Subsolidus equilibria and metastable phase development in the system ZrO₂-Al₂O₃-SiO₂

Subsolidus equilibria in the low silica portion of the system ZrO₂-Al₂O₃-SiO₂ were studied by X-ray diffraction of quenched samples fired at 1300°, 1400°, and 1480°C. Samples prepared as powders from combinations of a-Al₂O₃, Al(OH)₃, silicic acid, ZrO₂, and kaolinite reacted very slowly at 1300° and 1400°C and the trends toward equilibria are not apparent. A mixture of zircon and Al(OH)₃ reacted slowly at 1480°C to form mullite and ZrO₂, indicating that the latter phases are the stable assemblage rather than zircon and alumina. A gel prepared from aluminum nitrate, zirconyl nitrate, and colloidal silica reacted at 1480°C to form alumina, zirconia, zircon, and mullite; with continued firing, however, the zirconia and mullite developed further at the expense of aluminia and zircon. A tetragonal, pseudocubic, metastable phase developed in the calcined gel and was quite persistent, even at 1480°C. Hypothetical ternary liquidus surfaces have been proposed on the basis of available binary data from the literature --Abstract, page ii

Effect Of Cationic Charging Agent On The Bonding Strength Of Coarse Titanium Particles Deposited By Electrophoretic Deposition

Electrophoretic deposition (EPD) is a potential coating technique for surface hardening of steel when combined with a subsequent rapid sintering process. This process requires synergy between suspension particles and charging agent, particularly when the particles involved are noncolloidal in nature. The present work will investigate the effect of three commercially-available cationic charging agents; aluminium (III) chloride (AlCl3), polyethyleneimine (PEI) and poly(diallyldimethylammonium chloride) (PDADMAC) on the EPD of coarse Ti particles onto steel. The obtained Ti coatings were characterized by their surface microstructure, deposit yield, electrophoretic mobility and electrical conductivity. The key finding of the present study is the bonding strength of charging agent-adsorbed coarse Ti particles deposits predominantly controlled their deposit yield. Electrophoretic mobility of the Ti particles only played a lesser role in the deposit yield because of strong hindrance of gravity on the moving coarse particles. Charging agent, which gave the strongest to the weakest bonding strength is as follow: AlCl3, PDADMAC (Mw = 100,000 -200,000 amu), PDADMAC (Mw = 400,000 -500,000 amu), PEI

Electrophoretic Mobilities Of Dissolved Polyelectrolyte Charging Agent And Suspended Non-Colloidal Titanium During Electrophoretic Deposition

Coarse (<= 20 microns) titanium particles were deposited on low-carbon steel substrates by cathodic electrophoretic deposition (EPD) with ethanol as suspension medium and poly (diallyldimethylammonium chloride) (PDADMAC) as polymeric charging agent. Preliminary data on the electrophoretic mobilities and electrical conductivities on the suspensions of these soft particles as well as the solutions themselves as a function of PDADMAC level were used as the basis for the investigation of the EPD parameters in terms of the deposition yield as a function of five experimental parameters: (a) PDADMAC addition level, (b) solids loading, (c) deposition time, (d) applied voltage, and (e) electrode separation. These data were supported by particle sizing by laser diffraction and deposit surface morphology by scanning electron microscopy (SEM). The preceding data demonstrated that Ti particles of 1-20 microns size, electrosterically modified by the PDADMAC charging agent, acted effectively as colloidal particles during EPD. Owing to the non-colloidal nature of the particles and the stabilization of the Ti particles by electrosteric forces, the relevance of the zeta potential is questionable, so the more fundamental parameter of electrophoretic mobility was used. A key finding from the present work is the importance of assessing the electrophoretic mobilities of both the suspensions and solutions since the latter, which normally is overlooked, plays a critical role in the ability to interpret the results meaningfully. Further, algebraic uncoupling of these data plus determination of the deposit yield as a function of charging agent addition allow discrimination between the three main mechanistic stages of the electrokinetics of the process, which are: (1) surface saturation; (2) compression of the diffuse layer, growth of polymer-rich layer, and/or competition between the mobility of Ti and PDADMAC; and (3) little or no decrease in electrophoretic mobility of Ti, establishment of polymer-rich layer, and/or dominance of the mobility of the PDADMAC over that of Ti

Enhancement of Ce/Cr Codopant Solubility and Chemical Homogeneity in TiO 2 Nanoparticles through Sol–Gel versus Pechini Syntheses

International audienceThe Pechini method was more effective in fabricating Ce/Cr codoped TiO2 nanoparticles of superior photocatalytic properties in comparison with the sol-gel method. The improved efficiency was attributed to: a) formation of mixed-phase anatase + rutile with greater homogeneity of dopant incorporation, b) development of open agglomerated morphologies with significantly higher surface areas, and c) favourable valence changes (increased Ti 3+ concentrations) owing to electron exchange in inter valence and/or multi valence charge transfer reactions. Chen, Wen-Fan, et al. "Enhancement of Ce/Cr Codopant Solubility and Abstract Ce/Cr codoped TiO2 nanoparticles were synthesized using sol-gel and Pechini methods with heat treatment at 400°C for 4 h. A conventional sol-gel process produced well crystallized anatase while Pechini synthesis yielded less ordered mixed-phase anatase + rutile; this suggests that the latter method enhances Ce solubility, increases chemical homogeneity, but destabilizes the TiO2 lattice. Greater structural disruption from the decomposition of the Pechini precursor formed more open agglomerated morphologies while the lower levels of structural disruption from pyrolysis of the dried sol-gel precursor resulted in denser agglomerates of lower surface areas. Codoping and associated destabilization of the lattice reduced the binding energies in both powders. Cr 4+ formation in sol-gel powders and Cr 6+ formation in Pechini powders suggest that these valence changes derive from synergistic electron exchange from intervalence and/or multivalence charge transfer. Since Ce is too large to allow either substitutional or interstitial solid solubility, the concept of integrated solubility is introduced, in which the Ti site and an adjacent interstice are occupied by the large Ce ion. The photocatalytic performance data show that codoping was detrimental owing to the effects of reduced crystallinity from lattice destabilization and surface area. Two regimes of mechanistic behavior are seen, which are attributed to the unsaturated solid solutions at lower codopant levels and supersaturated solid solutions at higher levels. The present work demonstrates that the Pechini method offers a processing technique that is superior to sol-gel because the former facilitates solid solubility and consequent chemical homogeneity

Can Geopolymer Materials Be Optimized for Sustainable Building Envelope Applications? A Preliminary Development and Characterization Study

The materials used in the construction industry have a fundamental role in the development and achievement of low-carbon structures. This research aims to develop and characterize innovative and affordable renewable materials that can meet these low carbon requirements for building envelopes and internal element applications. These materials are geopolymers, which are being researched for fire-resistance, nuclear storage, and cement-based structural applications. Geopolymers are commonly fabricated from high-volume waste materials (e.g., fly ash and blast furnace slag) that are stored in landfill and tailings ponds and have the capacity to facilitate the transition toward a more sustainable and energy-efficient built environment while contributing to the circular economy in the building sector. Two geopolymers, a reference and a prototype, were fabricated and characterized in-lab for their thermal and optical properties to assess their performance for use as building envelope materials. Preliminary results suggest that the prototype has potential for use in green, lightweight building applications. Further optimization of the geopolymers&rsquo; thermal and optical performances will open new opportunities for the use of this material in buildings

Protonation Of The Polyethyleneimine And Titanium Particles And Their Effect On The Electrophoretic Mobility And Deposition

Proton activities of suspensions of Ti particles with added cationic polyelectrolyte as a function of acid additions have been investigated and compared in terms of the electrophoretic mobility and deposition yield. The proton activity in ethanol medium decreased with the addition of PEI polyelectrolyte and reduced further in the presence of Ti particles. The decrease in proton activity in the suspension indicates that protonation occurred on both the PEI molecules and Ti particles. It is proposed that the protonation of the amine groups of PEI and hydroxyl sites of Ti particle led to the formation of hydrogen bonding between the Ti particle and PEI molecules. Increase in the PEI and Ti with increasing acid addition translated to higher electrophoretic mobilities and deposition yield at low ranges of acetic acid addition (<0.75 vol%)

Synthesis and Structure-Chirality Relationship Analysis of Steroidal Quinoxalines to Design and Develop New Chiral Drugs

Of the utmost importance of chirality in organic compounds and drugs, the present work reports structure-chirality relationship of three steroidal quinoxalines, which were synthesised by condensing diaminobenzenes with cholestenone. All the compounds were purified and characterised by varying analytical tools prior to their chiroptical analysis by circular dichroism (CD) technique. The substituent groups on quinoxalines contributed to determining the chiroptical properties of the compounds. The positive Cotton effects have been observed in the CD spectra of unsubstituted and methyl-substituted quinoxalines, which indicated their P helicity. Importantly, chloro-substituent on quinoxalines produced different CD behaviour, which can be attributed to the presence of three lone pairs of electrons on Cl atom. The present work provides guidelines for determining the chiral properties of steroidal quinoxalines, which can be useful to design and develop potential molecules of biological importance

Long-Term Strength Evolution in Ambient-Cured Solid-Activator Geopolymer Compositions

The major downsides of cement manufacturing are the high CO2 emissions and high energy usage. Geopolymers, which are fabricated by activation of blends of fly ash (FA) and ground granulated blast furnace slag (GGBFS) using an alkaline activator, offer a promising solution to this issue. However, to enhance the replacement of cement in construction applications, geopolymer compositions have to be designed such that they can be activated on site by just adding water, similar to how cements are used. Therefore, the present work uses solid sodium metasilicate (MS, Na2SiO3) as the alkaline activator in order to design an add-water-style FA/GGBFS-based geopolymer composition. These compositions were designed by optimising the binder (FA/GGBFS) ratio, Na2SiO3/binder ratio, and water/binder ratio individually to assess the effects of these parameters on the setting times and mechanical (flexural and compressive) strengths over extended curing times (three months). The major factors affecting the strength development and setting times (initial and final) were the amounts of GGBFS and Na2SiO3, with the former demonstrating the more dominant effect. The consistent strength development with curing time was attributed to calcium aluminium silicate hydrate (CASH) gel formation in the early curing times which was affected by the slag addition levels, and sodium aluminium silicate hydrate (NASH) gel formation at later curing times which was influenced by the metasilicate addition levels. The metasilicate amounts were observed to impact on CASH gel formation in early stage curing. Geopolymer compositions with FA/GGBFS ratio of 35/65 and MS/water ratios of 0.2 showed high compressive strengths of ~70 MPa at 28 days, which are superior to values seen in conventional ordinary Portland cement (OPC) mixes for the same curing times

Long-Term Strength Evolution in Ambient-Cured Solid-Activator Geopolymer Compositions

The major downsides of cement manufacturing are the high CO2 emissions and high energy usage. Geopolymers, which are fabricated by activation of blends of fly ash (FA) and ground granulated blast furnace slag (GGBFS) using an alkaline activator, offer a promising solution to this issue. However, to enhance the replacement of cement in construction applications, geopolymer compositions have to be designed such that they can be activated on site by just adding water, similar to how cements are used. Therefore, the present work uses solid sodium metasilicate (MS, Na2SiO3) as the alkaline activator in order to design an add-water-style FA/GGBFS-based geopolymer composition. These compositions were designed by optimising the binder (FA/GGBFS) ratio, Na2SiO3/binder ratio, and water/binder ratio individually to assess the effects of these parameters on the setting times and mechanical (flexural and compressive) strengths over extended curing times (three months). The major factors affecting the strength development and setting times (initial and final) were the amounts of GGBFS and Na2SiO3, with the former demonstrating the more dominant effect. The consistent strength development with curing time was attributed to calcium aluminium silicate hydrate (CASH) gel formation in the early curing times which was affected by the slag addition levels, and sodium aluminium silicate hydrate (NASH) gel formation at later curing times which was influenced by the metasilicate addition levels. The metasilicate amounts were observed to impact on CASH gel formation in early stage curing. Geopolymer compositions with FA/GGBFS ratio of 35/65 and MS/water ratios of 0.2 showed high compressive strengths of ~70 MPa at 28 days, which are superior to values seen in conventional ordinary Portland cement (OPC) mixes for the same curing times