Glass formation in the system CaO-Al2O3-CaF2

The formation of glass in the system CaO-Al₂O₃-CaF₂ has been investigated in sealed platinum capsules having about one atmosphere pressure of AlF₃ vapour. Transparent colourless glass could be obtained in the low-fluoride moderate-alumina region of the system (Al₂O₃ 35 to 60%, CaF₂ 0 to 20%). With the concentration of CaF₂ exceeding 20% considerable amount of quench crystals of CaF₂ appeared in the glass. Moderate-alumina low-lime melts containing more than 35% CaF₂ occur in an immiscibility zone. At the low-fluoride periphery of the liquid immiscibility zone a small zone of metastable liquid immiscibility has been found. The results of electron microscopic and infra-red spectroscopic studies of a few selected glasses have been analysed in combination with the molar refractivity data to reflect upon the co-ordination characteristics of aluminium in these glasses.     

The join calcium monoaluminate-calcium fluoride

The system CaO-Al₂O₃-CaF₂ is important in cement and slag technology and in metallurgy. A section of this system, the pseudo-binary join CaO·Al₂O₃-CaF₂, has been studied and the phase diagram established. This join is of particular interest since CaO·Al₂O₃ is one of the main constituents of high alumina cement.Quenching in sealed platinum capsules followed by microscopic and X-ray examination was the principal method used. The only compound on the join is 3CaO·3Al₂O₃·CaF₂ which melts congruently at 1507±1.5° C and forms one eutectic with CaO·Al₂O₃ at 11% CaF₂ and 1498±5° C and another with CaF₂ at 97.5% CaF₂ and 1405±10° C. There is a wide zone of liquid immiscibility. The m.p. of CaF₂ was determined to be 1422±1° C.Attempts to use high temperature microscopy to study this system are described.     

The phase equilibrium diagram of the ternary subsystem CaO-CaO·Al2O3-11 CaO·7 Al2O3·CaF2

On the basis of the experimental results obtained from the study of high-temperature equilibrium relations of seven pertinent joins the phase diagram of the subsystem CaO-CaO·Al₂O₃-11 CaO·7 Al₂O₃·CaF₂ has been constructed. In this diagram the delineation of the boundary curves of the primary fields of CaO, 3 CaO·Al₂O₃, CaO·Al₂O₃ and the 11 CaO·7 Al₂O₃·CaF₂ solid solution has been improved over our previously published phase diagram of the system CaO-Al₂O₃-CaF₂ [1]. The isotherms have also been drawn more precisely to give a better idea about the topography of the portion investigated.     

Phase equilibria in the CaF2-Al2O3-CaO system

Computations of phase equilibria in the CaF₂-Al₂O₃-CaO system have been carried out on the basis of experimentally found thermodynamic properties of all intermediate phases and melts. Coordinates of the phase equilibrium boundaries were determined by solving a system of equations expressing equality of chemical potentials of the components in coexisting phases. The nature and quantity of the coexisting phases were established by a search for the Gibbs energy minimum of the system. All the phases of the CaF₂-Al₂O₃-CaO system were taken into consideration. Calculated phase diagrams of the CaO-CaF₂, CaO-Al₂O₃ and CaF₂-Al₂O₃ binary subsystems are in good agreement with the data available in the literature. Isotherms of the CaF₂-Al₂O₃-CaO system were calculated at 1600, 1650, 1723 and 1773 K. A wide region of liquid separation into two phases is observed in the system. One phase is composed of practically pure CaF₂ with additions of several mol% of CaO and Al₂O₃, and the other consists of 50 to 65 mol% of CaF₂ only. Eleven invariant points of the CaF₂-Al₂O₃-CaO system include seven ternary eutectics, two ternary peritectics and two points of four-phase monotectic transition. The primary fields of crystallization of all the phases are alongated toward the CaF₂ apex, the CaO field being the widest and the 3CaO·Al₂O₃ field the narrowest. Seven junctions of the CaF₂-Al₂O₃-CaO phase diagram were represented. Computed saturation lines of CaF₂-Al₂O₃-CaO melt with CaO, Al₂O₃, CaO·6Al₂O₃ and CaO·2Al₂O₃, and also the positions of a number of characteristic points, agree well with the experimental data available. The present calculations reveal a number of details and peculiarities of the constitution of the CaF₂-Al₂O₃-CaO phase diagram.     

Estimated immiscibility isotherms of the Li2O-Al2O3-SiO2 system

Using the immiscibility temperature estimation method, recently developed by the present author and Tomozawa, immiscibility isotherms of the Li₂O-Al₂O₃-SiO₂ system were estimated. High reliability of the estimated immiscibility isotherms was confirmed by observing the morphologies of phase-separated glasses, and also by comparing the estimated and observed immiscibility temperatures at several compositions. The determined immiscibility isotherms revealed that, in the Li₂O-Al₂O₃-SiO₂ system, only composition regions near the Li₂O-SiO₂ and Al₂O₃-SiO₂ binary edges are phase-separable. In composition regions where base glasses for commercial glass-ceramics are located, the immiscibility temperatures were much lower than the glass transition temperatures, implying that no phase separation actually occurs. Accordingly the phase separation in practical glasses for producing glass-ceramics may be attributed to increased immiscibility resulting from various additives.     

Some phase relationships within the system Na2O/B2O3/Nb2O5

Phase behaviour over regions of the ternary system Na₂O/B₂O₃/Nb₂O₅ has been explored.Liquidus temperatures and the stability regions of primary phases have been determined over selected composition ranges by high temperature microscopy. Crystallisation processes in melts and corresponding glasses have been followed using both conventional methods of thermal analysis and newly developed micro techniques combined with hot stage microscopy.An electron microscope has been employed to follow changes in the microstructure of quenched glasses after controlled heat treatments.It has been shown that the system contains a liquid immiscibility gap, and some attention is given in the discussions to the influence that can be assigned to cations in determining the extent of such gaps and general structural relationships in borate/oxide systems.     

Wetting behaviour of SiC ceramics: Part I. E2O3/Al2O3 additive system

Wettability is the most important phenomenon in SiC liquid phase sintering. This paper discusses the ceramic–ceramic wetting of E₂O₃/Al₂O₃ additives on SiC, where E₂O₃ is a mixture of rare earth oxide. A sphere-shaped sample of additive was put on a SiC plate and the set placed in a graphite resistance furnace and heated to the additive sphere melting point at a rate of 10 °C/min. The behaviour of the additive on the SiC plate was observed by means of an imaging system using a CCD camera, while the contact angle was measured and analyzed using QWin Leica software. The tests were performed in argon or nitrogen atmospheres. The wettability curves displayed a fast decline and good spreading. The E₂O₃/Al₂O₃ system, which approached a eutectic composition when compared with the phase diagram of the Y₂O₃/Al₂O₃ system, displayed better spreading. Measurements of the contact angle could not be made when the test was conducted in a nitrogen atmosphere because of the bubbles that formed in the liquid during the test, damaging the interfacial zone between E₂O₃/Al₂O₃ and SiC. The results of these tests indicate that the best sintering atmosphere for this additive system is argon.     

Glass formation and immiscibility in the TeO2-B2O3-Fe2O3-MnO system

The glass formation in the quaternary TeO₂-B₂O₃-MnO-Fe₂O₃ system and in its ternary systems was investigated. A range of liquid immiscible phases, located near to the binary TeO₂-B₂O₃ and B₂O₃-MnO systems was established. Using transmission electron microscopy, a trend to metastable liquid-phase separation in the single-phase glasses, located near to the boundary of immiscibility was observed. With an increase in the Fe₂O₃ and MnO content still in the process of cooling of the melts, it was possible for a fine glassy crystalline structure to be formed in them. It was shown that by changing the upper limit of the melting temperature and the cooling rate, the glassy crystalline structure and the Fe₃O₄ content could be modified.     

Microstructure Evolution and Isothermal Compaction in TiO2-Al-C Combustion Reaction

Combustion reaction in the TiO₂-Al-C system was investigated by the combustion wave freezing technique with a wedge-shaped copper-made quenching block. The combustion reaction was a combined process in which the aluminothermic reduction of TiO₂ (3TiO₂ + 4Al 2Al₂O₃ + 3Ti) and TiC formation reaction (Ti + C TiC) occur in series. First, the aluminothermic reduction was activated by wet spreading of molten Al into interspaces between TiO₂ particles to produce rounded Al₂O₃ grains embedded in the Ti-rich liquid phase. In the later combustion stage, the Ti-rich phase reacted with the reactant C to produce TiC grains in the Ti-rich liquid phase. The three-dimensionally interconnected Al₂O₃ structure typically shown in this system mainly originated from interconnection between the rounded Al₂O₃ grains due to the high combustion temperature from the high exothermic TiC formation reaction. With decreasing the combustion temperature and controlling the C and TiC content, the interconnected Al₂O₃ structure changed to isolated. The isolated Al₂O₃ structure showed superior isothermal compaction behavior to the interconnected. Finally, it is suggested that the microstructure of combustion reaction should be one of the important factors in the SHS compaction process.     

Subsolidus phase equilibria in the Al2O3-CeO2-PbO and Al2O3-CeO2-RuO2 systems

The subsolidus phase equilibria in air for the Al₂O₃-CeO₂-PbO and Al₂O₃-CeO₂-RuO₂ systems were studied with the aim of obtaining information on possible interactions between a CeO₂-based solid electrolyte in solid-oxide fuel cells (SOFCs) and other oxides. No ternary compound was found in either of the systems. The tie line in the Al₂O₃-PbO-CeO₂ system is between Al₂Pb₂O₅ and the CeO₂.     

Phase equilibria and immiscibility in the TeO2-P2O5 system

The phase equilibria and immiscibility of mutual glass formers up to 50 mol% P₂O₅ have been studied. Phase analysis indicates the formation of three new phases — incongruent melting Te₄P₂O₁₃, Te₂P₂O₉, and a supposed metacompound. Electron microscope investigations established stable and metastable phase separation. The immiscibility confines the tendency to glass formation up to 25.8 mol % P₂O₅. A reliable interpretation in relation to the morphology of liquid-liquid immiscibility and crystallization is considered.     

Diffusion bonding of Al2O3-TiC composite ceramic and W18Cr4V high speed steel in vacuum

Al₂O₃-TiC composite ceramic and W18Cr4V high speed steel were joined by diffusion bonding with a Ti-Cu-Ti multi-interlayer in a vacuum of 10⁻⁴-10⁻⁵ Pa. The interfacial microstructures of the Al₂O₃-TiC/W18Cr4V joint were investigated with optical microscope and scanning electron microscopy. The elemental concentration near the diffusion interface was evaluated by electron probe microanalysis. The results indicate that an obvious transition zone was formed between Al₂O₃-TiC and W18Cr4V during the vacuum diffusion bonding. The elements in the transition zone are mainly Ti and Cu with a small amount of Fe. Element Ti concentrates near the two interfaces of the Al₂O₃-TiC/transition zone/W18Cr4V. The microhardness of the transition zone is lower than that of Al₂O₃-TiC and higher than that of W18Cr4V. The formation process of the transition zone consists of five stages: (i) Formation of Cu-Ti liquid phase; (ii) Full melt of Cu; (iii) Full melt of Ti; (iv) Formation of reaction layer; (v) Formation of Cu-Ti solid solution and increment of reaction layer.     

Phase transition of Er3+-doped Al2O3 powders prepared by the non-aqueous sol–gel method

The Er³⁺-doped Al₂O₃ powders have been prepared by the non-aqueous sol–gel method using the aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium nitrate, as dopant under isopropanol environment. The phase structure and phase transition of the Er³⁺-doped Al₂O₃ powders were investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The phase contents diagram for the Er-doped Al–O system with the doping concentration up to 5 mol% was described at the sintering temperature from 550 to 1250 °C. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases, γ-, θ- and α-(Al, Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the Er–Al–O phase contents diagram. The Er³⁺ doping suppressed crystallization of the γ and θ phases and delayed phase transition of the γ → θ and θ → α. The increased Er³⁺ doping concentration and the elevated sintering temperature enhanced the precipitation of the ErAlO₃ and Al₁₀Er₆O₂₄ phases. The preparation procedure for the Er³⁺-doped Al₂O₃ powders in the non-aqueous sol–gel process, including chelating, hydrolysis, peptization, doping and gelation, has a significant effect on the phase formation and its transition for the Er³⁺-doped Al₂O₃ powders.     

Microstructural observation of ZnAl2O4 formation affected by the physical nature of Al2O3 in the presence of LiF

Microstructures of ZnA₂O₄ formation in the presence of LiF were observed for specimens including both coarse and dense alumina and agglomerates of fine alumina, and their morphological changes were compared with each other. LiF formed an intermediate liquid phase with ZnO and Al₂O₃. In the specimen with agglomerated Al₂O₃, the porous ZnAl₂O₄ layer was developed from the Al₂O₃ agglomerates. However, on firing at 700° C, zeta-lithium aluminate developed just inside the ZnAl₂O₄ layer and interfered with the spreading of the liquid including Al₂O₃ to the ZnO phase. However, on firing above 800° C, the formation of ZnAl₂O₄ was promoted by the rapid spreading of the fluoride liquid including Al₂O₃ to the ZnO phase through the porous ZnAl₂O₄ layer before LiAl₅O₈ formation. In the specimen with coarse and dense Al₂O₃, the dense ZnAl₂O₄ layer grew around the coarse and dense Al₂O₃ and the fluoride liquid phase was confined between the ZnAl₂O₄ layer and the Al₂O₃ particles. The dense ZnAl₂O₄ layer interrupted the spread of the liquid phase to the ZnO phase and interfered with further formation of ZnAl₂O₄. The confined liquid phase gradually reacted with Al₂O₃ to form LiAl₅O₈. It is necessary for the fluoride liquid including Al₂O₃ to spread out to the ZnO phase through the porous ZnAl₂O₄ layer to form ZnAl₂O₄, before the dense ZnAl₂O₄ or LiAl₅O₈ layer grew around Al₂O₃. The use of fine Al₂O₃ powder and a high firing temperature were effective in the promotion of ZnAl₂O₄ formation in the presence of LiF.     

Zone-melted unidirectional solidification of particulate dispersed composites

In order to solve the problem of particles settling and agglomeration in front of solidifying interface in unidirectional solidification (UDS) experiments, a zone-melted process has been utilized in this study. The experimental results show that, the melting zone could be kept in 30–40 mm width and the zone melted UDS experiments are realized with Al₂O₃ particle reinforced aluminum-matrix composites. But particle settling still occurs in the liquid, and becomes severe as the particle volume fraction decreases. However, when the volume fraction of the particles is more than 20–22 vol.%, no further settling occurs under a solidification rate of 8–16 mm/h. Investigation on the interaction of particles and solid/liquid interface reveals that the Al₂O₃ particles are rejected into liquid and pushed by the growing solid phase in Al₂O_3(P)/Al and Al₂O_3(P)/Al-0.23wt.%Ce composites. Some particles are mechanically entrapped between cells, and distributed along the crystal grain boundaries.     

Phase separation in tellurite glass-forming systems containing B2O3,GeO2,Fe2O3,MnO,CoO,NiO and CdO

The boundaries between the regions of single-phase and two-phase glasses were established in tellurite glass-forming systems containing B₂O₃ and one of the following oxides: GeO₂, Fe₂O₃,CoO, NiO, MnO and CdO. The character of the microstructures inside and outside the regions of stable phase separation were determined by electron microscopy. It was shown that the existing microheterogeneities may either result from incomplete liquid immiscibility during melting and supercooling or be due to typical metastable separation.     

Effect of a small amount of liquid-forming additives on the microstructure of Al2O3 ceramics

Sintering behaviour and microstructure of Al₂O₃ ceramics without additives and with 0.02–0.25 mol% CaO + SiO₂ (CaO/SiO₂ = 1) were investigated. When Al₂O₃ bodies were sintered at 1400 °C, the sinterability and the grain size decreased as the content of CaO + Si₂ increased. When Al₂O₃ ceramics with 0.05 – 0.25 mol% CaO + SiO₂ were sintered at higher sintering temperature, both CaO and SiO₂ reacted with Al₂O₃ to produce the liquid phase along grain boundaries, and exaggerated platelet Al₂O₃ grains, with an aspect ratio of about 4.5, were formed. Because the size of platelet grains decreased as the content of CaO + SiO₂ increased, the distribution of either SiO₂ particles or this intergranular phase of CaO – Al₂O₃ – SiO₂ might control the microstructure.     

Effect of Al2O3 particles on the growth kinetics of molybdenum grains in liquid nickel

Grain growth inhibition by inert third phase particles during liquid phase sintering has been investigated in Mo-Ni alloys containing Al₂O₃ particles sintered at 1460° C. The grain growth is inhibited slightly by the Al₂O₃ particles, and this result is explained in terms of the shielding of material precipitation at the contact area and of the suppression of material precipitation around the contact area, which is determined by the dihedral angle between the molybdenum grain and the Al₂O₃ particle. The result is compared to those obtained earlier with specimens containing highly soluble third phase particles.     

Study of the Ti/Al2O3 interface

The Ti/Al₂O₃ (1 ¯1 0 2) interface formation has been investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES). The results showed that when an active metal titanium was evaporated on to a room-temperature Al₂O₃ (1 ¯ 1 0 2) surface in ultrahigh vaccum, a Ti/Al₂O₃ interface region of about 200 nm was formed, and in the first several monolayers of titanium, the titanium was oxidized due to the active oxygen anions on the surface. Therefore, the pure Ti/Al₂O₃ interface was replaced gradually by a titanium oxides/Al₂O₃ interface, which has a stronger interaction than the former. The change of shape of the photoemission lines and the shift of binding energy of aluminium, oxygen and titanium with increasing coverage of titanium showed that the formation of the Ti-O bond at the interface is due to titanium transferring its electrons to Al³⁺ via O^2– anions in the Al-O bond, whereby the Al³⁺ was reduced to metallic aluminium, Al⁰. The AES intensity profile also proved the existence of the reduced species Al⁰. This suggests that the reaction layer consists of a multiphasic mixture: the Ti-O type phase, the (Ti, Al)₂O₃ phase and metallic aluminium phase.     

Novels patterns of hydration and new compounds in the ternary system of CaO-Al2O3-P2O5

 The patterns of hydrating and solidifying with the compositional variation of phosphorus-rich, phosphorus-calcium-rich and aluminum-calcium rich regions in ternary system CaO-Al₂O₃-P₂O₅ has been studied in detail, and two new ternary compounds L and H have been synthesized here. The results indicate that the region of 48–56% P₂O₅ doesn’t present cementitiousness, which contains mainly crystal phases of β-C₂P(2CaO·P₂O₅), α-C₃P(3CaO·P₂O₅) and AlPO₄; the phosphorus-calcium-rich region of 21–35% P₂O₅ exhibits substantial cementitiousness, which contains mainly crystal phase of α-C₃P and certain amount of CA(CaO·Al₂O₃) and new phases L/H; and the aluminum-calcium-rich region of 8–18% P₂O₅ is full of promise for cementitiousness. It contains mainly new crystal phase L and certain amount of α-C₃P and CA. The hydration and solidification mechanisms have been preliminarily analyzed by means of XRD, XPS and DTA. It appears that crystal phase CA might hydrate directly to the stable phase of C₃A·6H₂O in the phosphorus-rich case of 21–35% P₂O₅; new phase H has the behavior of rapid setting; and L, being a dominant phase, can prevent cement pastes from significant strength loss in long curing cycles. Received: 12 March 1998 / Accepted: 29 July 1998