min. Compared to the conventional sintering, the sintering temperature and holding time of microwave sintering were reduced by 14% and 89%, respectively. The microwave sintering made the sizes of some particles keep in nano-scale, which leaded to the formation of intragranular structures. The residual stress in the intragranular structures increased the ratio of grain boundary toughness to grain toughness of matrix (K_cb/K_cg), and thus the micro-Al₂O₃ grains were more inclined to transgranular fracture.     

Longer‐lasting Al2O3‐SiCw‐TiC cutting tools obtained by spark plasma sintering

In this work, an electrically conductive powder mixture of alumina, silicon carbide whiskers, and titanium carbide has been used to obtain cutting tools with a standard SNGN geometry after machining spark plasma sintered disks with 150 mm in diameter and 10 mm height. The obtained cutting inserts from the sintered blanks were used in real conditions in order to make a wear study of this composition compared to the alumina‐silicon carbide whiskers inserts available in the market. In parallel, pin on disk wear studies were performed. Results are discussed in terms of the different properties and microstructural features of the different ceramic composites.     

Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering

Hydroxyapatite (HAP) possesses excellent bioactivity/osteointegration properties. Nevertheless, its inferior flexural strength and fracture toughness limit its use in human weight-bearing parts. We investigated a microwave sintering technology which can be effectively used to develop titanium dioxide-hydroxyapatite (TiO₂-HAP) ceramics with different amounts of TiO₂ (0.8,1.6,2.4,3.2,4.0,4.8,5.6 and 6.4 wt%), which contribute to extremely high flexural strength (90–130 MPa) along with a good combination of elastic modulus and fracture toughness. The results of the Rietveld refinement show that multiphase bioceramics (HAP, β-TCP) can be achieved by doping nano-TiO₂ under microwave sintering. Despite the fact that the main phases of the sintered TiO₂-HAP ceramics are HAP and β-TCP, X-ray diffraction confirms the formation of the CaTiO₃ and CaTi₂O₄(OH)₂ phases. Furthermore, the sintering reactions to form these phases are discussed and the results show that an appropriate amount of nano-TiO₂ can not only effectively inhibit the growth of grain, but also change the fracture mode and increase the relative density. Finally, it is found that doping nano-TiO₂ by microwave heating is an effective technique for producing HAP/β-TCP composite load-bearing implants in clinical applications without coarsening the size of grain.     

The cutting performance of Al2O3 and Si3N4 ceramic cutting tools in the milling plywood

ABSTRACT

This research focuses on the cutting performance of Al₂O₃ and Si₃N₄ ceramic cutting tools in up-milling plywood, the results of which are as follows. First, whether the tool material is Al₂O₃ or Si₃N₄ ceramic, the cutting forces at low-speed cutting were less than those at high-speed cutting, and the machining quality at low-speed cutting was greater than that at high-speed cutting. Then, whether at low- or high-speed cutting, the cutting forces of Al₂O₃ cutting tools were higher than those of Si₃N₄ cutting tools, and the machining quality of plywood milled by Al₂O₃ ceramic cutting tools was poorer than that milled by Si₃N₄ ceramic cutting tools. Finally, Si₃N₄ ceramic cutting tools were more suitable to machine the wooden productions with much glue content than Al₂O₃ ceramic cutting tools for the better machined quality.     

Injection molding of ceramic cutting tools for wood-based materials

In this study a mutable mold for ceramic cutting tools with inserts of different cutting angles and two different injection positions was designed. Three alumina-based ceramic feedstocks with different types and amount of second phases were developed. A mold filling study was carried out for both sprue positions in order to prove the molding behavior of the feedstock and the functionality of the mold. Debindering and sintering of molded green parts was arranged for each composition, respectively. Mechanical properties, microstructure and achieved cutting edge sharpness of produced tools were investigated. Results show that the mold design and injection molding process play a key role in order to manufacture cutting tools of best possible sharpness enabling a wood machining process. Feedstocks exhibit a good mold filling behavior resulting in comparatively sharp cutting edges of ≈10 μm after sintering. Mechanical properties show high potential for application of wood machining cutting tools.     

Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young's modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ_compression = 531.3 MPa and K_IC = 1.12 MPa m^1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications.     

Structural and microstructural effects produced by microwave sintering over La2O3 doped TiB2-Ni composites

In this paper, an agglomerated composite powder of titanium diboride-nickel (TiB₂-Ni) was milled and spray-dried. The effects of the sintering method, sintering temperature, holding time, and La₂O₃-doped on the morphology, phase content and properties of the powder were investigated. The results showed that the microwave-sintered La₂O₃-doped powder had the best properties when sintered at 1273 K for 15 min. La doping reduced the oxygen content, inhibited the formation of the brittle Ni₃B phase, and improved the mechanical properties of the powder. The powder may be a suitable raw material for thermally-sprayed TiB₂-Ni coatings.     

Cold sintering assisted CaF2 microwave dielectric ceramics for C-band antenna applications

A cold sintering process is adopted to pre-densify CaF₂ ceramics from 85.7% at 300 MPa to 91.7% at 750 MPa. Subsequent post-annealings at 1000–1150 °C lead to further improvements in densification, where great enhancements of grain size and crystallinity are also observed from the scanning and transmission electron micrographs. Significant advances in Qf values are achieved in the post-annealed CaF₂ ceramics. The optimum Qf value (80,522 GHz) is achieved after cold sintering at 750 MPa and post-annealing at 1000 °C, which is three times higher than the conventional sintered one at 1000 °C (26,448 GHz). Moreover, the obtained low-ε_r (5.9–6.5) of CaF₂ ceramics suggests broad application prospects in the high-band microwave communications. A microstrip patch antenna is fabricated using the CaF₂ ceramics as the substrate, which operates at 7.89 GHz in the C-band, with an S₁₁ of ?13.4 dB, simulated high gain and efficiency of 6.41 dBi and ?0.56 dB, respectively.