Hydrothermal Hot-pressing Process Research Articles

Microstructural development of MnCO3 microsphere compacts through hydrothermal hot-pressing

Low-temperature sintering is a highly effective method for solidifying powders of thermally degradable materials, such as carbonates and hydroxides. Hydrothermal hot-pressing (HHP) is a low-temperature sintering method that uses a dissolution-precipitation reaction to water under hydrothermal conditions. Most sintering techniques, including HHP, were devised to fabricate high-density compacts; hence, varying the internal microstructure during solidification is challenging. The present study focuses on the microstructural development of MnCO3 microsphere compacts through HHP. Depending on the HHP process temperature, the formation of MnOOH fibers and Mn3O4 nanoparticles within the MnCO3 compacts or hollowing of the microspheres during the compaction was observed. The direct production of microstructure-controlled compacts by HHP will contribute to new structural designs and add functionalities to existing applications.

Synthesis of bulk lanthanum polyphosphate and other rare earth phosphates through hydrothermal hot-pressing

Bulk samples of lanthanum polyphosphate were synthesized through a hydrothermal hot-pressing (HHP) process. In this process, pressing temperature, pressure and volume of water were varied in order to improve the density and strength of the resulting materials. The strength of the bulk samples was estimated through drilling and ultrasonic treatments. In order to improve the strength of the materials, the use of microwave irradiation was examined. Lanthanum polyphosphate formed porous bulk samples with a filling factor of approximately 70%, which was calculated from real and theoretical densities. With respect to machinable strength, a drilled hole greater than 7.0 mm in diameter was obtained on some bulk samples, and the diameter of the samples was 14 mm. The HHP process is a useful method for obtaining bulk samples of lanthanum polyphosphate. Bulk lanthanum polyphosphate containing water crumbled easily to a powder form upon ultrasonication. However, these bulk samples retained their shape upon ultrasonication, despite containing water, after exposure to microwave irradiation, and also experienced minimal weight loss. Furthermore, to study the effect of microwave heating, bulk lanthanum orthophosphate, yttrium orthophosphate and polyphosphate were also examined.

Splitting Tensile Strength of Fly Ash Activated by Hydrothermal Hot-Pressing Process

Fly ash is normally solidified by strong alkaline solutions at room or moderate temperatures. In this study, an innovative process of recycling fly ash (both Class F and Class C) into potential construction materials with superior mechanical properties is investigated. This innovative process involves chemical activation, hydrothermal reaction (hence, heat and moisture), and hot pressing. The emphasis is on the synergistic effects. Several key parameters—such as treatment temperature, duration of process, and amount of chemical activator—are evaluated critically. The results confirm that fly ash can be solidified and specimens show splitting tensile strengths of about 1.0 MPa by hydrothermal hot pressing alone. With a small amount of chemical activator (NaOH), the tensile strength can reach as high as 5.4 MPa.

Hydrothermal hot-pressing induced phase transition in hexagonal boron nitride

The phase transition of hBN nanocrystals induced by hydrothermal hot-pressing process has been investigated by XRD, FTIR, TEM and HRTEM. It was found that a phase transition of hBN → tBN → aBN occurred with increasing hot-pressing temperature, i.e., hBN transformed into tBN at above 270 °C, and followed by another transformation from tBN to aBN at 310 °C. In addition, FTIR spectra and HRTEM images indicate that a small amount of cBN formed directly from the amorphous BN matrix at 75 MPa and 310 °C. This phenomenon is similar to what happened in conventional high temperature and high pressure method, which is believed to promote the phase transition from hBN to cBN.

Reaction of Hexagonal Boron Nitride Nano-crystals under Mild Hydrothermal Conditions

The reaction between hexagonal boron nitride (hBN) nano-crystals and water at low temperature and low pressure has been investigated. The results reveal that this reaction can be greatly promoted by increasing the hot-pressing temperature. However, when the temperature is above 280 °C, the reaction is too fast to be controlled by varying the hot-pressing pressure and time. On the other hand, stress and defects are induced in hBN nano-crystals by the hydrothermal hot-pressing process, resulting in a shift of the IR absorption bands and a deterioration of crystalline perfection. These results may be useful for synthesizing cBN by the hydrothermal method and converting hBN nanocrystals into cBN under moderate conditions.

Synthesis of Ca3Co4O9 ceramics by polymerized complex and hydrothermal hot-pressing processes and the investigation of its thermoelectric properties

Ca3Co4O9 ceramic powders were synthesized by the polymerized complex (PC) method and consolidated by the hydrothermal hot-pressing (HHP) technique. Observation by scanning electron microscope indicated that the form of powders by the PC process was fine and flaky. The density of the sintered body increased with an increase of the operating temperature and pressure during the HHP process. The electrical resistivity was greatly reduced with an increase of the operating pressure, but the thermal conductivity is prone to increase with an increase of the operating temperature and pressure. The Seebeck coefficient was hardly affected by the HHP condition. As a result, the sample treated by the HHP process under the condition of 573 K, 200 MPa, and 1 h showed a maximum ZT of about 0.29 at 1,073 K.

Transition from brittle to ductile behavior of fly ash using PVA fibers

In this study, short polyvinyl alcohol (PVA) fibers are employed to improve the mechanical properties of brittle fly ash samples, which are solidified by a small amount of sodium hydroxide (NaOH) and by a hydrothermal hot-pressing process. The mechanical properties of the PVA fiber reinforced fly ash composites are evaluated by splitting tensile test. Several key factors such as fiber content, processing temperature, and processing duration are investigated. The effect of different types of fly ash (Class C vs. Class F) is also studied. The experimental results demonstrate that the addition of short PVA fibers can significantly improve the ductility of the fly ash composites. The optimum fiber volume content is estimated to be 1.0% with Class C fly ash.

Synthesis of Ca<SUB>3</SUB>Co<SUB>4</SUB>O<SUB>9</SUB> Ceramics by Citric Acid Complex and Hydrothermal Hot-Pressing Processes and Investigation of Its Thermoelectric Properties

Ca 3 Co 4 O 9 ceramics powders were synthesized by the citric acid complex (CAC) method and consolidated by the hydrothermal hot-pressing (HHP) technique. The observation by the scanning electron microscope indicated that the powders obtained by the CAC process showed plate-like grains. The density and the Lotgering factor, which was estimated from the X-ray diffraction data, of the sintered body increased with an increase of the operating pressure during the HHP process. The electrical resistivity was much reduced with an increase of the operating pressure, but the Seebeck coefficient was hardly affected by the HHP conditions. As a result, the sample treated with the HHP under the condition of 573 K, 200 MPa and 3 h showed a maximum power factor.

Synthesis of Na xCo 2O 4 by the hydrothermal hot-pressing and its thermoelectric properties

Na x Co 2O 4 powders were synthesized by the citric acid complex (CAC) method and consolidated by the hydrothermal hot-pressing (HHP) technique. The measurement by the X-ray diffraction and the observation by the scanning electron microscope indicated that the sintered Na x Co 2O 4 ceramics with the plate-like grains aligned along the pressed plane can be obtained by the CAC and HHP methods, and the larger pressing pressure and higher treating temperature in the HHP process are favorable to obtaining the samples with high orientation. The relative density of the samples also increases with increasing pressure and temperature in the HHP process. The Seebeck coefficient ( S) and electrical resistivity ( ρ) of the samples treated by HHP are generally smaller than those of the sample untreated by HHP. As a result, the power factor ( S 2/ ρ) of the sample treated by HHP under 523 K and 200 MPa is slightly larger than that of the sample untreated by HHP.

Development of Recycling Technologies, and Functional Material Formation by Hydrothermal Processes

In order to establish a sustainable society, with practical material recycling, it is appropriate to focus on a range of guiding ``Earth principles,'' including the character and application of hydrothermal processes. In this paper, it is demonstrated how information obtained from the study of fundamental Earth principles can be used to inspire the development of new methods for material recycling, such as the following: 1) Hydrothermal hot-pressing processes, simulating the formation of sedimentary rock sequences, can be used for the solidification of toxic and hazardous materials. 2) Organic materials could be formed from CO2 under hydrothermal conditions (using Fe and Ni metals, and low valence Fe oxides), at temperature and pressure conditions consistent with a subduction (tectonic) setting, such as (under Japan) where the Pacific Plate sinks beneath the (Eurasian) continental plate. 3) Diamond-structured carbon may be formed from toxic chlorinated hydrocarbon, in very high pressure regions of the Earth, and at high alkaline hydrothermal conditions, where magma formation may occur. 4) High temperature dry steam in rock fractures (at near critical conditions -i.e. below saturated vapor pressure, but relatively high pressure) is not only a source/carrier of noble metals, including gold, silver and copper, but also ceramic materials, silica and alumina. Its laboratory simulation may also be a guide for the formation of thin layer silicate ceramics on metal plates (e.g. SUS-304 nickel alloy). 5) To design an underground boiler in hot dry rock, non-equilibrium dissolution and deposition hydrothermal processes may be studied using a tube reactor, which simulates fluid flow and temperature gradients in fractured hot rock.

Fabrication of colored CaCO3 compacts by hydrothermal hot-pressing method

Hydrothermal hot-pressing (HHP) technique was used to solidify shell powder dyed. Shell powder was mixed with dye with a ratio of 0.25 wt%, and was treated by HHP at 130 to 170° C, 45 MPa for 2 hours. It is showed that the powder can be dyed well by some basic dyes at 60° C, and can maintain the color of dyes after HHP process. The resulting compactness with a beautiful color appearance has high hardness and apparent density. Possible mechanisms for solidification and coloration were also discussed.