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1.
《Ceramics International》2016,42(16):18393-18401
This paper reports a comparative assessment of the chemical and structural stability of LSCM perovskite composites fabricated with 8YSZ and 10ScSZ. Role of oxidizing and reducing atmospheres are examined on the processing and electrochemical performance of the composites. Higher density is obtained during the sintering of LSCM mixed with 10ScSZ when compared to composite mixtures containing 8YSZ in oxidizing as well as reducing atmosphere. Above composites also densified more in Ar-3%H2-3%H2O atmosphere than air along with the formation of SrZrO3 in reducing atmosphere. MnCr2O4 formation is found only in LSCM-8YSZ composite in Ar-3%H2-3%H2O. Electrochemical tests conducted using symmetric cell configurations indicates low polarization resistance and higher performance for Gas (air/fuel)/LSCM-10ScSZ//8YSZ/LSCM-10ScSZ/Gas (air/fuel). Unlike cell containing LSCM-8YSZ composite, no significant changes are identified in the polarization resistance of LSCM-10ScSZ cell for 80 h. Sr-segregation on the surface of LSCM in electrically tested LSCM-8YSZ cell is attributed to performance degradation in the reducing atmosphere.  相似文献   

2.
Body mixes for stoneware containing a Brazilian red clay, nepheline syenite, and quartz with two different median particle sizes (∼2 and 18 μm) were fired under oxidizing and reducing atmospheres. The densification behavior was followed by dilatometry simulating the firing schedule, as well as by water absorption, linear shrinkage, and bulk density measurements on as-fired specimens. It was revealed that finer quartz led to interconnected pore closure at 1125°C when fired under an oxidizing atmosphere. Densification was systematically related to the uniformity of the compacts in the unfired state. Phase and microstructural examination by X-ray diffractometry, scanning electron microscopy/energy-dispersive spectroscopy (EDS), and transmission electron microscopy/EDS showed that mullite developed more extensively in the stonewares fired in an oxidizing atmosphere, along with cristobalite and haematite, whereas metallic iron was found in stonewares fired under a reducing atmosphere.  相似文献   

3.
Alkali resistance and thermo-mechanical properties of the transition zone of cement rotary kilns refractories are the key factors affecting their service life. Calcium alumino-titanate (CAT) containing bauxite-SiC composites were prepared using bauxite, SiC, CAT, Guang Xi white clay, α-alumina, and metallic silicon powder as starting materials and Al(H2PO4)3 as the binder. The effects of the CAT particle size on the phase composition, microstructure, thermo-mechanical properties, and alkali resistance of the CAT-containing bauxite-SiC composites during firing were investigated. The results reveal that the CAT particle size strongly affects the composites’ microstructure and sintering densification. With decreasing particle size, the particle-particle and particle-matrix interfacial bonding deteriorates gradually. When CAT particles are added, the specimens show higher strength, refractoriness under load, and residual rupture strength than the specimens with fine CAT powders. Specimens with fine CAT powders show lower coefficient of thermal expansion compared to the specimens with CAT aggregates. The alkali attack test confirms that the bauxite-SiC composite refractories with CAT aggregates show better alkali resistance than those with CAT fine powder. According to the alkali mechanism, 1) K vapors penetrate the specimen through the open and connected pores and cracks, 2) K vapors react with anorthite and corundum to form kalsilite accompanied by the formation of a liquid phase and new cracks.  相似文献   

4.
《Ceramics International》2017,43(6):4880-4887
To improve oxidation resistance of carbon/carbon (C/C) composites, a multiphase double-layer ZrB2-CrSi2-SiC-Si/SiC coating was prepared on the surface of C/C composites by pack cementation. Thermogravimetry analysis showed that the as-prepared coating could provide effective oxidative protection for C/C composites from room temperature to 1490 °C. After thermal cycling between 1500 °C and room temperature, the fracture behaviors of the as-prepared specimens changed and their residual flexural strengths decreased as thermal cycles increased. The specimen after 20 thermal cycles presented pseudo-plastic fracture characteristics and relatively high residual flexural strength (83.1%), while the specimen after 30 thermal cycles failed catastrophically without fiber pullout due to the severe oxidation damage of C/C substrate especially the brittleness of the reinforcement fibers.  相似文献   

5.
To take full advantage of the excellent properties of CA6 present in calcium alumino-titanate (CAT) and reduce the formation of the low melting point phase (anorthite), CAT-bauxite-SiC composite refractories were fabricated under buried sintering in order to achieve low thermal expansion, superior high-temperature performance, and increased alkali resistance. Furthermore, the corrosion mechanism of K vapor was investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results show that CA6 present in CAT can be partially retained and the hot strength of CAT-bauxite-SiC composites slowly decreases when the amount of CAT added is less than 21.6?wt%. The cold strength and bulk density decrease with the CAT content, and the residual ratio of MOR firstly decreases and subsequently increases with the CAT content. For the specimens with CAT additions, 43.2?wt% CAT results in the highest volume expansion at high temperatures. It is proposed that the corrosion mechanism of CAT aggregates under buried sintering is as follows: 1) K vapors penetrate into the CAT with high CA6 content through the lamellar CA6 gap and deposit on the inner regions of CAT; and 2) K vapors react with corundum and anorthite present in CAT and cause the microstructural destruction of CAT due to a decrease in the amount of the Al2O3-CaO-SiO2 liquid phase in the CAT. The alkali resistance of the CAT-bauxite-SiC composites decreases as the CAT content increases, which is attributed to poor sintering densification and high apparent porosity.  相似文献   

6.
《Ceramics International》2022,48(4):5162-5167
Calcium alumino-titanate (CAT), a secondary material obtained from ferrotitanium slag, was used as a hibonite source to prepare CaAl12O19–MgAl2O4–Al2O3 castables. The restructuring effect of CAT aggregate was compared by replacing tabular alumina aggregates with CAT aggregates of different particle sizes. The effects of CAT particle size on cold mechanical strength and thermal shock resistance of CaAl12O19–MgAl2O4–Al2O3 castables were studied. The results showed that CAT aggregates with particle size of 5–3 or 3–1 mm led to more internal cracks or pores and reduced the cold mechanical strength of the castable samples fired at 1600 °C for 3 h. The use of CAT aggregates with particle size of 1–0 mm led to the formation of a well-bonded CAT aggregate and matrix, improving the cold mechanical strength and thermal shock resistance of the castable samples fired at 1600 °C for 3 h. The enhancement mechanism of fine CAT aggregates in this process was proposed based on the sintering of the matrix–aggregate interface with the formation of Ca(Al, Mg, Ti)12O19.  相似文献   

7.
The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO2) or an oxidizing (air) atmosphere was performed by Mössbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe2SiO4), hercynite (FeAl2O4), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al2O3-SiO2 phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.  相似文献   

8.
《应用陶瓷进展》2013,112(6):280-283
Abstract

The purpose of the work reported in the present paper was to establish the correlation between the physical, mechanical, and microstructural properties of alumina matrix composites reinforced with (CeO2, Nd2 O3, Y2O3 )–PSZ (partially stabilised zirconia) depending on the processing and thermal treatment conditions. The composites obtained from fine powder mixtures were formed by hydraulic pressing, ceramic injection moulding, and hot pressing under various temperature and pressure conditions. The samples were fired at 1550–1770°C in an oxidising atmosphere and in vacuum depending on the forming conditions. Comparative microstructure investigations were made by TEM on sample surfaces. The XRD results were in accordance with the determined properties of the investigated compositions. The results highlighted that the best physical and mechanical properties and homogenous microstructure for the ZTA composites were obtained by firing in vacuum.  相似文献   

9.
A SiC fiber-reinforced composite containing a SiC-ZrB2 mixed matrix (SiCf/(SiC-ZrB2)) with high density and enhanced mechanical properties was fabricated. ZrB2 at 5 or 40?vol% was added to a (SiC + C) slurry to be infiltrated into the voids of 2D woven Tyranno?-SA grade-3 fabrics by electrophoretic deposition. Subsequent hot pressing at 1300?°C and 10?MPa for 1?h, followed by liquid silicon infiltration (LSI) at 1600?°C for 5?h in an Ar atmosphere resulted in the formation of the reaction-bonded SiC matrix, which revealed a composite density close to 97%. SiCf/(SiC-ZrB2) having open porosities of 0.2–0.6% showed peak strengths of 398 and 320?MPa for 5 and 40?vol% ZrB2 addition, respectively. The large mismatch in the coefficient of thermal expansion and Young's modulus between the SiC and ZrB2 phases was attributed to a reverse trend in the strength of composites. Brittle behavior of the composites in flexure can be explained by the strong bonding between the matrix and fibers formed by the reaction of interphase with molten Si during LSI. Strength retention after oxidation at 1000 and 1400?°C for 2?h was also compared in terms of ZrB2 amount contained in the composites.  相似文献   

10.
Chemical vapor-infiltrated self-healing SiC(f)/BN(i)/[SiC-B4C](m) composite specimens were exposed at 1300?°C for 300?h at atmospheric pressure under two different oxidizing atmospheres (i.e., wet (12%H2O:8%O2:80%Ar) and dry (0.01%O2:99.99%Ar)) representative of rich and poor oxidizing conditions, respectively. Mechanical testing, microstructural observations, and element analyses were performed on the treated specimens. The flexural strength retentions of the specimens were 47.9 and 39.4% under wet and dry oxygen conditions, respectively. The SiC and B4C matrices were severely oxidized under wet oxygen conditions, whereas the BN interphase remained intact. The BN interphase and the B4C layered phase were both partially oxidized under dry oxygen conditions. Thus, the SiC(f)/BN(i)/[SiC-B4C](m) composites exhibited improved oxidation resistance under wet oxygen atmospheres as compared to dry oxygen conditions as a result of the formation of borosilicate glasses. In addition, two different degradation mechanisms for the composites during the oxidation process were discussed.  相似文献   

11.
Huang Jian-Feng  Li He-Jun  Xiong Xin-Bo 《Carbon》2003,41(14):2825-2829
In order to exploit the unique high temperature mechanical properties of carbon/carbon (C/C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al2O3-mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C/C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C/C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface.  相似文献   

12.
By the oxidation of liquid poly(1,2‐butadiene) (LPB) with H2O2/HCOOH, epoxidate poly(1,2‐butadiene) (ELPB) was obtained as a toughening agent to prepare diglycidyl ether bisphenol‐A (DGEBA) epoxy composites by using V115 polyamide(PA) as a cross‐linking agent. DGEBA, ELPB, and the composites were effectively cured by PA at 100°C for 2 h followed by postcuring at 170°C for 1 h. Thermal gravimetric analysis results in air and nitrogen atmosphere showed that the thermal stability of composites could be improved by the addition of ELPB. Compared with DGEBA/PA, the composites exhibited a decrease in strength at yield but an increase in strain at break with the increase in ELPB amount. The composite with 10% ELPB exhibited both thermal stability and tenacity superior to those of DGEBA/PA and composites with 5 and 20% ELPB, respectively. The improvements in thermal and mechanical properties of composites depended on the formation of Inter Penetrating Networks (IPN) among DGEBA/PA/ELPB and their distributions in the matrix. At an appropriate ELPB amount, the IPN, mostly made of DGEBA/PA/ELPB, may be distributed more evenly in the matrix; less ELPB resulted in the formation of IPN mainly made of DGEBA/PA; excessive addition of ELPB resulted in the local aggregation of ELPB/PA and phase separations. The toughening mechanism was changed from chemically forming IPN made of DGEBA/PA/ELPB to physically reinforcing DGEBA/PA by ELPB/PA with the increase in ELPB addition. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009  相似文献   

13.
This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB2 ceramics and ZrB2-based composites. Hot-pressing is the most commonly used densification method for the ZrB2-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB2-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB2 ceramics and ZrB2-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB2-based composites is in the range of 2–6 MPa m1/2. Fine-grained ZrB2 ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi2. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB2-based ceramics. In addition, the ZrB2-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB2-based composites attractive candidates for high-temperature thermomechanical structural applications.  相似文献   

14.
Reducing atmosphere fired (K0.5Na0.5)NbO3‐based ceramics open up an important opportunity for manufacturing lead‐free multilayer piezoelectric devices cofiring with the base metal inner electrode. In this study, the effects of sintering atmosphere on the piezoelectric, dielectric, and insulating properties in Sn‐doped (Na0.52K0.44Li0.04)NbO3 (KNN) were investigated. To establish the relationship between the defect structure and electrical properties, the thermally stimulated depolarization current (TSDC) technique was implemented. The electrical properties degrade severely when the pure KNN ceramics are sintered in reducing atmosphere, compared with the ceramics sintered in air. The reason is that the oxygen vacancy concentration in reduced fired ceramics is much higher than that in air fired ceramics. However, the 0.6 mol% Sn‐doped KNN ceramics sintered in reducing atmosphere exhibit comparable electrical properties to the air fired ceramics. From the TSDC analysis, the reducing atmosphere fired ceramics have approximately the same oxygen vacancy concentration as the air fired ceramics because B‐site substituted Sn works as an acceptor.  相似文献   

15.
R.E. Bullock  E.L. McKague 《Carbon》1973,11(5):547-553
Two types of carbon/carbon composites were irradiated to a fast-neutron fluence of 2 × 1018 n/cm2 (E > 1 MeV) in a helium atmosphere at 175°C. These specimens were mechanically tested in flexure and in shear to determine resulting property changes. Shear strengths at room temperature were increased by 25 per cent or more by irradiation, and these increases in shear strengths allowed the composites to be flexed to higher stress levels (15–25 per cent) before undergoing permanent deformations. These property improvements were retained by the materials up to temperatures of at least 1000°C in an inert environment.  相似文献   

16.
In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb2Si2O7, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb2Si2O7 coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.  相似文献   

17.
Conclusions Cerium dioxide mixed with ZrO2 and in the form of tetragonal solid solution with ZrO2, preliminarily synthesized in an oxidizing atmosphere, is reduced in vacuum and in a reducing atmosphere to Ce2O3, and in this state forms solid solutions with ZrO2; CeO2 in vacuum and in a reducing atmosphere is partially reduced to the metal.At 1750–2000°C and with concentrations of 5.55–16.65 mol% Ce2O3 forms with ZrO2 cubic solid solutions with a crystal lattice of the fluorite type which during rapid cooling to room temperature preserves its structure, and with sufficiently slow cooling decomposes into a solid solution of Ce2Zr2O7-ZrO2 of the pyrochlore type, and monoclinic solid solution ZrO2-Ce2O3.During combined stabilization and sintering the composition ZrO2-Ce2O3 sinters badly; and furthermore the density and strength of the fired products diminish with increase in the content of stabilizing oxides. Dense and strong articles of composition ZrO2-Ce2O3 are obtained from presynthesized solid solutions ZrO2-CeO2 by intermediate fine grinding.Solid solutions ZrO2-Ce2O3 with an increase in temperature are oxidized in air, which leads to destruction of dense articles. The high (15% or more) porosity of the specimens contributes to the compensation of the volume changes in the changes during oxidation, so there are no essential changes in the strength of the articles.Translated from Ogneupory, No.1, pp. 40–46, January, 1970.  相似文献   

18.
《Ceramics International》2022,48(13):18739-18744
Modulating the thermal conductivity has been a pragmatic approach for the development of high-performance thermoelectric material and thereby a step forward towards commercialization. Despite some efforts, the reduction in thermal conductivity of SrTiO3 ceramic has not been fully realized. In this work, Ti3AlC2 in 3, and 7 vol% were uniformly incorporated in SrTiO3 through nanostructured powder processing. The pristine SrTiO3 and composites powders were consolidated by the spark plasma sintering at 1200 °C under uniaxial pressure of 50 MPa. Thermal properties of the bulk samples were evaluated from room temperature to 750 K through laser flash analysis. The thermal conductivity of SrTiO3 based composites decreases substantially with the addition of nanostructured Ti3AlC2 from the pristine SrTiO3 bulk sample. The reduction in thermal conductivity of 7 vol% composites is more than 30% at room temperature and even higher at elevated temperatures from the SrTiO3. The interface thermal resistance was estimated which indicates a dominant role in diminishing the thermal conductivities of the composites. The results suggest that the addition of Ti3AlC2 as a second phase and nanostructuring through ball milling has significantly altered the phonon scattering mechanisms through multiple factors and thereby contributed to reducing effective thermal conductivities of the composites. This, work provide a scalable and economical route for the development of high-performance thermoelectric material.  相似文献   

19.
The aim of the study is to evaluate the influence of surface-treatment methods with and without the use of a retention hole on the shear bond strength of a resin composite adhered to amalgam using an adhesive system. Amalgam specimens were divided into six groups. Group 1 (Bur) specimens were roughened with a diamond bur, Group 2 (Al2O3) specimens were sandblasted with a 50?μm aluminum oxide powder, Group 3 (CoJet®) specimens were sandblasted with 30?μm CoJet® Sand, Group 4 (Bur?+?Rh) specimen surfaces were prepared with a retention hole 1?mm in diameter and 1?mm deep and roughened with a diamond bur, Group 5 (Al2O3?+?Rh) specimens were also prepared with a retention hole and sandblasted with 50?μm aluminum oxide powder, and Group 6 (CoJet®?+?Rh) surfaces were prepared with a retention hole and sandblasted with 30?μm CoJet® Sand. Resin composite cylinders were bonded onto the amalgam surfaces using Xeno® IV, Optibond? All-In-One, Clearfil? SE Bond, Adper? Single Bond Plus, and Scotchbond? Multi-Purpose adhesive systems. In addition, silane (Monobond S) was used for Groups 5 and 6. The shear bond was determined and statistically analyzed using two-way analysis of variance and post hoc Tukey’s tests (p?≤?0.05). The surface treatment significantly affected the shear bond strengths of the adhesive systems. The shear bond strengths of Optibond? All-In-One (2.661?±?0.48?MPa) in Group 1 and Scotchbond Multi-Purpose (3.818?±?0.98) in Group 4 were significantly higher than those of the other adhesive systems. Silica coating of the amalgam surface significantly improved the shear bond strength of the resin composites. The addition of a retention hole on the amalgam affects the bonding strength of the composite adhesion.  相似文献   

20.
The effect of two compatibilizers, i.e. ethylene diamine dilaurate (EDD) and maleic anhydride grafted polypropylene (MAPP) on the mechanical properties, water absorption, morphology, and thermal properties of silica‐filled polypropylene (PP/Sil) composites were studied. The results show that the tensile, impact and flexural strengths (up to 2 php), Young's modulus, and elongation at break (Eb) increased with increasing EDD content. However, increasing MAPP content increases the tensile strength, Young's modulus, impact and flexural strengths, and water absorption resistance. At a similar compatibilizer content, EDD exhibits higher Eb, impact and flexural strengths but lowers tensile strength, Young's modulus, and water absorption resistance compared with MAPP. Scanning electron microscopy study of tensile fractured surfaces exhibits the evidence of better silica‐PP adhesion with MAPP and EDD compared with the similar composites but without compatibilizer. Fourier transform infra red spectra provide an evidence of interaction between EDD or MAPP with PP/Sil composites. Termogravimetry analysis results indicate that the addition of EDD or MAPP slightly increases the thermal stability of PP/Sil composites. Differential scanning calorimetry also indicates that PP/Sil composites with EDD or MAPP have higher heat fusion (ΔHf(com)) and crystallinity (Xcom) than similar composites but without compatibilizer. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers  相似文献   

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