Hydrothermal technology for nanotechnology K. Byrappa a, * , T. Adschiri b a University of Mysore, DOS in Geology, P.B. 21, Manasagangotri P.O., Mysore-570 006, India b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980 8577, Japan Abstract The importance of hydrothermal technology in the preparation of nanomaterials has been discussed in detail with reference to the processing of advanced materials for nanotechnology. Hydrothermal technol- ogy in the 21st century is not just confined to the crystal growth or leaching of metals, but it is going to take a very broad shape covering several interdisciplinary branches of science. The role of supercritical water and supercritical fluids has been discussed with appropriate examples. The physical chemistry of hydrothermal processing of advanced materials and the instrumentation used in their preparation with re- spect to nanomaterials have been discussed. The synthesis of monodispersed nanoparticles of various metal oxides, metal sulphides, carbon nanoforms (including the carbon nanotubes), biomaterials, and some selected composites has been discussed. Recycling, waste treatment and alteration under hydrother- mal supercritical conditions have been highlighted. The authors have discussed the perspectives of hydro- thermal technology for the processing of advanced nanomaterials and composites. Ó 2007 Elsevier Ltd. All rights reserved. PACS: 82.Rx; 61.46.þw; 81.40.Àz; 81.10.Dn; 82.60.Lf; 35.Rh Keywords: A1. Nanostructures; A1. Morphology control; A2. Hydrothermal technology; A2. Solvothermal; A2. Super- critical fluid technology; A2. Nanoparticles fabrication 1. Introduction The hydrothermal technique is becoming one of the most important tools for advanced materials processing, particula rly owing to its advantages in the processing of nanostructural * Corresponding author. Tel.: þ91 821 2419720; fax: þ91 821 2515346. E-mail addresses: byrappak@yahoo.com (K. Byrappa), ajiri@tagen.tohoku.ac.jp (T. Adschiri). 0960-8974/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2007.04.001 Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 www.elsevier.com/locate/pcrysgrow materials for a wide variety of technological applications such as electronics, optoelectronics, catalysis, ceramics, magnetic data storage, biomedical, biophotonics, etc. The hydrothermal technique not only helps in processing monodispersed and highly homogeneous nanoparticles, but also acts as one of the most attractive techniques for processing nano-hybrid and nanocom- posite materials. The term ‘hydrothermal’ is purely of geological origin. It was first used by the British geologist, Sir Roderick Murchison (1792 e 1871) to describe the action of water at ele- vated temperature and pressure, in bringing about changes in the earth’s crust leading to the formation of various rocks and minerals. It is well known that the largest single crystal formed in nature (beryl crystal of >1000 g) and som e of the large quantity of single crystals created by man in one experimental run (quartz crystals of several 1000s of g) are both of hydrothermal origin. Hydrothermal processing can be defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions. Definition for the word hydrothermal has undergone several changes from the original Greek meaning of the words ‘hydros’ meaning water and ‘thermos’ meaning heat. Recently, Byrappa and Yoshimura define hydrothermal as any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-aqueous) above the room temperature and at pressure greater than 1 atm in a closed system [1]. However, there is still som e confusion with regard to the very usage of the term hydrothermal. For example, chemists prefer to use a term, viz. solvothermal, meaning any chemical reaction in the presence of a non-aqueous solvent or solvent in super- critical or near supercritical conditions. Similarly there are several other terms like glycother- mal, alcothermal, ammonothermal, and so on. Further, the chemists working in the supercritical region dealing with the materia ls synthesis, extraction, degradation, treatment, alteration, phase equilibria study, etc., prefer to use the term supercritical fluid technology.However,ifwe look into the history of hydrothermal research, the supercritical fluids were used to synthesize a variety of crystals and mineral species in the late 19th cent ury and the early 20th century itself [1]. So, a majority of researchers now firmly believe that supercritical fluid technology is nothing but an extension of the hydrothermal technique. Hence, here the authors use only the term hydrothermal throughout the text to describe all the heterogeneous chemical reactions taking place in a closed system in the presence of a solvent, whether it is aqueous or non-aqueous. The term advanced material is referred to a chemical substance whether organic or inorganic or mixed in composition possessing desired physical and chemical properties. In the current context the term materials processing is used in a very broad sense to cover all sets of technol- ogies and processes for a wide range of industrial sectors. Obviously, it refers to the preparation of materials with a desired application potential. Among various technologies available today in advanced materials processing, the hydrothermal technique occupies a unique place owing to its advantages over conventional technologies. It covers processes like hydrothermal synthesis, hydrothermal crystal growth leading to the preparation of fine to ultra fine crystals, bulk single crystals, hydrothermal transformation, hydrothermal sintering, hydrothermal decomposition, hydrothermal stabilization of structures, hydrothermal dehydration, hydrothermal extraction, hydrothermal treatment, hydrothermal phase equilibria, hydrothermal electrochemical reac- tions, hydrothermal recycling, hydrothermal microwave supported reactions, hydrothermal mechanochemical, hydrothermal sonochemical, hydrothermal electrochemical processes, hy- drothermal fabrication, hot pressing, hydrothermal metal reduction, hydrothermal leaching, hydrothermal corrosion, and so on. The hydrothermal processing of advanced materials has 118 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 lots of advantages and can be used to give high product purity and homogeneity, crystal sym- metry, metastable compounds with unique properties, narrow particle size distributions, a lower sintering temperature, a wide range of chemical compositions, single-step processes, dense sin- tered powders, sub-micron to nanoparticles with a narrow size distribution using simple equip- ment, lower energy requirements, fast reaction times, lowest residence time, as well as for the growth of crystals with polymorphic modifications, the growth of crystals with low to ultra low solubility, and a host of other applications. In the 21st century, hydrothermal technology, on the whole, will not be just limited to the crystal growth, or leaching of metals, but it is going to take a very broad shape covering several interdisciplinary branc hes of science. Therefore, it has to be viewed from a different perspec- tive. Further, the growing interest in enhancing the hydrothermal reaction kinetics using micro- wave, ultrasonic, mechanical, and electrochemical reactions will be distinct [2]. Also, the duration of experiments is being reduced at least by 3e4 orders of magnitude, which will in turn, make the technique more economic. With an ever-increasing demand for composite nano- structures, the hydrothermal technique offers a unique method for coating of various com- pounds on metals, polymers and ceramics as well as for the fabrication of powders or bulk ceramic bodies. It has now emerged as a frontline technology for the proce ssing of advanced materials for nanotechnology. On the whole, hydrothermal technology in the 21st century has altogether offered a new perspective which is illustrated in Fig. 1. It links all the important technologies like geotechnology, biotechnology, nanotechnology and advanced materials tech- nology. Thus it is clear that the hydrothermal processing of advanced materials is a highly in- terdisciplinary subject and the technique is popularly used by physicists, chemists, ceramists, hydrometallurgists, materials scientists, engineers, biologists, geologists, technologists, and Bio-Technology Advanced Materials Technology Geo-Technology Nano- Technology Hydrothermal Technology Fig. 1. Hydrothermal technology in the 21st century. 119K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 so on. Fig. 2 shows various branches of science either emerging from the hydrothermal tech- nique or closely linked with the hydrothermal technique. One could firmly say that this family tree will keep expanding its branches and roots in the years to come. The hydrothermal processing of materials is a part of solution processing and it can be described as super heated aqueous solution processing. Fig. 3 shows the PT map of various ma- terials processing techniques [3]. According to this, the hydrothermal processing of advanced materials can be considered as environmentally benign. Besides, for processing nanomaterials, the hydrothermal technique offers special advantages because of the highly controlled diffusiv- ity in a strong solvent media in a closed system. Nanomaterials require control over their phys- ico-chemical char acteristics, if they are to be used as functional materials. As the size is reduced to the nanometer range, the materials exhibit peculiar and interesting mechanical and physical properties: increased mechanical strength, enhanced diffusivity, higher specific heat and electrical resistivity compared to their conventional coarse grained counter-parts due to a quantization effect [4]. Hydrothermal technology as mentioned earlier in a strict sense also covers supercritical wa- ter or supercritical fluid technology, which is gaining momentum in the last 1½ decades owing to its enormous advantages in the yield and speed of production of nanoparticles and also in the disintegration, transformation, recycling and treatment of various substances including toxic or- ganics, waste s, etc. In case of supercritical water technology, water is used as the solvent in the Fig. 2. Hydrothermal tree showing different branches of science and technology. 120 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 system, whereas supercritical fluid technology is a general term when solvents like CO 2 and several other organic solvents are used, and because these solvents have lower critical temper- ature and pressure compared to water this greatly helps in proce ssing the materials at much lower temperature and pressure conditions. Hence, chemists use the term green chemistry for materials processing using supercritical fluid technology. K. Arai, T. Adschiri, M. Goto (all from Japan) and V.J. Krukonis, J. Watkins, P. Savage, T. Brill (USA), M. Poliakoff (UK), M. Perrut, F. Cansell (France), Buxing Han (China), K.P. Yoo and Y.W. Lee (South Ko- rea), etc., have done extensive studies in the area of supercritical fluid technology. Supercritical water (SCW) and supercritical fluids (SCF) provide an excellent reaction me- dium for hydrothermal processing of nanoparticles, since they allow varying the reaction rate and equilibrium by shifting the dielectric constant and solvent density with respect to pressure and temperature, thus giving higher reaction rates and smaller particles. The reaction products are to be stable in SCF leading to fine particle formation. The hydrothermal technique is ideal for the processing of very fine powders having high purity, controlled stoichiometry, high qual- ity, narrow particle size distribution, controlled morphology, uniformity, less defects, dense par- ticles, high crystallinity, excellent reproducibility, controlled microstructure, high reactivity with ease of sintering and so on. Further, the technique facilitates issues like energy saving, the use of larger volume equip- ment, better nucleation control, avoidance of pollution, higher dispersion, higher rates of reac- tion, better shape control, and lower temperature operations in the presence of the solvent. In nanotechnology, the hydrothermal technique has an edge over other materials processing tech- niques, since it is an ideal one for the processing of designer particulates. The term designer particulates refers to particles with high purity, high crystallinity, high quality, monodispersed and with controlled physical and chemical characteristics. Today such particles are in great de- mand in the industry. Fig. 4 shows the major differences in the products obtained by ball Fig. 3. Pressure temperature map of materials processing techniques [3]. 121K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 milling or sintering or firin g and by the hydrothermal method [5]. In this respect hydrothermal technology has witnessed a seminal progress in the last decade in processing a great variety of nanomaterials ranging from microelectronics to micro-ceramics and composites. Here the au- thors discuss the progress made in the area of hydrothermal technology for the past one decade in the processing of advanced nanomaterials. These materials, when put into proper use, will have a profound impact on our economy and society at least in the early part of 21st century, comparable to that of semiconductor technology, information technology or cellular and molec- ular biology. It is widely speculated that the nanotechnology will lead to the next industrial rev- olution [6]. Though it is widely believed that commercial nanotechnology is still in its infancy, the rate of technology enablement is increasing in no small part, as substantial government mandated funds have been directed toward nanotechnology [7,8]. It is strongly believed that hydrothermal technology has a great prospect especially with respect to nanotechnology research. 2. History of nanomaterial processing using hydrothermal technology Gold nanoparticles have been around since Roman times. As per the literature data, Michael Faraday was the first scientist to seriously experiment with gold nanoparticles starting in the 1850s. They have recently become the focus of researchers interested in their electrical and op- tical properties. Similarly, the history of hydrothermal processing of nanomaterials is very inter- esting. It must have begun in 1845, when Schafthaul prepared fine powders of sub-microscopic Fig. 4. Difference in particle processing by hydrothermal and conventional techniques [5]. 122 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 to nanosize quartz particles using a papin’s digester containing freshly precipitated silicic acid [9]. Majority of the early hydrothermal experiments carried out during the 1840s to the early 1900s mainly dealt with the nanocrystalline products, which were discarded as failures due to the lack of sophisticated electron microscopic techniques available during that time to observe such small sized products. Thus the whole focus was on the processing of bulk crystals or bulk materials. Many times when bulk crystals or single crystals were not obtained as products of several millimeter size the experiments were considered failures and the materials were washed away. Prior to X- ray techniques, chemical techniques were mainly employed in identifying the products. It was only after the application of X-rays for crystal studies that the researc hers slowly began to study the powder diffraction patterns of the resultant products and by the 1920s a systematic understanding of the products began. Before that the experiments were considered as failures. The experiments were concluded by stating that the solubility was not suitable for growing crystals. Until the works of Giorgio Spezia in 1900, hydrothermal technology did not gain much importance in the growth of bulk crystals, as the products in majority of the cases were very fine grained without any X-ray data [10]. Even the use of seeded growth was initiated by Spezia during that time. Morey [11] quotes in his classical work that the early hydrothermal experimenters used to have horrible experiences since sometimes experiments lasted for 3e6 months without any bearing on petrogenesis and phase equilibria, and ended up with very fine product whose status was not clear. The experiments were simply discarded as failures [11]. Gradually, from the late 1920s to the late 1950s, the products were being analyzed as fine crystalline materials. During this period a great variety of phosphates, silicates, germinates, sulphates, carbonates, oxides, etc., even without natural analogues, were prepared. However, no special significance was attached to such fine crystalline products except for the phase equilib- ria studies. In fact, the experimental duration was also enhanced in several cases to transform these fine crystalline products into small or bulk single crystals, whenever it was possible. Thus the interest on the growth of bulk crystals was revived during the 1960s and it survived until the 1990s. However, such attempts failed again because of the lack of knowledge on the hydrother- mal solution chemistry. It was only during the 1950s and 1960s; some attempts were made to understand the hydrothermal solution chemistry and kinetics of the hydrothermal reactions. It was during the 1970s that some attempts were made to observe the hydrothermal reactions us- ing sapphire windows in the autoclaves. However, owing to the extreme PT conditions these works were not encouraging and the in situ observation of the growth processes was later aban- doned. But today, it has become one of the most attract ive aspects of hydrothermal researc h technology. Combination of advanced h ydrothermal reactor design with the new sophisticated analytical techniques like Laser Raman, FTIR, synchrotron, HR-SEM, etc. has greatly aided the observation of nucleation and materials processing in situ. With the availability of high resolu- tion SEM from 1980 onwards hydrothermal researchers started observing such fine products which were earlier discarded as failures. The hydrothermal research in the 1990s marks the b e- ginning of the work on the processing of fine to ultra fine particles with a controlled size and morphology. The advanced ceramic materials prepared during that time justify this statement. In the last two decades these sub-micron to nanosized crystalline products have created a rev- olution in science and technology under a new terminology, ‘Nanotechnology’. Today hydro- thermal researchers are able to understand such nanosized materials and control their formation process, which in turn, give the desired prope rties to such nanomaterials. Thus hy- drothermal technology and nanot echnology have a very close link ever since this hydrothermal technology was proposed. 123K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 The recent advances in the hydrot hermal solution chemistry through the principles of ther- modynamics, kinetics, and chemical energy have created a new trend in materials processing. For example, the materials synthesized under extreme PT conditions in the earlier days could be well crystallized presently under much lower PT conditions. Table 1 gives the recent trends in hydrothermal research. Such trends have greatly helped in processing advanced materials at rel- atively lower PT conditions and at a much faster rate, thus having a great bearing on nanotech- nology of the 21st century. Also, the trends shown in Table 1 take hydrothermal technology towards green technology for sustained human development since it consumes less energy with no or little solid waste/or waste liquid/gases and involves no recovery treatment, no hazardous process materials, high selectivities, a closed system of processing, etc. The important subjects of technology in the 21st century are predicted to be the balance of environmental and resource and/or energy prob- lems. This has led to the development of a new concept related to the processing of advanced materials in the 21st century, viz. industrial ecology or science of sustainability [12]. Several researchers have already used the terms green hydrothermal process, green hydrothermal tech- nology, green hydrothermal route, etc., since the last one decade [13,14]. 3. Physical chemistry of hydrothermal processing of advanced materials for nanotechnology Physical chemistry of hydrothermal processing of materials is perhaps the least known as- pect in the literature. The Nobel Symposium organized by the Royal Swedish Academy of Sci- ences during 1978, followed by the First International Symposium on hydrothermal reactions organized by the Tokyo Institute of Technology in 1982, helped in setting a new trend in hy- drothermal technology by attracting physical chemists in large number [15,16]. The hydrother- mal physical chemistry toda y has enriched our knowledge greatly through a proper understanding of hydrothermal solution chemistry. The behaviour of the solvent under hydro- thermal conditions dealing with aspects like structure at critical, supercritical and sub-critical conditions, dielectric constant, pH variation, viscosity, coefficient of expansion, density, etc. is to be understood with respect to pressure and temperature. Similarly, the thermodynamic studies yield rich information on the behaviour of solutions with varying pressure temperature conditions. Some of the commonly studied aspects are solubility, stability, yield, dissolutione precipitation reactions and so on, under hydrothermal conditions. Hydrothermal crystallization Table 1 Current trends in hydrothermal technology [5] Compound Earlier work Author a Li 2 B 4 O 7 T ¼ 500e700  C T ¼ 240  C P ¼ 500e1500 bars P ¼ <100 bars Li 3 B 5 O 8 (OH) 2 T ¼ 450  C T ¼ 240  C P ¼ 1000 bars P ¼ 80 bars NaR(WO 4 ) 2 , R ¼ La, Ce, Nd T ¼ 700e900  C T ¼ 200  C P ¼ 2000e3000 bars P ¼ <100 bars R:MVO 4 , R ¼ Nd, Eu, Tm; M ¼ Y, Gd Melting point >1800  C T ¼ 100  C P ¼ <30 bars LaPO 4 Synthesized at >1200  C T < 120  C P < 40 bars a From the works of Prof. K. Byrappa. 124 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 is only one of the areas where our fundamental understanding of hydrothermal kine tics is lacking due to the absence of data related to the intermediate phases forming in solution. Thus our fun- damental understanding of hydrothermal crystallization kinetics is in the early stage although the importance of kinetics of crystallization studies was realized with the commercialization of the synthesis of zeolites during the 1950s and the 1960s itself. In the absence of predictive models, we must empirically define the fundamental role of temperature, pressure, precursor, and time on crystallization kinetics of various compounds. Insight into this would enable us to understa nd how to control the formation of solution species, solid phase s and the rate of their formation. In recent years, the thermochemical modeling of the chemical reactions under hydrothermal conditions is becoming very popular. The thermochemcial computation data help in the intelli- gent engineering of the hydrothermal processing of advanced materials. The modeling can be successfully applied to very complex aqueous electrolyte and non-aqueous systems over wide ranges of temperature and concentration and is widely used in both industry and academy. For example, OLI Systems Inc., USA provides the software for such thermochemical modeling, and using such a package aqueous systems can be studied within the temper ature range À50 to 300  C, pressure ranging from 0 to 1500 bar and concentration 0e30 m in molal ionic strength; for the non-aqueous systems the temperature range covered is from 0 to 1200  C and pressure from 0 to 1500 bar with species concentration from 0 to 1.0 mole fraction. A key limitation to the conventional hydrothermal method has been the need for time- consuming empirical trial and error methods as a mean for process development. Currently, research is being focused on the development of an overall rational engineering-based approach that will speed up process development. The rational approach involves the following four steps: 1. Compute thermodynamic equilibria as a function of chemical processing variables. 2. Generate equilibrium diagrams to map the process variable space for the phases of interest. 3. Design hydrothermal experiments to test and validate the computed diagrams. 4. Utilize the processing variables to explore opportunities for controlling reac tions and crys- tallization kine tics. Such a rational appro ach has been used quite successfully to predict the optimal synthesis conditions for controlling phase purity, particle siz e, size distribution, and particle morphology of lead zirconium titanates (PZT), hydrox yapatite (HAp) and other related systems [17e19]. The software algorithm considers the standard state properties of all system species as well as a comprehensive activity coefficient model for the solute species. Table 2 gives an example of thermodynamic calculations and the yield of solid and liquid species outflows at T ¼ 298 K, P ¼ 1 atm., I ¼ 0.049 m, and pH ¼ 12.4. Using such a mode ling approach, theoretical stability field diagrams (also popularly known as the yield diagrams) are constructed to get 100% yield. Assuming the product is phase-pure, the yield Y can be expressed as: Y i ¼ 100 À m ip i À m eq i Á m ip i % where m ip and m eq are the input and equilibrium molal concentrations, respectively, and sub- script i the designated atom. Figs. 5 and 6 show the stability field diagrams for the PZT and HA systems. From Fig. 5 it is observed that the region with vertical solid lines represents the 99% yield of PZT although the PZT forms within a wide range of KOH and Ti concentrations. The figure 125K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 illustrates clearly the region where all the solute species transform towards 100% product yield. Similarly from Fig. 6, it is observed that all the Ca species participate in the reaction to form HA and thus leading to 100% yield of HA in the region denoted by a black square. Thick dotted lines indicate the boundary above which 99% Ca precipitates as HA. The other regions mark the mixed phase prec ipitation like hydroxyapatite, monatite and other calcium phosphate phases. Such thermodynamic studies help to intelligently engineer the hydrothermal processing and also to obtain a maximum yield for a given system. This area of research has a great potential application in advanced materials processing including nanomaterial s. 4. Instrumentation in hydrothermal processing of nanomaterials Material processing under hydrothermal conditions requires a pressure vessel capable of containing a highly corrosive solvent at high temperature and pressure. Hydrothermal Table 2 Thermodynamic calculations for HAp system Species name Inflows moles Outflows Liquid/mol Solid/mol H 2 O 55.51 55.51 8.10 Â 10 À2 Ca(OH) 2 0.1 7.2 Â 10 À6 CaO Ca 2+ 1.5 Â 10 À2 Ca(OH) + 4.0 Â 10 À3 H + 4.45 Â 10 À13 OH À 3.41 Â 10 À2 Total 55.61 55.56 8.10 Â 10 À2 0 -0.8 -1.6 -2.4 -3.2 0 2 4 6 8 10 12 14 [KOH] (mol/kg H 2 O) Log [Ti] No PZT PbO 0% < Yield < 99% 180°C • • • PZT 70/30, yield > 99% Fig. 5. Calculated stability field diagram for the PZT system at 180  C with KOH as the mineralizer [17]. 126 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials 53 (2007) 117e166 [...]... (2007) 117e166 Fig 20 Hydrothermally synthesized ZnO particles [62] were considered for the hydrothermal synthesis The hydrothermal experimental duration was increased from 5 to 50 h and its effect on the formation of pure ZnO phase was studied It was found that the formation of pure ZnO phase under such low temperature condition requires a minimum duration of 40 h Fig 20 shows the hydrothermally synthesized... several other reactors popularly used for materials processing under hydrothermal conditions with special provisions for microwave, mechanochemical, electrochemical or sonochemical energies, flow reactors, rocking autoclaves, and so on, which greatly help in providing enhanced kinetics for hydrothermal reactions Figs 11e14 show the photographs of these four special reactors For the laboratory scale as well... reactors are used to protect from the solvent medium Therefore, the corrosion resistance of any metal under hydrothermal conditions is very important For example, turbine engineers have long known that boiler water with pH > 7 is less corrosive than slightly acidic water, especially for alloys containing silicon The commonly used reactors in the hydrothermal processing of advanced nanomaterials are listed... discs are commercially available for various ranges of bursting pressure The most important arrangement is that provision should be made for venting the live volatiles out in the event of rupture Proper shielding of the reactor should be given to divert the corrosive volatiles away from the personnel working nearby 5 Hydrothermal processing of advanced materials and nanotechnology There are hundreds of... Microwave hydrothermal reactors Mechanochemicalehydrothermal Piston cylinder apparatus Belt apparatus Opposed anvil Opposed diamond anvil Figs 7 and 8 show the most popular autoclave designs such as general purpose autoclaves, Morey autoclaves, modified Bridgman autoclaves and TuttleeRoy autoclaves In most of these Fig 7 General purpose autoclave popularly used for hydrothermal treatment and hydrothermal. .. C for 48 h and by synthesizing by the direct reaction of bismuth salts with thioacetamide at 140  C for 48 h in the aqueous solution (Photos: Courtesy Prof Y.T Qian) Hydrothermal synthesis of NiS, NiS2, NiS7, CuS has been carried out by several authors using some surfactants, which assisted in controlling their size and shape [111e113] Thus sulphides occupy a prominent place in both hydrothermal technology. .. shape [111e113] Thus sulphides occupy a prominent place in both hydrothermal technology and nanotechnology owing to their unique properties From the above discussion it is clear that solvothermal route is a more preferred one than the aqueous solution route for sulphides 5.5 Hydrothermal synthesis of carbon nanoforms The synthesis of different carbon polymorphs such as graphite, diamond, amorphous carbon... attracted considerable interest for a long time because of their importance in science and technology There are uncertainties about the phase stabilities of these polymorphs, as some of them do not find a place in the carbon pressureetemperature (PeT) diagram and are also known for their contrasting physical properties The exact physico-chemical phenomena responsible for their formation are yet to be understood... of Jacobson et al [130] indicated that the formation of free carbon is expected in the low water to carbide ratio Further, in the high pressure metal-carbon experimental system the free excess water in the system inhibits the formation of diamond [131], and the formation of graphite is more favourable The sealed capsules were placed in the autoclaves for hydrothermal treatment After the experimental... dispersibility Hence, a great variety of modifications are used in the hydrothermal technique However, for the sake of convenience, the synthesis of the most popular metal oxides such as TiO2 and ZnO will be discussed separately Perrotta and Al’myasheva et al have reviewed the hydrothermal synthesis of corundum nanoparticles under hydrothermal conditions [13,34] A high specific surface area corundum has . technologists, and Bio -Technology Advanced Materials Technology Geo -Technology Nano- Technology Hydrothermal Technology Fig. 1. Hydrothermal technology in. for the fabrication of powders or bulk ceramic bodies. It has now emerged as a frontline technology for the proce ssing of advanced materials for nanotechnology.