Preparation of ceramics by hydrothermal hot-pressing

Abstract

The densification mechanism of borosilicate glass, silica gel and amorphous titania powders by hydrothermal hot-pressing is described. The glass powders were densified by a viscous flow mechanism, and fully dedfied compacts were obtained by hydrothermal hot-pressing. On the other hand, porous ceramics were produced from silica gel and amorphous titania. The pore size distribution of these ceramics could be controlled by hydrothermal hot-pressing conditions. In the case of silica gel, it remained amorphous, but amorphous titaaia was crystallized to anatase by hydrothermal hot-pressing. The pore diameter and mechanical strength of the compacts prepared from silica gel increased with reaction time. The densification of the amorphous titania was improved by increasing temperature and pressure.     

Hydrothermal hot pressing for recycling of waste glass

Abstract

Recycling of waste glasses has been conducted by a hydrothermal hot-pressing method to produce high density compacts. Our research was focussed on the intluence of various reaction variables (temperature, time, water content and loading pressure) during compaction of several types of waste glasses. The compaction of all glasses starts around 120°C and proceeds rapidly with increasing temperature. Under hydrothermal hot pressing conditions, the compaction of the waste glass is most strongly controlled by temperature and water content, and affected to a lesser degree by reaction time and loading pressure. A maximum tensile strength of 65 MPa was obtained for ‘blue’ glass at 200°C, 10wt% water content, after applying 60 MPa loading pressure for 2 hours.     

热压工艺参数对n型和p型Bi2Te3基赝三元热电材料电学性能的影响

通过熔炼/研磨/热压方法制备了n型和p型赝三元Bi₂Te₃基的热压合 金样品，测量了由不同工艺参数(热压温度、热压压力)制备的样品Seebeck系数和电导率.分析了热压参数对热电性能产生的影响.特别是发现了增加热压压力和热压温度会使n型和p型热压样品的Seebeck系数和电导率都有所提高，这与单晶和取向晶体材料的Seebeck系数和电导率变化趋势相反的规律显然不同，其结果对热压样品的电学性能提高有积极的影响. 关键词： 热电材料 热压 Seebeck系数 工艺参数     

Development of porous silica production by hydrothermal method

Abstract

A new process for porous silica production has been developed using a hydrothermal method. Hydrothermally synthesized calcium silicate was used as the starting material in this study, which was produced from a mixture of Ca(OH)₂ and amorphous silica (white carbon) under hydrothermal conditions of 140°C and 0.4 MPa, for 8 hours. The calcium silicate was subsequently treated with an acid solution, facilitating the leaching of Ca ions. After washing with pure water, the multant Ca²⁺ -free silica powder was allowed to dry. The Ca²⁺ -free silica powder was found to have an amorphous structure, with 0.9 ml/g pore volume, up to 610m²/g BET specific surface area, and an average 5 ～ 8 nm pore size. Our hydrothermal process is simple and low cost, and is anticipated to have numerous applications to the petrochemical industry.     

Morphology control of ATiO3 (A=Ba,Sr, Ca) by hydrothermal soft chemical process

Abstract

A hydrothermal soft chemical process for controlling the morphology of ATiO₃ (A = Ba, Sr, Ca) perowkites is proposed, and used for the synthesis of fibrous particles. In this process. a fibrous hydrous tetratitanate (H₂Ti₄O₉ · nH₂O) which has a layered structure was used as the precursor, and hydrothermally treated with Ba(OH)₂, Sr(OH)₂and Ca(OH)₂ solutions. The hydrous tetratitanate was transformed to the perowkite structure, but retained the fibrous morphology after the hydrothermal reactions.     

Synthesis of lithium-manganese oxide spinel in hydrothermal conditions

Abstract

Lithium-manganese oxide spinel LiMn₂O₄ was synthesized in hydrothermal conditions (400°C, 20 MPa) in the course of thermovaporous treatment mixtures of MnO₂ and LiOH/or Li₂CO₃. The conditions of synthesis of the spinel as monophase product were determined. The obtained product has been characterized by means of various physical and chemical methods. The spinel has been used for manufacturing cathodes of rechargeable lithium cells. The cells discharged in the potential range 2.8-3.5 V. The specific capacity was 100-140mAh/g.     

Data for scaling-up hydrothermal waste water treatment process

Abstract

Hydrothermal oxidation of wastes is developed as an alternative technique in order to limit the toxic end-product formation, the waste volume and the energy supply. We are now working on the transfer of this technology, and so on the determination of data for scaling-up the hydrothermal oxidation process. The main data concern the knowledge of reactive pathway, the determination of reaction kinetic and thermal parameter. Kinetic and thermal data have been determined for the hydrothermal oxidation of acetic acid as well as the reactive pathway of hydrothermal oxidation of a C, H, O, N compound, the fenuron (C₆H₅-NH-CO-N(CH₃)₂).     

Reductive hydrothermal synthesis of La(OH)<Subscript>3</Subscript>:Tb<Superscript>3+</Superscript> nanorods as a new green emitting phosphor

A reductive hydrothermal process with use of hydrazine hydrate as a protecting agent is proposed to synthesize La(OH)₃:Tb³⁺ (Tb mol% = 0, 1, 5, 10, and 20) nanorods. The oxidation of Tb³⁺−Tb⁴⁺ was effectively prevented in the presence of hydrazine hydrate; hence the La(OH)₃:Tb³⁺ nanorods exhibited much stronger green photoluminescence than the product prepared by the normal hydrothermal process. X-ray diffraction and transmission electron microscopy were employed to characterize the products, the results of which revealed that all the products were one-dimensional rod-like nanostructures of hexagonal structure (∼20 nm in diameter). The reductive hydrothermal process is desirable for the synthesis of other efficient Tb³⁺ doped nanophosphers.     

Hydrothermal synthesis of spinel Li1+x Mn2O4 as cathode material for rechargeable lithium battery

Abstract

The hydrothermal synthesis of Li-Mn spinel oxide (Li_1+xMn₂O₄) was undertaken in order to develop high quality, low cost cathode material for a rechargeable lithium battery. In our experiments, γ-MnOOH, LiOH · H₂O and H₂O₂ were used as starting materials to synthesize Li-Mn spinel oxide under hydrothermal conditions of 180-230°C and about 1.0-2.8 MPa. The chemical composition and particle size of the Li_1+xMn₂O₄ is easily controlled in the hydrothermal reaction. The Li_1+xMn₂O₄ produced was characterized by X-ray diffraction, with the spinel phase having a Li/Mn ratio of 0.50-0.60. There is convincing evidence, as a result of this work, that our synthesis process is most suitable for producing high quality cathode material that can be used in a rechargeable lithium battery.     

The use of the high pressure hydrothermal method for tailored synthesis of zeolites without structure directing agents. Instance: Synthesis of natural zeolites with 5-1 building units

Abstract

The synthetic counterparts of natural alumosilicate zeolites containing 5-1 structural building units have been synthesized by simulating the natural hydrothermal formation conditions. Synthetic glasses of the respective zeolite composition were used as starting materials, while distilled water under hydrothermal conditions was used as pressure and reaction medium. No structure directing agents, additional solutions or sol-gel material has been employed.

The experience of former investigations on the tailored synthesis of alumosilicate zeolites showed, that 220°C is the expected temperature of maximum gain. Synthesis time was always 60 d. Synthesis pressure for mazzite (MAZ), mordenite (MOR), fer-rierite (FER), epistilbite (EPI) and dachiardite PAC) was 1 kbar while bikitaite (BIK) could only be synthesized at 2 kbar pressure.

The results of the investigations on hand show, that this process can be applied for the tailored synthesis of zeolites with completely new properties (e.g., semiconducting frameworks).     

Growth and piezoelectric characterisation of GeO2 single crystals with quartz structure

Abstract

The first results for growth of germanium dioxide single crystals with the crquartz type structure (α-GeO₂) from refluxed hydrothermal solutions are reported. The success of this method is due to the existence of metastable α-GeO₂ in aqueous solutions at relatively low temperature and to the structural similarity between α-GeO₂ and α-SiO₂. The continuous partial evaporation of the heated solvent, vapour condensation, saturation of the condensate by nutrient α-GeO₂ and growth of α -GeO₂ single crystals on quartz seeds are considered as the successive stages of the process. Mzre homogeneous crystals of α -GeO₂ were obtained on seeds parallel to {0001}, {1120} and {1121} faces using sodium hydroxide and ammonium fluoride solutions in the range of temperature 90-130°C and at pressure equivalent to the fugacity of saturated vapor.

Piezoelectric, elastic and dielectric properties of the mentioned above crystals for the first time were studied.     

A novel method to synthesize homogeneous and mono-dispersible CeO<Subscript>2</Subscript> nanospheres: two-step hydrothermal process

In this study, homogeneous mono-disperse CeO₂ nanosphere was successfully synthesized through two-step hydrothermal process, and the degree of sphericity was also improved comparing with the sample synthesized through one-step hydrothermal process. In the whole reaction, crystallization and ripening were regarded as two independent stages. Upper-layer transparent solution obtained through centrifugation after first-step hydrothermal process contained a large number of CeO₂ crystal seeds, indicating that crystallization has taken place in this stage. Then, these seeds participated in the second hydrothermal process and ripened into homogeneous spheres finally. In addition, poly(vinylpyrrolidone) was applied as surfactant to facilitate the oriented aggregation of small CeO₂ nanoparticles into nanospheres. The formation mechanism was discussed through scanning electron microscopy and transmission electron microscopy.     

Co3O4-ZnO hierarchical nanostructures by electrospinning and hydrothermal methods

A new hierarchical nanostructure that consists of cobalt oxide (Co₃O₄) and zinc oxide (ZnO) was produced by the electrospinning process followed by a hydrothermal technique. First, electrospinning of a colloidal solution that consisted of zinc nanoparticles, cobalt acetate tetrahydrate and poly(vinyl alcohol) was performed to produce polymeric nanofibers embedding solid nanoparticles. Calcination of the obtained electrospun nanofiber mats in air at 600 °C for 1 h, produced Co₃O₄ nanofibers with rough surfaces containing ZnO nanoparticles (i.e., ZnO-doped Co₃O₄ nanofibers). The rough surfaced nanofibers, containing ZnO nanoparticles (ZnNPs), were then exploited as seeds to produce ZnO nanobranches using a specific hydrothermal technique. Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to characterize the as-spun nanofibers and the calcined product. X-ray powder diffractometery (XRD) analysis was used to study the chemical composition and the crystallographic structure.     

Low-frequency Raman characterization of size-controlled anatase TiO<Subscript>2</Subscript> nanopowders prepared by continuous hydrothermal syntheses

The advantages of a recently presented continuous hydrothermal elaboration route for size-controlled anatase TiO₂ nanoparticles are investigated. Nanopowders prepared by this route and using a soft chemistry route are characterized using X-ray diffraction, surface area measurements, high-resolution electron microscopy and Raman spectroscopy with an emphasis on the determination of the size distribution with low-frequency Raman measurements. The continuous hydrothermal route is shown to be more suitable for producing narrower size distributions.     

Study on preparation and properties of molybdenum alloys reinforced by nano-sized ZrO<Subscript>2</Subscript> particles

The nano-sized ZrO₂-reinforced Mo alloy was prepared by a hydrothermal method and a subsequent powder metallurgy process. During the hydrothermal process, the nano-sized ZrO₂ particles were added into the Mo powder via the hydrothermal synthesis. The grain size of Mo powder decreases obviously with the addition of ZrO₂ particles, and the fine-grain sintered structure is obtained correspondingly due to hereditation. In addition to a few of nano-sized ZrO₂ particles in grain boundaries or sub-boundaries, most are dispersed in grains. The tensile strength and yield strength have been increased by 32.33 and 53.76 %.     

A novel determination of activation quantities and reaction order(s) for the hydrothermal destruction and its applications

Abstract

A novel determination method of activation quantities and reaction order(s) of hydro-thermal reactions was developed in order to achieve economically, easily and speedily research and development of new hydrothermal plants. This method is called the NIT method (abbreviation of ‘Nonisothemal method’) in the case of a batch reactor and the F-NIT method (abbreviation of ‘nonisothermal method for flow reactor’) in the case of a plugged flow reactor. The main merit of the NIT method (or the F-NIT method) is to give access to the determination of activation quantities and/or reaction orders by measuring the changes in amount of chemical species under nonisothermal conditions.

After summary of the recently published NIT method, the newly F-NIT method is described briefly. Then the two types of hydrothermal fluids (hydrothermal steam and supercritical/subcritical water) are applied to the destruction of phenol under nonisothermal conditions. The activation quantities can be obtained at once by use of these methods and are compared with the reported values in the scWO.     

Hydrothermal etching as a method of quartz quality control of density and distribution of dislocations

Abstract

In this paper the effective non-destructive method of hydrothermal etching of crystals is considered to carry out estimation of density and place of dislocations in large scaled crystals of synthetic quartz. In this process of etching there are specific etch figures which appear as faceted holes of a millimetre up to several millimetres in size on all quartz facets. The size of holes depends on concentration of solution, duration of the process of etching and degree of solution saturation by silicon dioxide. The configuration of the holes on the facets of different indexes is various. Pits of symmetry, having facets, correspond to each facet there. Zcrystals are basic in quartz production. In sectional operation the expedients of defection of dislocations in pits generated on a facet are considered (0001).     

Morphology-control of VO2 (B) nanostructures in hydrothermal synthesis and their field emission properties

VO₂ (B) nanostructures were synthesized via a facile hydrothermal process using V₂O₅ as source material and oxalic acid as reductant. Three nanostructures of nanorods, nanocarambolas and nanobundles were found existing in the products, and a continuous changing of morphology was found in the synthesis process, during which the proportion of these three types of nanostructures can be adjusted by altering the concentrations of oxalic acid. The microstructures were evaluated using X-ray diffraction and scanning and transmission electron microscopies, respectively. FE properties measurement of these three types of nanostructures showed that the nanobundles have the best field emission performance with a turn-on field of ∼1.4 V/μm and a threshold field of ∼5.38 V/μm. These characteristics make VO₂ (B) nanostructures a competitive cathode material in field emission devices.     

Massive spin-momentum entanglement measured in an atomic beam spin echo experiment

In this paper we present an experiment performed with an atomic beam spin echo interferometer, in which massive intraparticle entanglement is demonstrated. In the longitudinal Stern-Gerlach arrangement the nuclear spin and linear momentum of ³He particles are inextricably linked, such that the overall system state cannot be written as the tensor product of the corresponding Hilbert spaces. The measured data show maximal entanglement between ℋ _I and ℋ _p . This hybrid system of one quantum and one classical degree of freedom is a textbook example of entanglement between discrete and continuous observables.     

Characteristics of LiFePO4 obtained through a one step continuous hydrothermal synthesis process working in supercritical water

The olivine-like material LiFePO₄ was prepared via a continuous hydrothermal synthesis process working from subcritical to supercritical water conditions. The influence of some processing parameters–temperature and reaction time–was investigated in terms of material purity, grain size and morphology. Supercritical conditions were found to be attractive to synthesize in one step a well-crystallized material without impurities. The primary particles size was in the nanometric range. They showed a natural tendency to form micron size agglomerates, which were supposed to be the cause of the limited capacity, as demonstrated through a cross study using laser particle size distribution analysis, electrochemical measurements and XRD at different Li contents.