Obtaining zircon-based ceramic material

The behavior of zircon concentrates from various producers is compared while obtaining ceramics based on them. It is shown that the presence of impurities affects not only the thermal dissociation of zircon, but also the processes during its sintering. The use of cold uniaxial and cold isostatic pressing to form zircon powders unambiguously affects the strength characteristics of ceramics based on them. A fractographic analysis of the fracture surface of the zircon ceramics showed that the material strength is in many aspects determined by the inclusions of a phase consisting of SiO₂, ZrO₂, and Al₂O₃ of the variable composition.     

Compaction and grain growth in alumina ceramics under microwave sintering

The laws governing the transformations of the porous and grain structures that occur in Al₂O₃-based ceramics during microwave sintering are investigated. These processes are very similar to analogous processes that occur during coherent compaction of powders. Under microwave sintering the ceramic preforms retain a wide range of porosity and microstructure for a long time. As a result strong capillary forces exist throughout the process, causing active compaction. On the basis of the experimental data we suggest that the heating rate and the bulk mode of the heating are not the only factors that accelerate the consolidation of the ceramics under microwave sintering. Institute of Problems in Material Science, National Academy of Sciences of Ukraine, Kiev. Institute of Applied Physics, Russian Academy of Sciences, Nizhnii Novgorod. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 8–13, July–August, 1997.     

Effects of heating rate on densification and grain growth during field-assisted sintering of α-Al2O3 and MoSi2 powders

Two types of powders, electrically conductive MoSi₂ and insulating α-Al₂O₃, were sintered by a field-assisted sintering technique (FAST) using heating rates from 50 °C to 700 °C/min. The Al₂O₃ powders were sintered to 99 pct density at 1100 °C for 2 minutes under 45 Mpa pressure. For Al₂O₃, no exaggerated grain growth was observed and the final grain size inversely scaled with the heating rate. Such a grain growth behavior fits the literature models based on multiple transport mechanisms for constant-heating-rate sintering. The density reached by MoSi₂ under similar sintering conditions was 91 pct. The grain size was independent of the heating-rate value. Specific electrical field and pressure effects are shown to contribute to enhanced densification and minimal coarsening in each material.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Multistage sintering approach to prepare Ni3Al/α-Al2O3 composites

The nickel aluminide intermetallic matrix composites (IMC), Ni₇₆Al₂₄B_0.1 with either 5 or 10 vol pct α-Al₂O₃, were synthesized through a multistage sintering approach from the elemental powders of Ni, Al, and oxide of α-Al₂O₃. An electroless nickel-boron (Ni-B) plating process was adopted to improve the contacted interface between the reinforced oxide ceramics and the metal matrix, as well as to supply the atomic scale boron in the metallic matrix of the IMCs. The entire process comprises steps involving preparing a powdery starting material, sealing it within a metal sheath or can, compacting or cold deforming it, preliminarily heating the compacted material at a relatively low temperature, executing a pore-eliminating (mechanical deforming) process to eliminate the pores resulting from the preceding heating step, and sintering the material at a relatively high temperature to develop a transient liquid phase to heal or to eliminate any microcracks, crazes, or collapsed pores from the previous steps. Most of all, it is important that contact with a heat absorbent material, such as a metal sheath, produces the Ni₂Al₃ phase during preliminary heating. This new phase is a brittle and crispy material with a low melting point (1135 °C). It has been found to play an important role in preventing any significant cracks during the pore-eliminating process and in developing a transient liquid phase in the following 1200 °C sintering step. This multistage sintering with a heat absorbent process is beneficial for producing a product that has large dimensions, a desirable shape, good density, and excellent mechanical properties. The resulting elongation of tensile tests in air reaches 14.6 and 8.9 pct for the present 5 and 10 vol pct powder metallurgy IMCs, respectively.     

Low-Temperature Densification Sintering and Properties of Monoclinic-SrAl2Si2O8 Ceramics

The dense monoclinic-SrAl₂Si₂O₈ ceramics have been prepared by a two-step sintering process at a sintering temperature of 1173 K (900 °C). Firstly, the pre-sintered monoclinic-SrAl₂Si₂O₈ powders containing small SiO₂·Al₂O₃ crystal phases were obtained by continuously sintering a powder mixture of SrCO₃ and kaolin at 1223 K (950 °C) for 6 hours and 1673 K (1400 °C) for 4 hours, respectively. Subsequently, by the combination of the pre-sintered ceramic powders with the composite flux agents, which are composed of a SrO·3B₂O₃ flux agent and α-Al₂O₃, the low-temperature densification sintering of the monoclinic-SrAl₂Si₂O₈ ceramics was accomplished at 1173 K (900 °C). The low-temperature sintering behavior and microstructure evolvement of the monoclinic-SrAl₂Si₂O₈ ceramics have been investigated in terms of Al₂O₃ in addition to the composite flux agents. It shows that due to the low-meting characteristics, the SrO·3B₂O₃ flux agent can urge the dense microstructure formation of the monoclinic-SrAl₂Si₂O₈ ceramics and the re-crystallization of the grains via a liquid-phase sintering. The introduction of α-Al₂O₃ to the SrO·3B₂O₃ flux agent can apparently lead to more dense microstructures for the monoclinic-SrAl₂Si₂O₈ ceramics but also cause the re-precipitation of SiO₂·Al₂O₃ compounds because of an excessive Al₂O₃ content in the SrO·3B₂O₃ flux agent.     

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Nonisothermal Conditions. Part V. Nonisothermal Sintering of Barium Titanate Powders of Different Dispersion

A comparative study has been made of the processes involved in the consolidation of nanosized barium titanate powders by nonisothermal sintering at a linear heating rate, by rate-controlled sintering, and high-pressure sintering (up to 5 GPa). The use of linear heating and high-pressure has been found to be ineffective for obtaining nonporous ceramics (residual porosity of about 2%) and for miniziming grain growth. The application of external pressure does not prevent coalescent grain growth controlled by surface diffusion. When rate-controlled sintering is employed a densification/grain growth optimum can be attained with a relative density of 99.9% of the theoretical value and a grain size of about 100 nm.     

Sintering kinetics of YAG ceramics

Solid state reactive （SSR） sintering kinetics was observed for YAG ceramics. There were two densification stages in sin- tering process due to its reaction. After the first stage, samples began to expand, then, the second densification stage began. At a heat- ing rate of 10 ℃/min, the sample warped down and warped back to straight. The apparent activation energy of the first densification process was about 522 kJ/mol for the initial shrinkage of A1203 and Y203 mixed powder green-body, which increased in the follow- ing process due to the solid state reaction. In the second densification stage, synthesis reaction of YAG still worked. Green-bodies processed with higher heating rate got more shrinkage at the same temperature than lower heating rate green bodies. And its kinetic field diagram was abnormal, compared with that of other reported ceramics, such as Al203. It was found that the reaction of YAG provided positive effect to the sintering driving force. The apparent activation energy for densification of SSR YAG sintered in ArH5 atmosphere was 855 kJ/mol at temperature holding sintering. And the apparent activation energy for grain growth was 1053 kJ/mol.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

Liquidlike sintering behavior of nanometric Fe and Cu powders: Experimental approach

Nanometric Fe and Cu powders were sintered in vacuum, He, and H₂ atmospheres after uniaxial cold pressing. The shrinkage behavior of samples was studied using three different dilatometric techniques: constant heating rate, isothermal annealing, and the Dorn method. Density greater than 90 pct was obtained at sintering temperatures of 900 °C. In nanometric powders, densification and grain coarsening occurred in a narrow temperature interval. Despite the low oxide content in the starting powders (1.5 to 4 wt pct), the reducing atmosphere plays a relevant role in the sintering process. The self-diffusion activation energies obtained for nanometric Fe were 116 and 60 kJ/mole in vacuum and H₂, and those obtained for nanometric Cu were 70 and 43 kJ/mole in He and H₂. According to the present results, the activation energies obtained from both nanometric powders in H₂ could be associated with those for self-diffusion in liquid Fe (65 kJ/mole) and Cu (41 kJ/mole).     

Refractory and ceramic materials

The paper examines the consolidation of 95 mole% ZrO₂-2 mole% CeO₂-3 mole% Y₂O₃ nanocrystalline powder in cold uniaxial double-action pressing, cold isostatic pressing (60 and 120 MPa), and sintering. Five starting powders are produced by processing a suspension after hydrothermal decomposition in different conditions. It is established that a homogeneous microstructure forms only in a material from the powder subjected to two homogenizing grindings. After cold uniaxial pressing and cold isostatic pressing, the sintered samples reach a relative density of 0.96 to 0.94. The bending strength is 600 to 660 MPa. The efficient consolidation of ceramics requires comprehensive processing of starting nanocrystalline powders to modify their morphology. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 7–8 (456), pp. 45–58, 2007.     

A two-stage approach of manufacturing FeAl40 iron aluminides by self-propagating synthesis and pressureless sintering

A two-stage sintering process was successfully used to sinter FeAl to densification levels of just above 95% at a temperature of 1300°C. In the first stage, mixed iron and aluminium powders were synthesised at 750°C via self-propagating high-temperature synthesis (SHS) to form brittle and porous Fe₂Al₅. Then the pellets were crushed and milled to various sizes and mixed with iron powders in the nominal composition of FeAl40 and pressurelessly sintered at a higher temperature to obtain a higher densification by taking advantage of the less violent exothermic reaction of Fe₂Al₅ and Fe. The intermediate and end products in SHS and sintering were characterised by SEM/EDX and XRD. The porosity level of the final FeAl40 product was controlled by the heating rate and powder size, which was also strongly influenced by the temperature, holding time and the ratio of the two powders.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5–FeAl2 powders Part 2 – Densification mechanism analysis

Abstract

The use of Fe₂Al₅–FeAl₂ prealloyed powders and heating rates >150 K min^?1 overcomes the formation of density restricting Kirkendall porosity in the Fe–Al system. X-ray diffraction, electron probe micro analysis and differential thermal analysis suggest that the absence of a persistent liquid, experienced when liquid phase sintering with elemental powders, is overcome. Homogenisation is greater during heating at a rate of 20 K min^?1 than for 150, 250 or 400 K min^?1 and homogenous Fe₃Al forms across the compact at temperatures below the melting point of the liquid forming constituent, indicating that a liquid will not form under such processing conditions. The maximum density achieved under the processing conditions in the present study is 92% of theoretical density. The presence of large pores shortly after liquid formation suggests that the remaining porosity is largely due to powder agglomeration during mixing.     

Study on Agglomeration and Densification Behaviors of Gadolinium-Doped Ceria Ceramics

By synthesizing reactive powders via a self-sustaining combustion synthesis, the glycine-nitrate process, the gadolinium-doped celia （GDC） with the chemical formula Ce0.8Gd0.2O1.9 was prepared. The resultant powders were dispersed with the terpineol as the dispersant through different methods such as ball milling and high-shear dispersing. Coagulation factor （CF） was used to mark the degree of agglomeration on the nano-scale GDC in this work. The effect of agglomeration on the densification behavior at different sintering temperatures was investigated. The studies indicated that agglomeration retarded the densification at the sintering stage. The powders with better dispersion exhibited a higher sintered density at the same temperature. After effective dispersion treatment, GDC could be fully densified at the sintering temperature of 1300 ℃. The densification temperature was significantly lower than those reported previously. The high sintering kinetics of the ceramics was obtained based on the agglomeration control.     

Studies on BaO-Y2O3-TiO2 Microwave Dielectric Ceramics

BaO-Y₂O₃-TiO₂ microwave dielectric ceramics with the rich area of TiO₂ were fabricated by a solid-state reaction method using BaCO₃, Y₂O₃, TiO₂ powders as starting materials. The sintering characteristics, phase composition, micro-structures and microwave dielectric properties of BaO-Y₂O₃-TiO₂ microwave dielectric ceramics with different k values sintered at different temperatures were investigated. The results showed that the sintering temperature of BaO-Y₂O₃-TiO₂ microwave dielectric ceramics was lower (about 1240 °C), and the sintered ceramics with the major phase of Y₂Ti₂O₇ had excellent dielectric properties. When k = 4, ɛ_r and tanδ were about 78.3 and 3 × 10⁻³ respectively. When k=5, ɛ_r and tanδ were about 53 and 9 × 10⁻⁴ respectively.     

Inorganic interfacial engineering: processing of hard materials

Abstract

Inorganic interfacial engineering may be regarded as the core of powder metallurgical processing of hard materials. The present paper reviews recent results from an interdisciplinary research effort, BRIIE (the Brinell Centre for Inorganic Interfacial Engineering), a joint effort between five industrial companies, three universities, two research institutes and VINNOVA (the Swedish Agency for Innovation Systems). The research involves experimental work on the aqueous processing of powders and the use of surface actants is reviewed as well as the colloidal processing of ceramics. Pressing and sintering of agglomerated powders have been studied both theoretically and experimentally. Models for the simulation of pressing and sintering of hard metal powders are developed. Results on ceramic materials obtained by spark plasma sintering and their resistance to thermal shock are reported.     

Nitrogen Ceramics: Liquid Phase Sintering

Abstract

The role of a minor silicate eutectic liquid phase as a transport medium in sintering hot–pressed silicon nitride (β Si₃N₄) ceramics was identified in the 1970s. A similar mechanism is applicable to hot–pressed Si–Al–O–N ceramic alloys which offer an advantage in control of the final liquid volume and hence in superior high temperature mechanical properties. By increasing the liquid volume it is possible to densify ceramic alloys without application of pressure at the sintering temperature and hence to fabricate components of complex shape. The Lucas Syalon ceramics typify the new range of pressureless–sintered ceramics based on the β Si₃N₄ structure. They are fabricated from the ultrafine compound powders α Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and a polytypoid phase (a substitute for A1N). The ceramics consist of submicrometre solid solution crystals of general composition Si_3?xAl_xO_xN_4?x(x < 1) within a minor matrix phase which may be either a glassy Y–Si–Al oxynitride or be crystallized to form yttrogarnet. Analysis of matrix glass compositions shows them to be residues of liquids near to a ternary eutectic in the Y₂O₃–SiO₂–Al₂O₃ system which is well below the sintering temperature of ～ 1800°C. Sintering models, based on particle rearrangement due to dissolution of the major α Si₃N₄ component in the eutectic liquid and its reprecipitation as a β Si₃N₄ solid solution, are discussed. Properties and current applications of Syalon ceramics are surveyed briefly. PM/0266     

Preparation and Properties of Ba6-3xEu8+2xTi18O54 Ceramics via Ethylenediaminetetraacetic Acid Precursor

Ba_6-3xEu_8+2xTi₁₈O₅₄ (x=2/3) (BET) ceramic powders were synthesized by the Pechini method using ethyl-enediaminetetraacetic acid (EDTA) as a chelating agent. A milk-white, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized BET ceramic powders were prepared by sintering and smashing ceramics samples, without the formation of any intermediate phases. Meanwhile, the crystal structure, which was determined by X-ray diffraction, had important effect on the microwave dielectric properties of BET. The BET ceramics had good microwave dielectric characteristics: ɛ_r = 72.13, Q_f = 7111 GHz, τ_f = −36.53 × 10⁻⁶/°C.     

In-Situ Reactive Synthesis of Si2N2O Ceramics and Its Properties

Si₂N₂O is considered as a new great potential structural/functional material in place of Si₃N₄ for high-temperature applications. In the present work, Si₂N₂O ceramics were in-situ reactive synthesized by a nitridizing powder mixture of Si and SiO₂ using an optimized two-step sintering process according to thermodynamic analyses. The results showed that the purity of Si₂N₂O in the produced ceramics increased with an increase in final sintering temperature, while the shrinkage and Vickers hardness decreased. After final sintering above 1923?K (1650?°C), pure nanograined Si₂N₂O ceramics can be obtained. Flexural strength and fracture toughness both showed peak values at 1873?K (1600?°C). The reaction mechanism was proposed and then the difference of the produced ceramics was discussed.