Effects of microwave sintering temperature and soaking time on microstructure of WC-8Co

WC-8Co cemented carbide samples were processed via microwave irradiation in a 2.45 GHz, high-power multi-mode microwave cavity. The densification of the compacts and the microstructures of the prepared alloys were studied. The results demonstrate that the liquid phase is formed around 1300 ℃ and nearly full densification is obtained at 1450 ℃ for 5 min via microwave irradiation. The microstructures of microwave sintered samples have finer and more uniform WC grains than those of vacuum sintered samples. Besides, the WC grain size and distribution are only decided by the sintering temperature. Holding time has negligible effects on them. No matter how holding time is, the mean grain size is 2.7 pan when the sintering temperature is kept at 1450 ℃.

Decarburization and improvement of ultra fine straight WC-8Co sintered via microwave sintering

Cemented tungsten carbide with ultra fine grains was prepared via microwave sintering.η phase（W3Co3C） was formed on the surface of the samples during the preparation process.Extra carbon black was premixed and the influence of carbon content on mechanical properties was studied.The results show that the maximum value of hardness and transverse rupture strength are HRA 93.2 and 3396 MPa respectively when the carbon black content is 0.45%.The microstructure investigated by SEM show that the WC grains growth mainly occurs during the early stage of microwave sintering by the coalescence of grains.

Microwave sintering of Mo nanopowder and its densification behavior

In order to prepare high-performance Mo with fine and homogeneous microstructure to meet the demand of high-technology applications such as metallurgical,mechanical,national defense,aerospace and electronics applications,the microwave sintering process and densification mechanism of Mo nanopowder were studied.In this experiment,Mo nanopowder and micropowder were used for conventional sintering and microwave sintering at different sintering temperatures and sintering time,respectively.The results showed that with the increase in the sintering temperature,the increase rates of the relative density and hardness increased rapidly at first and then slowed down.The relative density rapidly reached 95%,followed by a small change.Mo nanopowder with a relative density of 98.03% and average grain size of 3.6 μm was prepared by microwave sintering at 1873 K for30 min.According to the analysis of the sintering kinetics,its densification is attributed to the combination of volumetric diffusion and grain boundary diffusion mechanisms.The calculated sintering activation energy of Mo nanopowder was 203.65 kJ/mol,which was considerably lower than that in the conventional sintering,suggesting that the microwave sintering was beneficial to the enhancement in the atom diffusion and densification for the powder.The results confirm that the microwave sintering is a promising method to economically prepare molybdenum with high properties.

Process−microstructure−properties relationship in Al−CNTs−Al2O3 nanocomposites manufactured by hybrid powder metallurgy and microwave sintering process

Al−2CNTs−xAl2O3 nanocomposites were manufactured by a hybrid powder metallurgy and microwave sintering process.The correlation between process-induced microstructural features and the material properties including physical and mechanical properties as well as ultrasonic parameters was measured.It was found that physical properties including densification and physical dimensional changes were closely associated with the morphology and particle size of nanocomposite powders.The maximum density was obtained by extensive particle refinement at milling time longer than 8 h and Al2O3 content of 10 wt.%.Mechanical properties were controlled by Al2O3 content,dispersion of nano reinforcements and grain size.The optimum hardness and strength properties were achieved through incorporation of 10 wt.%Al2O3 and homogenous dispersion of CNTs and Al2O3 nanoparticles(NPs)at 12 h of milling which resulted in the formation of high density of dislocations and extensive grain size refinement.Also both longitudinal and shear velocities and attenuation increase linearly by increasing Al2O3 content and milling time.The variation of ultrasonic velocity and attenuation was attributed to the degree of dispersion of CNTs and Al2O3 and also less inter-particle spacing in the matrix.The larger Al2O3 content and more homogenous dispersion of CNTs and Al2O3 NPs at longer milling time exerted higher velocity and attenuation of ultrasonic wave.

Microstructure and properties of Y_2O_3-doped steel-cemented WC prepared by microwave sintering

Steel-cemented WC was prepared by ball milling, cold compacting and microwave sintering with Fe powder as the matrix, WC as the hard phase and the addition of rare earth Y2O3. The results show that the interface of the WC particles and Fe matrix exhibits excellent wettability and liquidity when the microwave sintering temperature reaches 1,280℃. The density and mechanical properties of the steel bonded WC carbides could be greatly improved, the hard phases become finer and more uniform dispersed owing to the addition of Y2O3. With the increase of the Y2O3 contents, the grain becomes uniform and fine first, and then gathers and grows up. The relative density, microhardness and bending strength all rise first, reaching the maximum values of 97.29 %, HV1024 and 1,267.60 MPa at 0.5 % Y2O3, respectively, and then decrease. Moreover, the relative density and mechanical properties of the steel-cemented WC with nano-Y2O3 are higher than that with micron-Y2O3, which indicates that the effect of nano-Y2O3 is better than that of the micron-Y2O3.

Parameter optimization of microwave sintering porous Ti-23%Nb shape memory alloys for biomedical applications

Porous Ti-23%Nb(mole fraction)shape memory alloys(SMAs)were prepared successfully by microwave sintering with excellent outer finishing(without space holder).The effects of microwave-sintering on the microstructure,phase composition,phase-transformation temperature,mechanical properties and shape-memory effect were investigated.The results show that the density and size of porosity vary based on the sintering time and temperature,in which the smallest size and the most uniform pore shape are exhibited with Ti-23%Nb SMA after being sintered at 900°C for 30 min.The microstructure of porous Ti-Nb SMA consists of predominantα',α,andβphases in needle-like and plate-like morphologies,and their volume fractions vary based on the sintering time and temperature.Theβphase represents the largest phase due to the higher content ofβstabilizer element with little intensities ofαandα'phases.The highest ultimate strength and its strain are indicated for the sample sintered at 900°C for 30 min,while the best superelasticity is for the sample sintered at 1200°C for 30 min.The low-elastic modulus enables these alloys to avoid the problem of“stress shielding”.Therefore,microwave heating can be employed to sinter Ti-alloys for biomedical applications and improve the mechanical properties of these alloys.

Preparation and properties of ultra-fine TiCN matrix cermets by vacuum microwave sintering

Ultra-fine titanium carbonitride （TiCN） matrix materials with a grain size less than 1μm were successfully prepared by vacuum microwave sintering. The milling process for raw TiCN particles and the microstructure and properties of cermets produced with a composition of 15wt.%WC-17wt.%（Co＋Ni）-9wt.%Mo2C-59wt.%Ti0.TN0.3 and sintered by vacuum microwave were analyzed by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. The results show that a ball-to-powder mass ratio of 8：1 and a milling time of 50 h provided appropriate conditions for the production of ultra-fine TiCN solid solution powders. The use of vacuum microwave sintering produced cermets with much finer grain and black core structures and higher relative density and hardness than those produced by vacuum sintering technology.

Preparation and characterization of porous NiTi alloys synthesized by microwave sintering using Mg space holder

To obtain the lightweight,high strength,and high damping capacity porous NiTi alloys,the microwave sintering coupled with the Mg space holder technique was employed to prepare the porous NiTi alloys.The microstructure,mechanical properties,phase transformation behavior,superelasticity,and damping capacity of the porous NiTi alloys were investigated.The results show that the porous NiTi alloys are mainly composed of the B2 NiTi phase with a few B19'NiTi phase as the sintering temperature is lower than or equal to 900℃.With increasing the sintering temperature,the porosities of the porous NiTi alloys gradually decrease and the compressive strength increases first,reaching the maximum value at 900℃,and then decreases.With increasing the Mg content from 1 wt.%to 7 wt.%,the porosities of the porous NiTi alloys increase from 37.8%to 47.1%,while the compressive strength decreases from 2058 to 1146 MPa.Compared with the NH4HCO3 space holder,the phase transformation behavior of the porous NiTi alloys prepared with Mg space holder changes,and all of the compressive strength,superelasticity,shape memory effect and damping capacity are greatly improved.

Spark plasma and microwave sintering of Al6061 and Al2124 alloys

Despite the importance of aluminum alloys as candidate materials for applications in aerospace and automotive industries, very little work has been published on spark plasma and microwave processing of aluminum alloys. In the present work, the possibility was explored to process A12124 and A16061 alloys by spark plasma and microwave sintering techniques, and the microstructures and properties were compared. The alloys were sintered for 20 rain at 400, 450, and 500℃. It is found that compared to microwave sintering, spark plasma sintering is an effective way to obtain homogenous, dense, and hard alloys. Fully dense （100%） A16061 and A12124 alloys were obtained by spark plasma sintering for 20 rain at 450 and 500℃, respectively. Maximum relative densities were achieved for A16061 （92.52%） and A12124 （93.52%） alloys by microwave sintering at 500℃for 20 min. The Vickers microhardness of spark plasma sintered samples increases with the increase of sintering temperature from 400 to 500℃, and reaches the values of Hv 70.16 and Hv 117.10 for A16061 and A12124 alloys, respectively. For microwave siutered samples, the microhardness increases with the increase of sintering temperature from 400 to 450℃, and then decreases with the further increase of sintering temperature to 500~C.

Effects of Phosphorus Additions on Microwave Sintering Properties of Uitrafine WC-10Co Cemented Carbides

Using the microwave sintering technology, the effects of phosphorus （P） additions on the microstructure and properties of the ultrafine WC-10Co alloys were investigated. The experimental results show that with only 0.3wt% P additions, full density WC-10Co cermets were obtained at temperature of 1250℃, which is 70 ℃ lower than that of the undoped counterparts. Lower sintering temperature can result in finer WC grain growth; therefore, the P-doped WC-10Co alloys exhibited higher hardness than the undoped ones. But at the same time, P doping could lead to sacrifice of fracture toughness ofWC-10Co cemented carbides.

Microwave sintering of Ca_(0.6)La_(0.2667)TiO_3 microwave dielectric ceramics

Ca0.6La0.2667TiO3 ceramics were prepared by conventional and microwave sintering techniques and their sinterability, microstructure, and microwave dielectric properties were investigated in detail for comparison. Densified Ca0.6La0.E667TiO3 ceramics were obtained by microwave sintering at 1350℃ for 30 min and by conventional sintering at 1450℃ for 4 h. An unusual phenomenon was found that some larger grains （grain size range： 8-10 pan） inclined to assemble in one area but some smaller ones （grain size range： 2-4 μm） inclined to gather in another area in the microwave sintered ceramics. The microwave dielectric properties of Ca0.6La0.2667TiO3 ceramics prepared by micro- wave sintering at 1350℃ were as follows＂ dielectric constant （er） = 119.6, quality factor （Qf） = 17858.5 GHz, and temperature coefficient of resonant fi＇equency （rf）= 155.5 ppm/℃. In contrast, the microwave dielectric properties of the ceramics prepared by conventional sintering at 1450℃ were er = 117.4, Qf= 13375 GHz, and rf= 217.2 ppm/℃.

Preparation of BaAl_2O_4 by microwave sintering

The desulfurater(BaAl2O4) was successfully synthesized with BaCO3 and Al(OH)3 powders as raw materials by microwave sintering method.The mass loss of raw materials and the characterization of the outcome were investigated by means of TG-DSC,XRD and optical microscopy.The reaction mechanism was discussed.The experimental results show that synthesized BaAl2O4 by microwave sintering method is feasible.Compared with conventional sintering method,microwave sintering is a better way to synthesize BaAl2O4 with advantages of low temperature sintering,short time sintering and high synthesis rate.

Microwave Sintering of W-15Cu Ultrafine Composite Powder Prepared by Spray Drying & Calcining-continuous Reduction Technology

The effects of microwave sintering and conventional H2 sintering on the microstructure and properties of W-15Cu alloy using ultrafine W-15Cu composite powder fabricated by spray drying ＆ calcining-continuous reduction technology were investigated. In comparison to the conventional HE sintering processing, microwave sintering to W-15Cu can be achieved at lower sintering temperature and shorter sintering time. Furthermore, higher performances in microwave sintered compacts were obtained, but high microwave sintering temperature or long microwave sintering time could result in coarser microstructures.

Microwave Sintering of Al_2O_3-ZrO_2-WC-Co Cermets

Composite powders of nanocrystalline WC-10Co （15wt%）,Y2O3 （8mol%） stabilized nanocrystalline ZrO2 （30wt%）,industrial cobalt powder （4.5wt%） and submicron Al2O3 （55wt%） composite powders were fabricated by high-energy ball-milling process.The nanocomposite powders were consolidated by microwave sintering process at temperature ranged 1300℃-1550℃ for 15min,respectively.The optimum consolidation conditions,such as temperature,were researched during microwave sintering process.Vickers Hardness of the consolidated cermets was measured by using a Vickers indentation test,and density of specimens was also determined by Archimedes＇ principle.Microwave sintering process could not only increase the density of Al2O3-ZrO2-WC-Co cermets and reduce the porosity,but also inhibit abnormal grain growth.

Preparation of Porous Mullite Composite by Microwave Sintering

Microwave sintering method was carried out to prepare porous mullite composite. An insulation structure based on hybrid heating mode was well designed with the wall of mullite and the aided heaters of SiC. The obtained samples were characterized by XRD analysis, apparent porosity detection, and bending strength measurement. SEM was used to observe the microstructure of the sample. It is found that the porous mullite composite could be prepared through the microwave sintering within 2 h at relatively low temperatures around 1000 ℃. The lasted samples show comparatively superior properties to the products prepared by conventional processing.

Preparation and Properties of Si2N2O Ceramics for Microwave Sintering Furnaces

Si2N2O ceramics were prepared using amorphous Si3N4 as the raw material and Li2CO3 as the sintering additive through vacuum multi-stage sintering.The influence of the Li2CO3 addition(0%,1%,2%,3%,and 5%,by mass)on the phase composition,the microstructure,the porosity,the mechanical properties,the dielectric constant and the tangent of the dielectric loss angle of the porous Si2N2O ceramics was investigated.The results reveal that a suitable addition of Li2CO3 can promote the generation of Si2N2O but excessive or inadequate Li2CO3 causes decomposition of Si2N2O ceramics.The prepared porous Si2N2O ceramics have good mechanical properties,good thermal shock resistance,and low dielectric properties,which have excellent potential for application in microwave sintering furnaces.

RESEARCH ON MICROWAVE SINTERING ALUMINA CERAMICS

This paper introduces the characteristic of microwave sintering,introduces and analyses the process of microwave sintering of thealumina ceramics, moreover, compared with the experiments ofconventional sintering method. Based on the research, some resultsare obtained, and experimental basis for microwave sintering ofceramics is put forward.

Facile synthesis of Ca-SiAlON:Eu^(2+) phosphor by the microwave sintering method and its photoluminescence properties

Pure Ca-SiAlON:Eu2+ was synthesized by microwave sintering method at a relatively low temperature of 1550℃.Photoluminescence intensity of the resultant phosphor was higher than those of the samples synthesized by conventional gas-pressure sintering technique at 1750℃.When it was excited at 450 nm,the as-prepared yellow Ca-SiAlON:Eu2+ sample had an external quantum efficiency of 42%,comparable to the sample synthesized at 1750℃ under 0.5 MPaN2 gas pressure by the GPS method reported in reference.The experimental results demonstrated that the microwave sintering method was also an interesting approach for synthesizing nitride phosphors,which promises lower firing temperature than those by carbothermal reduction and nitridation (CRN) methods,higher heating rate and shorter duration time compared with those by gas-pressure sintering.

In situ investigation of SiC powder's microwave sintering by SR-CT technique

Microwave sintering is being developed as a novel technique for the preparation of dense structural ceramics,but the mature theory has not been established due to the technical difficulties.The synchrotron radiation X-ray computed tomography(SR-CT) technique was introduced for the first time into the study of microwave sintering to in-situ observe the microstructure evolution of silicon carbide(SiC) material in this paper.By applying the SR-CT technique,the reconstructed 2D and 3D images of the specimen were obtained and the double logarithm curve of mean neck size and time(Ln(x)-Ln(t)) were obtained from these reconstructed images.Various sintering phenomena including sintering neck growth during microwave treatment were observed from the reconstructed images.Furthermore,the differences in microstructure evolution and sintering kinetics between microwave and conventional sintering were analyzed based on the reconstructed images and the Ln(x)-Ln(t) curve.1) The sharp surface of grains near the contact region distinctly grew blunt and the sintering neck growth between these grains were obviously observed at the early stage.Besides,the larger particles grew faster than smaller ones.The main reason for these phenomena may be the micro-focusing effect of electric fields.2) During each of the three sintering stages,the sintering kinetics curve of double logarithm relationship between mean neck size and time shows a good linear relationship,but at the middle stage the slope of the curve increases dramatically,which is quite larger than conventional sintering.The preliminary interpretation for these extraordinary phenomena has been discussed in details.

Luminescence enhancement of CaMoO_(4):Eu^(3+) phosphor by charge compensation using microwave sintering method

CaMoO_(4):Eu^(3+)and CaMoO_(4):Eu^(3+),A+(A=Li,Na,K)phosphors for light-emitting diode(LED)applications have been prepared by microwave sintering method(MSM),and their structure and luminescence properties are investigated.The influences of microwave reaction time and concentration of different kinds of charge compensation A+and Eu^(3+)on luminescence have also been discussed.The samples emit a red luminescence at 615 nm attributed to the^(5)D0→^(7)F2 transition of Eu^(3+)under 464 nm excitation.It is observed that adding charge compensation A+in the sample synthesis increases luminescence intensity.The optimized sample made with 32 mol%Li+and 32 mol%Eu^(3+)has an enhancement factor of 4 in photoluminescence compared to the sample made without charge compensation.The CIE(Commission Internationale de l'Eclairage)coordinates of Ca0.36MoO_(4):0.32Eu^(3+),0.32Li+are x=0.661 and y=0.339,which indicate that the obtained phosphor can be a promising red color candidate for white LED fabrications.