Formation Temperature Of Liquid Phase Research Articles

Reduction of liquid phase formation temperature of TiC–Ni composite by sintering activator addition

Abstract In this work sintering activators (phosphorus: P, copper: Cu and tin: Sn) were added to the TiC–Ni powder mixes. The mixed powders were compacted and then sintered at relatively low sintering temperatures (1200–1300 °C). From differential thermal analysis (DTA) results, it was found that addition of the activators could reduce sintering temperatures needed for densification of the TiC–Ni powder mixes. Reduction of the liquid phase forming temperatures was caused either by melting of eutectic or solid solution materials formed from Ni and the activators powders. Amongst the investigated activators, phosphorous addition showed an effect on increase of mechanical properties of the TiC–Ni composites. In contrast, the addition of other sintering activators showed no evidences of microstructure and mechanical property enhancement.

Diffusion Behaviour of the Grain-Growth Inhibitor Vc in Hardmetals

Diffusion couples of the type WC-Co/VC-Co were employed to observe the diffusion behaviour of the grain-growth inhibitor (GGI) VC within nanoand ultrafine-grained WC-10wt.% Co samples. Since diffusion of the GGI during heating occurs already in the solid-state regime, interrupted sintering experiments were performed up to the liquid phase formation temperatures. By the use of light-optical and scanning electron microscopy the impact of GGI concentration on the microstructure as a function of depth from the interface was investigated. Electron Probe Microanalysis (EPMA) was used in order to quantify the amount of vanadium diffused into WC-Co as a function of distance. In first approximation, an activation energy of 3.45 eV (332,6 kJ/mol) for vanadium transport was determined.

Phase Analysis and Study of the Microstructure of Cordierite-Si3N4 Composite Synthesis by Nano- and Micro-Sized Silicon Particles

In this study the effect of in situ Si3N4 formation on the microstructure changes of cordierite-Si3N4 composite prepared using nano silicon powders, which obtained in a high energy planetary ball mill, and micro silicon powder was investigated. Different mixtures of the cordierite with nano and micro silicon powders were prepared and sintered in a tube furnace at different temperatures in a nitrogen atmosphere. Microstructural characteristics, thermal properties and phase analysis of these composites were carried out by using scanning electron microscopy (SEM), simultaneous thermal analysis (STA) and X-ray diffraction analysis (XRD). It was found that micro and nano silicon particles have different reaction characterizations with the nitrogen in the matrix of cordierite at various temperatures. Results showed that by using nano silicon powder the formation of silicon nitride bonding will be increased which could be related to the acceleration of nitride phase at temperatures lower than the temperature of liquid phase formation.

Efeitos do carbono na evolução de segundas-fases e na densificação do nitreto de alumínio com Y2O3

Foi investigado o efeito da adição do carbono na evolução de segundas-fases e na densificação do AlN com 4% em massa de Y2O3. A mudança de composição da segunda-fase do AlN com Y2O3 foi induzida pela adição de 0,5% e 1,0% em massa de carbono. A sinterização sob atmosfera de nitrogênio foi realizada em forno com elemento resistivo de tungstênio entre 1650 ºC e 1850 ºC. A evolução de segunda-fase mostrou uma tendência para formar fases mais ricas em ítrio com o aumento do teor de carbono, o que atrasou a densificação do AlN com Y2O3 devido ao aumento da temperatura de formação de fase líquida. O efeito prejudicial causado pela adição de carbono diminuiu com o aumento da temperatura de sinterização, pois todas as amostras atingiram quase completa densificação após sinterização a 1800 ºC. A adição de carbono induziu uma evaporação significativa de compostos durante a sinterização, mas o comportamento de densificação foi pouco influenciado pela redução na fração de fase líquida existente na temperatura de sinterização e pelo gás aprisionado no interior dos poros fechados.

Effect of CuO on the formation of clinker minerals and the hydration properties

The purposes of this study are to explore the mechanisms of Cu element in clinker burning and hydration processes and to make effective use of waste containing copper in cement production. The effect of CuO on clinker mineral composition, C 3S polymorph and size, Cu element distribution and state, compressive strengths, hydration products, non-evaporable water quantity and hydration heat release rate was analyzed by XRD, SEM, DTA, isothermal heat-conduction calorimetry, etc. Results show that as the amount of CuO increases the formation and growth of C 3S grain are accelerated, R C 3S is gradually transformed into M 3 and the content of C 4AF increases; a small quantity of CuO increases the 3-day and 28-day strengths and the hydration degree of clinker, but excessive CuO has adverse effects. Those effects of CuO on clinker burning process are attributed to the formation of low-melting Cu 2O and the dissolution of CuO in C 4AF which decrease the formation temperature of liquid phase and increase its quantity. The effects on hydration process result from the combined action of the following factors: the induction period is prolonged; the hydration reactions in the initial and acceleration periods are accelerated.

Microstructure of boron carbide pressureless sintered in an Ar atmosphere containing gaseous metal species

97.4% of theoretical density was obtained for boron carbide (B 4C) ceramics by heating up to 2226 °C in an Ar atmosphere containing gaseous Al and Si species without external pressure. Impurities and secondary phases in the sintered B 4C samples were examined by X-ray fluorescence and X-ray diffraction analyses respectively, which revealed that both Al and Si elements infiltrated into the green compacts and reacted with B 4C to form SiC, Al 4C 3 and Al 4SiC 4 during the sintering. Triple junctions observed in the polished surfaces of the densified samples were filled by the secondary phases, indicating formation of liquid phase during heating. Dilatometric measurements at a constant heating rate in the Ar gas with the metallic gas species demonstrated that the shrinkage started at around 1700 °C, which was the liquid-phase formation temperature for the system reported in the previous studies. It was supposed that the liquid phase might be responsible for the densification.

Formation of Varistor Characteristics by the Grain-Boundary Penetration of ZnO-PrOx Liquid into ZnO Ceramics

Penetration of a liquid (ZnO-PrOx) into the grain boundaries of sintered, cobalt-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1-2 V range and nonlinearity coefficients of 22-37. The varistors were fabricated by spreading a thin layer of Pr6O11 powder paste on the surface of ZnO pellets and heating to various temperatures (1200°-1400°C) and times (0-60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat-treated above liquid-phase formation temperature.

Inversion boundary induced grain growth in TiO 2 or Sb 2O 3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO 2/Bi 2O 3 ratio. The selection of a composition with an appropriate TiO 2/Bi 2O 3 ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi 2O 3 is important as the grain growth is slowed down by a larger amount of Bi 2O 3-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 TiO 2 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 Zn 2TiO 4 spinel phase only affects grain growth in compositions with a TiO 2/Bi 2O 3 ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb 2O 3 trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb 2O 3/Bi 2O 3 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 Sb 2O 3/Bi 2O 3 ratios.

The effect of additives on sintering behavior and strength retention in silicon nitride with RE-disilicate

Abstract Four kinds of Si 3 N 4 –RE 2 Si 2 O 7 (RE: Nd, Sm, Y, Yb) ceramics have been fabricated by hot pressing at 1725–1750°C for 2 h in N 2 gas flow and post-annealing at 1450°C for 4 h. The effect of rare-earth oxide additions on the densification process was investigated by measuring the shrinkage of the compact. Results revealed that the rare-earth oxides affect the densification process of Si 3 N 4 –RE 2 Si 2 O 7 ceramics in which the shrinkage temperature varies with the types of rare-earth additives. The Si–RE–O–N liquid phase formation temperature can be estimated from the onset temperature of final shrinkage stage, and the temperature is in the order of Sm 2 gas flow. The high temperature strength of Si 3 N 4 –Yb 2 Si 2 O 7 or Si 3 N 4 –Y 2 Si 2 O 7 ceramics is better than that of Si 3 N 4 –Sm 2 Si 2 O 7 or Si 3 N 4 –Nd 2 Si 2 O 7 ceramics. There is a roughly linear relation between the strength retention and the liquid phase formation temperature. Present results indicate that Si 3 N 4 –RE 2 Si 2 O 7 ceramics with high strength retention can be fabricated by adopting additives with small ionic radius and high ionic charge.

Investigation of the solidus boundaries and microstructure in the ZnO–PrO1.5–CoO system

ZnO ceramics used as varistors are prepared with cobalt oxide as an essential additive to improve nonohmic properties. Because some of its effects during the liquid-phase sintering remain unexplained, we characterize the liquid-phase formation temperatures and phase reactions in the system ZnO–PrO1.5–CoO. Using differential thermal analysis (DTA) during sintering, we detect new thermal phenomena. An addition of cobalt oxide to ZnO–PrO1.5 mixtures (ZnO–5 mol% PrO1.5–10 mol% CoO) significantly decreases the liquid-phase formation temperature to 1272 ± 5 °C, which is about 110 °C lower compared to those in the ZnO–PrO1.5 system. Characterization of ceramics quenched during sintering allows us to describe an isoplethal section with PrO1.5 contents of 5 mol% and solubility limit of Co in ZnO.

Sintering of injection-moulded WC–7 wt% Ni in a hydrogen atmosphere

Sintering injection-moulded WC–7 wt% Ni in a hydrogen atmosphere at temperatures higher than 900°C resulted in an inhomogeneous microstructure, as Ni2W4C (η phase) preferentially developed near the surface of specimens. Subsequent to sintering at temperatures higher than the liquid-phase formation temperature, the surface of specimens was virtually composed of η phase. This arose from the excessive decarburization occurring near the surface of specimens, and the exudation of the Ni-based metal binder towards the surface of specimens. Primarily caused by the dissolution of W into the Ni binder phase, the saturation magnetization of WC–7 wt% Ni decreased at temperatures lower than 1000°C.

Subsolidus grain growth in donor doped barium titanate

Grain growth in donor doped BaTiO3 was studied below the temperature of liquid phase formation. It was found that a high donor concentration and high oxygen partial pressure effectively inhibit grain growth rate during sintering. Defect chemistry, derived from a model consistent with electrical measurements, well describes the grain growth properties of donor doped BaTiO3 in the subsolidus grain growth region.

Electrical properties of air-fired Nb-doped SrTiO3 with excess titania

Abstract Electrical properties of Nb-doped SrTiO3 ceramics sintered above and below the liquid phase formation temperature in air were examined. High apparent dielectric constants were obtained when the donor level was ≥0.8 at%. The high dielectric constants and relatively high loss tangent, along with the I-V characteristics and color of the samples, implied conductive grains and weakly insulative grain boundaries. A weak dielectric relaxation was observed at the low radio frequencies (10-105 Hz) in the temperature region -50[ddot] -125[ddot]C. The ionic polarization model proposed by Skanavi was applicable to the dielectric relaxation, which supports strontium vacancy compensation for Nb donors.

Phase transformation and microstructure of a dense zircon-zirconia composite

Dissociated zircon mixed with 10 wt% monoclinic zirconia was densified above the liquid-phase formation temperature (≥ 1676°C) to greater than 97% theoretical density. The dense material was then heat treated between 1450 and 1650°C to increase zircon-phase content through the association reaction of amorphous silica with zirconia. Optical microscopy, SEM, TEM and quantitative XRD were used to study pore evolution due to the association reaction and to measure the degree of zircon-phase formation. The reaction of amorphous silica and zirconia has reaction kinetics that can be represented as a transformation-temperature-time (TTT) diagram. The diagram yields transformation curves in a ‘C’ shape which imply a nucleation and growth mechanism for the reaction with a maximum reaction rate near 1600°C. Pore volume was dependent on the extent of zircon association, and homogeneously distributed in the matrix. However, below 1500°C, an interface development influenced the transformation and gave a morphology with pore cluster near mm in size. Maximum open-pore volume was 10% after heat treatment at 1550°C for 4h. Eine Mischung aus dissoziiertem Zirkon und 10 Gew.% monoklinem Zirkondioxid-Zusatz wurde oberhalb der Bildungstemperatur der flüssigen Phase (≥ 1676 C) zu einer Dichte oberhalb 97% der theoretischen Dichte verdichtet. Das verdichtete Material wurde anschließend bei Temperaturen zwischen 1450 und 1650°C wärmebehandelt, um den Anteil an Zirkonium-Phase über die Assoziationsreaktion von amorphem Siliziumdioxid mit Zirkoniumdioxid zu erhöhen. Um die Entwicklung der Porosität aufgrund der Assoziationsreaktion zu untersuchen und um den Grad der Bildung von Zirkonium-Phase zu bestimmen, wurden Lichtmikroskopie, REM, TEM und quantitative XRD eingesetzt. Die Reaktion zwischen amorphem Siliziumdioxid und Zirkoniumdioxid zeigt eine Reaktionskinetik, die in der Form eines Zeit-Temperatur-Umwandlungs-diagramms (ZTU) dargestellt werden kann. Diese Auftragung führt zu C-förmigen Umwandlungskurven. Damit können Keimbildungs- und Keimwachstumsmechanismen für die Reaktion impliziert werden. Die maximale Reaktionsrate liegt bei 1600°C. Das Porenvolumen war gleichmäßig über die Matrix verteilt und ist abhängig von dem Ausmaß der Zirkonium-Assoziation. Unterhalb 1500°C beeinflußte jedoch die Ausbildung der Grenzflächen die Umwandlung und führte zu einer Morphologie mit Poren-Clustern im mm-Größenordhung. Das maximale Volumen offener Porosität einer Wärmebehandlung von 4 h bei 1500°C entsprach 10%. Du zircon (ZrSiO 4 ) dissocié mélangé avec 10% en masse de zircone monoclinique a été densifié à plus de 97% de la densité théorique au dessus de la température de formation de la phase liquide (≥ 1676°C). Le matériau dense a été par la suite traité thermiquement entre 1450 et 1650°C pour augmenter la teneur en zircon par la réaction entre de la silice amorphe et la zircone. On a utilisé la microscopie optique, le MEB, le MET et l'analyse quantitative par diffraction de RX pour étudier l'évolution des pores due à la réaction et pour mesurer le degré de formation du zircon. La réaction entre la silice amorphe et la zircone a une cinétique de réaction qui peut être représentée par un diagramme transformation-température-temps (TTT). Le diagramme présente des courbes de transformation en forme de C, ce qui implique une nucléation et un mécanisme de croissance pour la réaction, avec une vitesse maximale de réaction vers 1600°C. Le volume des pores était dépendant du degré d'association et présentait une distribution homogène dans la matrice. De plus en dessous de 1500°C, le développement d'une interface a influencé la transformation et a donné une morphologie avec des regroupements de pores d'une taille proche du mm. Le volume maximum de porosité ouverte a été de 10% pour un traitement thermique à 1550°C pendant 4 h.

Activated sintering of chromium-carbide-nickel alloys

1. Small phosphorus additions (0.1–0.2%) activate the initial state of sintering of chromium-carbide-nickel alloys, as a result of which a volume shrinkage of specimens amounting to 80–90% of the maximum attainable shrinkage is observed at a sintering temperature as low as 1100°C. 2. When the amount of phosphorus in the nickel binder corresponds to the eutectic composition in the Ni-P system, full sintering of KKhNF alloys is observed at 1100°C, i.e., a temperature 100–150°C lower than that for phosphorus-free alloys. 3. The low temperature of liquid phase formation in chromium-carbide-nickel-phosphorus alloys enables them to be hot-pressed at 950–1070°C, i.e., at temperatures 150–200°C lower than the hot-pressing temperatures of alloys without a phosphorus addition.

Activation of densification in the sintering of some powder metallurgy alloys

1. The addition to a mixture of copper and aluminum powders of an element (such as silver) capable of forming binary eutectics with the mixture components activated densification and reduces specimen growth in the temperature range 480–800°C. The added element produces this effect by lowering the temperature of liquid-phase formation, widening the temperature range in which the liquid phase is present, and increasing the amount of the latter in compacts. 2. The activated densification of a Cu-Al alloy at high temperatures (800–1000°C) is linked principally with the effect exerted by the silver addition on solid-phase sintering. The partial diffusion coefficients of silver and copper are unequal, and the preferential direction of the diffusion flux is from the addition to the matrix metal. This results in the formation of diffusion porosity in the surface layers of the silver particles, which activates shrinkage by facilitating the propagation of diffusional creep. 3. With rise in silver concentration in a Cu-Al alloy, the extent of shrinkage increases and volume growth diminishes. Alloying with silver substantially accelerates the homogenization process of the alloy.

Effect of Metal Additions on the Microstructure of Zirconia

The diffusion of titanium into zirconia was studied to determine the mechanism by which increased sintering occurred in the metal‐modified zirconias. Results revealed that the increased sintering occurred only when compositions above the solubility limit were fired considerably above the melting point of the titanium. Examination of the system Ti‐Zr‐0 revealed that considerable liquid was present at these firing temperatures. This liquid phase promoted the sintering of the zirconia. The increased sintering did not occur for compositions above the solubility limit fired below the temperature of liquid phase formation, or for compositions below the solubility limit fired considerably above or below the melting point of titanium.