**4.1.3 Hydrothermal synthesis**

of CaHPO4 at temperatures higher than 164°C in acidic environment and Ca3(PO4)2 precipi‐ tated from the solution at nearly neutral conditions. Hydroxylapatite again predominates at higher pH and Ca(OH)2 does not appear at higher temperatures and the pH below 14 (the

194 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

The phase equilibrium in the system CaO-P2O5-H2O was extensively studied by the solid-state reaction method under the atmospheric pressure of water vapor by VAN WAZRER [70] and in aqueous systems at temperatures lower than 100°C by BROWN et al [71],[72]. BIGGAR [73] studied the CaO-P2O5-H2O system in the temperature range from 700 to 950°C and the pressure of 1 kbar. FENG and ROCKETT [74 ] studied the system CaO-P2O5-H2O at 1000 bar with 50%wt. and

**precipitation method** uses the precursor mixed with low melting

point salt such as NaCl, KCl, or their eutectics. Upon the melting of the mixture, reactant oxides dissolve in the salt and desired compound precipitates due to its low solubility in molten salt.

36 Fused salts are widely used in many industrial processes requiring to free the limitations arising from the use of aqueous solutions. Their thermal stability and generally low vapor pressure enable fast reaction rates and ability to dissolve many inorganic compounds making them useful solvents in electrometallurgy, metal coating, treatment of by-products, and energy conversion. It is recalled that one of the most important chemicals produced worldwide, sulfuric acid, is made by the molten salt catalysis. The electrolysis of molten salt is a technique used by H. MOISSAN for the isolation of element fluorine from the melt of KF·2HF (Moissan's method is used for industrial production of fluorine). It was also used by H. DAVY to discover several new elements (sodium, potassium, alkali metals) and to prove the chlorine as a new element (originally discovered by C.W. SHEELLE who considered it as "*dephlogisticated marine acid*"). Today the industri‐ al production of Li and Na is based on the electrolysis of eutectic melt of LiCl–KCl (or CaCl2) and NaCl–KCl (or CaCl2), respectively. The production of K, Rb and Cs is based on the reduction of molten KCl, RbCl and CsCl by Na at the temperature of 600°C. Molten salt method also plays significant role in the development of energy resources, including the reprocessing of nuclear wastes, molten carbonate/solid oxide fuel cells (**Section 10.4**), and high temperature molten salt batteries. Fused alkali nitrates/nitrites are valuable materials for the heat transport and storage in solar plants. Molten salt bathes remain of large use in industry for the treatment of steel and variety of other metals as well as nonmetals, such

same as for **Fig. 8**).

200°C (**Fig. 12**).

**Fig. 12.** Phase diagram of Ca(OH)2-Ca3(PO4)2-H2O system [21].

The **molten** (**fused**) **salts**<sup>36</sup>

as glass, plastics and rubber [75].

The original hydrothermal37 method involves heating of the reactants in a closed vessel, an autoclave, with water (heterogeneous reaction). Autoclave is usually constructed from thick stainless steel to withstand the high pressures and is fitted with safety valves; it may be lined with nonreactive materials, such as noble metals, quartz or Teflon. When the autoclave is heated, the pressure increases and the water remains liquid above its normal boiling temper‐ ature of 100°C, so-called superheated water. These conditions, in which the pressure is raised above atmospheric pressure and the temperature is raised above the boiling temperature of water are known as hydrothermal conditions (high-pressure-high-temperature, HPHT). HPHT conditions enable to dissolve and recrystallize (recover) the materials which are relatively insoluble under ordinary conditions. The methods enable [21],[24],[67]:

**1.** Synthesis of new phases or stabilization of new complexes.

<sup>37</sup> The term hydrothermal is of purely geological origin. It was first used by British geologist, SIR RODERICK MURCHISON, to describe the action of water at elevated temperature and pressure in bringing about changes in the Earth´s crust, and leading to the formation of various rocks and minerals. Materials scientists popularized the technique, particularly during 1940s. The first hydrothermal synthesis was performed by SCHAFHAUTL in Papin's digester, who obtained quartz crystals upon hydrothermal treatment of freshly precipitated silic acid [21].


Several definitions of hydrothermal synthesis use aqueous solvent under HPHT conditions [21]:


Depending on the type of solvent used in the heterogeneous reaction the glycothermal, alcothermal, ammonothermal, lyothermal, carbothermal, etc., methods are recognized. According to applied solvent and condition, the hydrothermal methods can be further divided as follows [21].


Hydrothermal conditions exist in nature, and numerous minerals including naturally occurring zeolites and gemstones, are formed by this process. The term has been extended to other systems with moderately raised conditions and temperatures lower than those typical‐ ly used in ceramics and sol-gel syntheses. Lower temperatures used are one of the advantag‐ es of the method. Other methods include the preparation of compounds in unusual oxidation states or phases, which are stabilized by raised temperature and pressure [24].

<sup>38</sup> The temperature and the pressure at critical point of water are 373.946°C and 22.064 MPa, respectively.

**Fig. 13.** Tree showing the interdisciplinary nature of hydrothermal technology [21].

**2.** Crystal growth of several inorganic compounds.

196 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**6.** Decomposition, alteration, corrosion and technique.

morphology for specific applications.

**5.** Leaching of ores in metal extraction.

developed by such solution [77].

the pressure higher than 1 bar [78].

critical to supercritical conditions.

a few atmospheres [79].

as follows [21].

nanomaterials.

[21]:

**3.** Preparation of finely divided materials and microcrystallites with well-defined size and

**4.** In situ fabrication of materials with desired size, shape and also dispersity in case of

Several definitions of hydrothermal synthesis use aqueous solvent under HPHT conditions

**a.** In hydrothermal synthesis the material is subjected to the action of water, at tempera‐ tures generally near, though often considerably above the critical temperature38

**b.** Hydrothermal synthesis is a heterogeneous reaction in aqueous media above 100°C and

**c.** Hydrothermal synthesis involves water as a catalyst and occasionally as a component of solid phases in the synthesis at elevated temperature (>100°C) and pressure greater than

Depending on the type of solvent used in the heterogeneous reaction the glycothermal, alcothermal, ammonothermal, lyothermal, carbothermal, etc., methods are recognized. According to applied solvent and condition, the hydrothermal methods can be further divided

**iii. Supercritical hydrothermal** methods use aqueous and nonaqueous solvent under

**iv. Multienergy hydrothermal methods** combine hydrothermal method with addition‐ al microwave, electrochemical, sonar, mechanochemical, etc. energy.

Hydrothermal conditions exist in nature, and numerous minerals including naturally occurring zeolites and gemstones, are formed by this process. The term has been extended to other systems with moderately raised conditions and temperatures lower than those typical‐ ly used in ceramics and sol-gel syntheses. Lower temperatures used are one of the advantag‐ es of the method. Other methods include the preparation of compounds in unusual oxidation

**i. Conventional hydrothermal techniques**, which use aqueous solvent.

**ii. Solvothermal techniques or methods**, which use nonaqueous solvent.

states or phases, which are stabilized by raised temperature and pressure [24].

38 The temperature and the pressure at critical point of water are 373.946°C and 22.064 MPa, respectively.

(~370°C) in closed bombs, and therefore, under the corresponding high pressures

of water

Hydrothermal synthesis was used industrially39 to prepare large crystals of quartz and synthetic gemstones. It is useful in metal oxide systems, where oxides are not soluble in water at atmospheric pressure but dissolve in superheated water under hydrothermal conditions. Where even these temperatures and pressures are insufficient to dissolve the starting materi‐ als, alkali or metal salts as mineralizers can be added, the anions of which form complexes with the solid and render it soluble [24].

<sup>39</sup> The first successful commercial application of hydrothermal technology was in mineral extraction or in ore beneficiation. The method was used to leach bauxite by sodium hydroxide by KARL JOSEF BAYER in 1892. The product of so-called Bayer ´s process, aluminum hydroxide, is then converted to Al2O3 and used to produce aluminum metal or in ceramics [21].

Throughout the course of evolution of hydrothermal synthesis from the geoscientific applica‐ tions to modern technologies, the hydrothermal technique has captured the attention of scientists and technologists from different branches of science. The hydrothermal technique is popularly used by geologists, biologists, physicists, chemists, ceramists, hydro-metallurgists, materials scientists, engineers, etc. **Fig. 13** shows different branches of science either emerg‐ ing out from the hydrothermal technique or closely linked up with the hydrothermal techni‐ que. One could firmly say that this family tree will keep expanding its branches and roots in the years to come [21].

The hydrothermal techniques for the preparation of compounds with the structure of apatite should be divided to:


The hydrothermal synthesis of all three normal apatite end-members was reported by BAUMER and ARGIOLAS [80]. They prepared crystallites of sizes from 50 to 500 μm. The synthesis of chlorapatite at 400°C and the pressure <3 kbar proceeds via the reaction:

$$110\text{ CaCl}\_2 + 6\text{ H}\_3\text{PO}\_4 \Leftrightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{Cl}\_2 + 18\text{ HCl} \tag{21}$$

The synthesis and the stability of carbonate-fluorapatite were examined by JAHNKE [81]. The carbonate-apatite phase is stable in solutions relatively rich in carbonate such as sea-water. When exposed to low-carbonate solutions, the carbonate-apatite should lose the CO3 2− ion [82].

During the hydrothermal synthesis of HAP whisker, the acetamide was used by ZHANG and DARVELL [83] as an agent to drive homogeneous precipitation at temperatures below 100°C. Acetamide shows low hydrolysis rate in both acidic and basic conditions, releasing acetate and ammonia:

$$\text{CH}\_3\text{CONH}\_2 + \text{H}\_3\text{O}^+ \rightarrow \text{CH}\_3\text{CO}\_2\text{H} + \text{NH}\_4^+ \tag{22}$$

which do no substitute in HAP lattice. The precipitation of hydroxylapatite from the solu‐ tion of Ca(NO3)2·4H2O and (NH4)2HPO4 in 0.05 mol·dm−3 (Ca:P = 1.67) treated to the tempera‐ ture of 180°C for 10–15 hours yielded to large rod-like and well-crystallized particles of hydroxylapatite.

The stoichiometric single crystals of hydroxylapatite nanorods with mono-dispersion and narrow-size distribution in diameter were successfully synthesized by LIN et al [84] via the hydrothermal microemulsion method [85].40 The microemulsion was prepared using CTAB as the surfactant and *n*-pentanol as the cosurfactant. First, 0.5 M Ca(NO3)2 solutions and 0.3 M (NH4)2HPO4 solutions were obtained by dissolving Ca(NO3)2·4H2O and (NH4)2HPO4 in

<sup>40</sup> The emulsification consists in dispersing of one fluid in another, non-miscible one, via the creation of interface [85].

distilled water, respectively, and the pH of both solutions was adjusted to 11.0 by adding ammonia solution. These aqueous solutions were used as the water phase and n-hexane was used as oil phase. The mixture of surfactants [86]41 and Ca(NO3)2 solution was stirred, ultraso‐ nicated and optically transparent microemulsion was obtained. The solution of (NH4)2HPO4 was drop wisely added into the Ca(NO3)2 microemulsion solution to obtain a suspension, and the pH of the suspension was maintained at 11.0 using ammonia solution. Then the suspen‐ sion of the microemulsion was transferred into stainless steel autoclaves and maintained at 180°C for 18 h. Washed hydroxylapatite powder was then calcined at 600°C for 2 h [84].
