Synthesis of Lead Titanate: Thermodynamic Modeling and Experimental Verification

A comprehensive thermodynamic model has been developed to determine the reaction conditions favoring hydro-thermal synthesis of lead titanate (PbTiO₃). The model combines standard-state thermodynamic data for solid and aqueous species and an excess Gibbs energy model to account for the nonideality of the solution. The method has been used to generate phase stability diagrams that indicate the ranges of pH and reagent concentrations for which various species predominate in the system at a given temperature and pressure. Also, yield diagrams have been constructed that indicate the concentration, pH, and temperature conditions for which different yields of crystalline PbTiO₃ can be obtained. The stability and yield diagrams have been used to predict the optimum synthesis conditions (e.g., reagent concentrations, pH, and temperature). Subsequently, these predictions have been experimentally verified. As a result, phase-pure perovskite PbTiO₃ has been obtained at temperatures ranging from 413 to 433 K using lead acetate or lead nitrate and commercial TiO₂ powder. Also, PbTiO₃ has been synthesized at lower temperatures (353 T 363 K) by using lead acetate and hydrous (reactive) TiO₂ and calcining the obtained amorphous product.     

Thermogravimetric Determination of Carbon, Nitrogen, and Oxygen in Aluminum Nitride

A thermogravimetric method for the measurement of carbon, nitrogen, and oxygen in aluminum nitride is described. Since carbon oxidation occurs at lower temperatures than the oxidation of the aluminum compounds, thermogravimetric analyses in the respective temperature regimes can yield measurement of the oxygen, nitrogen, and carbon content. Thermogravimetric analyses were performed on mixtures of aluminum nitride and carbon with various concentrations in order to evaluate the accuracy of this method. The results were compared with those obtained by the hot gas extraction method. Errors produced by overlapping of the temperature ranges of the reactions were evaluated and experimental conditions were optimized to minimize error. This method can produce reliable results at elemental concentrations higher than a few percent. The absolute errors of the C/AI, N/Al, and O/Al atomic ratios were less then 0.02. The detection limit was evaluated to 0.2 wt% of each element. The applicability of the method to the study of the kinetics of the nitridation reaction was demonstrated.     

The role of ammonium citrate washing on the characteristics of mechanochemical–hydrothermal derived magnesium-containing apatites

The role of citrate washing on the physical and chemical characteristics of magnesium-substituted apatites (HAMgs) was performed. HAMgs were synthesized by a mechanochemical–hydrothermal route at room temperature in as little as 1 h, which is five times faster than our previous work. Magnesium-substituted apatites had concentrations as high as 17.6 wt% Mg with a corresponding specific surface area (SSA) of 216 m²/g. A systematic study was performed to examine the influence of increasing magnesium content on the physical and chemical characteristics of the reaction products. As the magnesium content increased from 0 to 17.6 wt%, magnesium-doped apatite crystallite size decreased from 12 to 8.8 nm. The Mg/(Mg + Ca) ratio in the product was enriched relative to that used for the reacting precursor solution. During mechanochemical–hydrothermal reaction, magnesium doped apatites co-crystallize with magnesium hydroxide. Citrate washing serves to remove the magnesium hydroxide phase. The concomitant increase in surface area results because of the removal of this phase. Possible mechanisms for magnesium hydroxide leaching are discussed to explain the measured trends.     

Pyrolysis of Titanium-Metal-Filled Poly(siloxane) Preceramic Polymers: Effect of Atmosphere on Pyrolysis Product Chemistry

Pyrolysis of unfilled as well as Ti-metal-filled poly(siloxane) preceramic polymers, heated at 1350^deg;C in N₂, Ar, or NH₃, was evaluated in terms of the effect of atmosphere on pyrolysis product chemistry. Pyrolysis of the unfilled poly(siloxane) in N₂ or Ar resulted in the formation of a metastable, amorphous SiO_xC_y phase with a residual turbostratic carbon phase, with the evolution of CH₄ and H₂ as the main pyrolysis products. However, pyrolysis of the unfilled poly (siloxane) in NH₃ resulted in a partial substitution of nitrogen from the atmosphere for network carbon to form a binary mixture of amorphous SiO_xC_y and SiO_xN_y phases. Pyrolysis of the Ti-filled poly(siloxane) in NH₃ resulted in the reaction of the Ti particles with the atmosphere to form nearly stoichiometric TiN. In Ar, the Ti particles react with either gaseous hydrocarbon, i.e., CH₄, or the carbon pyrolysis products of the poly(siloxane), to form slightly nonstoichiometric TiC along with a partial reduction of the SiO_xC_y matrix. Finally, in N₂, the Ti particles react with both carbon from the poly(siloxane) and nitrogen from the atmosphere to form a solid solution of TiC and TiN.     

An Approach Towards Amelioration of an Efficient VM Allocation Policy in Cloud Computing Domain

Cloud computing is on the horizon of the domain of information technology over the recent few years, giving different remotely accessible services to the cloud users. The quality-of-service (QoS) maintaining of a cloud service provider is the most dominating research issue today. The QoS embraces with different issues like virtual machine (VM) allocation, optimization of response time and throughput, utilizing processing capability, load balancing etc. VM allocation policy deals with the allocation of VMs to the hosts in different datacenters. This paper highlights a new VM allocation policy that distributes the load of VMs among hosts which improves the utilization of hosts’ processing capability as well as makespan and throughput of cloud system. The experimental results are obtained by utilizing trace based simulation in CloudSim 3.0.3 and compared with existing VM allocation policies.     

Solvothermal Synthesis of Acmite Conversion Coatings on Steel

Acmite (NaFeSi₂O₆) films were formed on steel coupons via solvothermal reaction of silica, sodium hydroxide, and 1, 4‐butanediol in an autoclave under autogenous pressure. Systematic variation in processing variables led to homogenous coatings comprised of pinacoidal acmite grains with an average grain size of ~33 μm. The coatings were produced on the steel coupons from reactant conditions of 0.635 m SiO₂, 2.546 m NaOH, and 3.087 m 1,4‐butanediol for 72 h at 240°C.     

A New Glycothermal Process for Barium Titanate Nanoparticle Synthesis

Barium titanate (BaTiO₃) nanoparticles were synthesized at the low temperature of 80°C through a glycothermal reaction using Ba(OH)₂·8H₂O and amorphous titanium hydrous gel as precursors and a solution of 1,4-butanediol and water as solvent. This processing method provides a simple low-temperature route for producing BaTiO₃ nanoparticles, which could also be extended to other systems. It is demonstrated that the size of BaTiO₃ particles can be controlled by reaction conditions, such as reaction temperature and various volume ratios of 1,4-butanediol/water.     

Lithium aluminum-layered double hydroxide chlorides (LDH): Formation enthalpies and energetics for lithium ion capture

Layered aluminum double hydroxide chloride sorbents, LiCl∙Al₂(OH)₆.nH₂O, Li-LDH, have shown promising application in selective Li extraction from geothermal brines. Maintaining LiCl uptake capacity and retaining a long cycle life are critical to widespread application of sorbent materials. To elucidate the energetics of Li capture, enthalpies of LDH with different Li content have been measured by acid solution calorimetry. The formation enthalpies generally become less exothermic as the Li content increases, which indicates that Li intercalation destabilizes the structure, and the enthalpies seem to approach a limit after the Li content x = 2Li/Al exceeds 1. To improve stability, metal doping of the aluminum LDH structure with iron was performed. Introduction of a metal with greater electron density but a similar ionic radius was postulated to improve the stability of the LDH crystal structure. The calorimetric results from Fe-doped LDH samples corroborate this as they are more exothermic than LDH-lacking Fe. This suggests that Fe doping is an effective way to stabilize the LDH phase.     

Thermochemistry of stoichiometric rare earth oxyfluorides REOF

Rare earth oxyfluorides (REOF) have potential applications in luminescent devices and energy storage and are important for synthesis and properties of RE-doped oxyfluoride glasses. This study evaluates the thermodynamic properties of stoichiometric REOF with varying RE (RE = La, Nd, Gd, Ho, Y, and Yb) elements using high temperature oxide melt solution calorimetry and investigates the phase transitions of YbOF by differential scanning calorimetry. The formation enthalpies from binary oxides and fluorides are exothermic and become less exothermic with the decrease of ionic radius of RE³⁺, showing that REOF compounds are energetically stable relative to equal molar mixtures of RE₂O₃ and REF₃ and the RE–O interactions contribute to most of their thermodynamic stabilities. Different from REOF with large RE ions (La–Er and Y) having the rhombohedral phase, REOF with small cations (e.g., Yb) possesses the monoclinic structure as the more energetically stable phase at room temperature. However, the slow transition between the monoclinic and cubic phases easily generates the rhombohedral phase, which is favored by kinetics.