Advanced ceramics have progressed significantly in the last few years. This progress is closely related to the development of new synthetic routes providing not only the preparation of known materials with improved properties, but also the discovery of novel compounds. This chapter gives an overview of established synthetic methods to prepare oxide- and non-oxide-based ceramics from precursors in condensed phases. Typical synthetic strategies performed either in liquid or solid phases are summarized. In addition, innovative synthesis methods such as sol–gel and polymer-derived pyrolysis technique to develop advanced ceramics with exceptional properties are considered.
KeywordsCeramic Synthesis Condensed phases
Advanced ceramics have been technically used worldwide over the past century and stimulated current research activities in many emerging research areas ranging from fundamental science to applications in sustainable technology, energy conversion, and environmental issues.
Improving and designing or tailoring the properties of advanced ceramic materials for a wide range of applications, e.g., from engineering, resource processing and power generation to aerospace, medicine and defense-oriented applications, are the challenging tasks of modern material science. The modern industry urgently needs the development and implementation of new ceramic-based materials with enhanced quality and reliability to be used in the next-generation technologies and devices. For example, there are demands for advanced ceramic material applications such as wear-, corrosion- and thermal shock-resistant parts for oil, gas, mining, mineral and chemical industries, power generation, engine components, filter and catalyst supports and some other parts for automotive manufacturing, biomedical implants, armor parts and structures, filter and catalytic systems for chemical and environmental uses and many others.
In general, the properties and performance of materials strongly depend on the applied synthesis, phase composition and microstructure. Traditionally, a high-temperature ceramic synthesis, also denoted as ceramic method or shake and bake, is used to prepare inorganic solids. In this approach, solid precursors are intimately grounded in stoichiometric quantities and heated at extreme temperatures (>1000 °C) for a period ranging from hours to days to facilitate interdiffusion of the solid reagents. This method has been widely used to synthesize ceramic materials like ferroelectric BaTiO3 or materials with a higher degree of complexity like cuprate-type superconductors, in which oxides contain up to six different components, such as Tl1−y PbyY1−x CaxSr2Cu2O7 . In spite of some benefits, the ceramic method faces the following drawbacks: (1) long synthesis protocol (lasting from hours up to days), (2) difficulties in controlling particle size and shape of the product and (3) only thermodynamically stable phase crystallized by this method. Therefore, in an attempt to overcome these shortcomings new synthesis routes have been intensively investigated in the past years.
Synthesis from the solid phase
High-pressure/high-temperature (HP-HT) synthesis
High-pressure and high-temperature (HP–HT) materials synthesis has been developed in recent years to form new compounds/phases under solid-state conditions that are not achievable under ambient pressure. Under extreme HP–HT conditions, novel polymorphs of well-known compounds as well as completely new compounds with high coordination numbers and high oxidation states for elements with multiple valances (e.g., Fe, Zr or Ce) can be achieved.
The aforementioned apparatuses can be joined by large-scale shock-wave facilities for studies of properties and synthesis under dynamic compression, and by a group of advanced theory and simulation techniques. As promising new materials are identified and shown to be recoverable to ambient pressure with their useful properties intact, the preparation can be scaled up by devising a static method, such as in situ observations for “large volume” experiments including electrical conductivity and calorimetric measurement. Besides that, improvements in press design and detectors coupled with synchrotron experiments permit high precision X-ray diffraction, infrared (IR)/Raman spectroscopy and image studies on solids and liquids such as in situ monitoring of the course of synthesis reactions and the kinetics of strain relaxation in materials .
In contrast, dynamic shock waves generated by compressed gas, explosives or lasers provide the most extreme HP–HT conditions . Pressures into the multi-megabar range (several hundred of MPa) and extending up to 50 TPa with simultaneous heating to thousands or tens of thousands of °C can be achieved. Resistive- and laser-heating techniques enable controlled heating of samples from a few hundred up to several thousand degrees, while they are held at high pressure. Despite its small sample volume, laser-heated (LH) DAC remains the essential tool for the synthesis and exploration of new high-pressure materials. There are two types of continuous-wave IR lasers for sample heating: i) solid state Nd:YAG lasers used for heating semiconductors, metals and insulators containing transition metals and ii) CO2 lasers used in experiments with non-conducting inorganic (oxides, silicates, nitrides, etc.) and organic materials.
Molten salt (MS) synthesis
Physicochemical properties of a molten NaCl at 850 °C and a solution of NaCl 10−1 M at 25 °C
Ion concentration (mol L−1)
Density (g cm−3)
Electrical conductivity (S cm−1)
Molten NaCl at 850 °C
NaCl 10−1 mol L−1 at 25 °C
1.07 × 10−1
Melting points and compositions of some commonly used metal halides, hydroxides and oxosalt systems
Composition (mol %)
Melting point (°C)
During the reaction, the molten salts provide the following advantages: (1) increase the reaction rate and reduce the reaction temperature by increasing the contact area of the reactant particles (precursors) and the mobility of the reactant species in the liquid molten salt; (2) increase the degree of homogeneity (the distribution of the constituting elements in the solid solution); (3) allow the control of the particle size and shape by controlling the rate of the Ostwald ripening mechanism (temperature and heating time); and (4) prevent the direct contact between the particles by covering their surfaces allowing the control of the aggregation state.
The solubility of oxides in molten salts varies greatly from less than 1 × 10–10 mol fraction to more than 0.5 mol fraction, typically 1 × 10–3–1 × 10−7 mol fraction. In many cases, the formation reaction occurs in the presence of solid reactant particles. In this sense, molten salt is somewhat different from ordinary solvents, which dissolve all reactant particles and the product precipitates from a homogeneous liquid phase.
Polymer-to-ceramic transformation synthesis
Preceramic polymer synthesis
The preceramic polymers represent inorganic/organometallic systems that provide ceramics with a tailored chemical composition and a defined nanostructural organization. Common preceramic polymers for the preparation of PDCs are polysilanes, polycarbosilanes and polysiloxanes, as well as polysilazanes and polysilylcarbodiimides (Fig. 8).
The preceramic polymers have to meet the following requirements: (1) they should possess a high molecular weight to avoid volatilization of low molecular components; (2) they should have appropriate rheological properties and solubility for the shaping process; and (3) they must have latent reactivity (presence of functional groups) for the cross-linking and curing steps. The molecular structure and type of the preceramic polymer is one of the key issues in PDCs due to influence on (1) the overall composition, (2) the number of phases and the phase distribution and (3) the microstructure of the final ceramic product. Thus, the macroscopic chemical and physical properties of PDCs can be tailored by the design of the molecular precursor.
During cross-linking, the precursors are converted into organic/inorganic materials with enhanced molecular weight, which reduces the loss of low-weight components of the precursor and minimizes fragmentation processes during the ceramization step. Since the cross-linked polymers are basically infusible materials they retain their shape upon pyrolysis and do not melt during ceramization. Depending on the chemistry of the precursor polymer and the reaction conditions, different chemical reactions can take place during the cross-linking process:
Polycarbosilanes can be cross-linked by either oxygen or electron beam (e-beam) curing. In the presence of oxygen, the cross-linking process occurs via radical mechanism which involves Si–H and Si–CH3 groups to form Si–OH, Si–O–Si and C=O units. The ceramization of those cross-linked polymers yields silicon carbide-based materials (e.g., SiC fibers) containing high oxygen content (e.g., 10–12 %). Cross-linking of polycarbosilanes in the absence of oxygen (using e-beam) involves reactions of Si–H bonds with Si–CH2 groups, leading to Si–CH–Si linkages. Silicon carbide materials cross-linked via e-beam radiation show low oxygen content (0.2–0.3 %).
Polysiloxanes can be cross-linked via condensation, transition metal-catalyzed addition and by free radical initiation mechanisms. The cross-linking of polysiloxanes containing either methyl or vinyl groups can be performed thermally, by using peroxides. Furthermore, cross-linking reactions can occur between silicon hydride units and Si–vinyl groups, either thermally or via metal salt catalysis. In case of polysiloxanes containing hydroxyl and alkoxy groups, condensation of Si–OH units as well as hydrolysis of alkoxy groups leads to the formation of the Si–O–Si bonds.
Polysilazanes can be cross-linked either thermally or using chemical reagents, such as catalysts and peroxides. During the thermal cross-linking process of appropriate substituted polysilazanes, four major reactions can occur: (1) hydrosilylation in polysilazanes, which are composed of Si–H and vinyl groups, at low temperatures (starting at 100–120 °C) leads to the formation of Si–C linkages; (2) vinyl polymerization is an additional process, which occurs at temperatures higher than 300 °C; (3) transamination occurs at temperatures above 200 °C and is characterized by a mass loss (i.e., evolution of amines, ammonia or oligomeric silazanes) which leads to a decrease in the nitrogen content of the ceramic materials upon pyrolysis; and finally, (4) dehydrocoupling reactions between Si–H and N–H units as well as between Si–H and Si–H units at higher temperatures (ca. 300 °C) yield the formation of new Si–N bonds or Si–Si bonds, respectively.
Finally, the cross-linked precursors are treated at elevated temperatures (i.e., 600–1000 °C). Within this temperature range, thermolysis and evolution of organic groups of the cross-linked polymers occur and, consequently, amorphous covalently bonded ceramics are attained . The reactions that occur during pyrolysis may be investigated by means of ex situ solid-state nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and Raman spectroscopy, as well as thermogravimetric analysis (TGA) coupled with in situ mass spectrometry (MS) and FTIR spectroscopy.
Polycarbosilanes can be pyrolyzed at temperatures ranging from 800 to 1000 °C, where the polycarbosilanes are transformed into inorganic materials with the evolution of hydrogen and methane. The ceramics obtained upon pyrolysis at T > 800 °C can be described as hydrogenated silicon carbide with excess carbon. The amorphous ceramic begins to crystallize into silicon carbide at temperatures exceeding 1000 °C with the simultaneous evolution of hydrogen [15, 16].
Cross-linked polysiloxanes are converted into silicon oxycarbide (SiOC) glasses upon pyrolysis at temperatures between 600 and 1000 °C [17, 18, 19]. The ceramization occurs via the evolution of hydrocarbons and hydrogen. Additionally, various redistribution reactions between Si–O, Si–C and Si–H bonds can occur leading to the evolution of low molecular weight silanes  and, consequently, decreasing the ceramic yield. The achieved materials consist of amorphous SiOC and residual free carbon phases.
The processes that occur upon pyrolysis of polysilazanes and poly(carbo)silazanes are rather complex. While the ceramization of cross-linked perhydropolysilazane (carbon-free precursor) yields binary amorphous or polycrystalline Si3N, the pyrolysis of either polyhydrido(organo)silazanes or polysilylcarbodiimides leads to the formation of amorphous SiCN with different microstructures. SiCN derived from polyhydrido(organo)silazanes is an amorphous phase and exhibits a mixed bond configuration (tetrahedrally coordinated silicon from SiC4, SiC3N, SiC2N2 and SiCN3 to SiN4) as well as free carbon, whereas the pyrolysis of polysilylcarbodiimides ends up with an amorphous nanocomposite composed of Si3N4, SiC and free carbon phase [21, 22, 23, 24, 25, 26, 27].
Active fillers are metallic or intermetallic powders that react with the decomposition gases generated during pyrolysis, or with the heating atmosphere or (less frequently) with the ceramic residue obtained from the preceramic polymer [28, 29]. For handling and safety reasons, the active fillers are normally quite coarse (in the micrometer range), as small metallic particles may exhibit pyrophoricity. Typical products of the chemical reactions between the preceramic polymer and the filler particles are carbides, nitrides or silicide phases, with a significant impact on the overall shrinkage. In fact, the metal-to-ceramic transformation generally occurs with a large volume expansion, due to a large density decrease, which compensates for the shrinkage associated with the conversion of the polymers to the ceramic material. Solid particles and the in situ reaction with the filler reduce the amount of gases generated and the local gas pressure, respectively, therefore enabling the fabrication of near-net shape, bulk, uncracked ceramic components .
To tune functional applications of polymer-derived ceramics, direct chemical modification of preceramic polymers using metal alkoxides or other chemical precursors (liquid or gaseous) has been studied in recent years. Metal alkoxides have been added to the preceramic polymers for various purposes, e.g., to enhance the cross-linking of Si-based polymers, control crystallization processes of the resulting ceramic residue, improve the high-temperature stability of PDCs or afford functional properties such as sensoric, magnetic or catalytic features. From the microstructural point of view, these additions result in Si-based ceramic residues, containing additional amorphous or crystalline oxidic or non-oxidic phases. In some cases, the additions have allowed the preceramic polymers to retain their plastic shaping capability, while in other cases the increase of the degree of cross-linking prevents their viscous flow.
The chemical modification of the polymers by the sol–gel technique, with metal alkoxides, represents an excellent opportunity to extend the temperature stability of the amorphous residues. As an example, Al-containing alkoxide compounds, such as alumatrane (C6H12NOAl), added to commercial silicones, led to an SiAlOC residue, stable up to 1300 °C . The potential of the additives, however, does not simply rely on avoiding phase separation, but also on allowing controlled crystallization with the development of new phases. Zr- and Hf-containing materials also represent an interesting example for the excellent high-temperature stability of the related nanocomposites [31, 32, 33]; zirconia and hafnia, being particularly resistant against carbothermal reduction, may form silicates, such as zircon (ZrSiO4) and hafnon (HfSiO4), stable up to 1600 °C. Moreover, SiZrOC and SiHfOC are more resistant under hydrothermal conditions up to 250 °C than pure SiOC facilitating a synergic effect, i.e., while zirconia and hafnia have a relatively low, but appreciable, solubility in water under the testing conditions, the SiCO matrix protects the dispersed phases from the water-induced tetragonal-to-monoclinic transformation .
Synthesis from the liquid phase
Synthesis based on wet chemical methods is a special synthetic approach that allows the attainment of advanced ceramics with controlled size (ranging from micro- to nanoscale) and shape (e.g., powder, fibers, films or monoliths), and with high chemicophysical reactivity as well as high purity control. This approach can be used to synthesize (1) oxidic and non-oxidic, (2) binary, ternary and multicomponent, (3) pure and doped, (4) stable and metastable ceramic materials. Besides that, the liquid phase synthesis allows the formation of fine powders, thin fibers, films and aerogels.
In most cases, wet synthetic methods involve the precipitation (the product contains two elements) or coprecipitation (the product contains more than two elements) of solid particles from soluble precursors in aqueous solutions caused by pH, temperature and precursor concentration changes or by the addition of external agents (oxidizing, reducing and/or stabilizing agents). To be able to tailor the powder properties for specific applications, a certain understanding of the basic mechanisms of nucleation, growth and agglomeration is essential.
Sol–gel procedures have been employed in the search of new low-temperature synthetic routes to produce solid-state materials from chemically homogeneous precursors. By trapping the “randomness of the solution state” and thereby ensuring atomic level mixing of reagents, one should be able to produce complex inorganic materials (such as multinary oxides) at lower processing temperatures and shorter synthesis times. Furthermore, sol–gel chemistry should enable control over particle morphology and size. However, producing a homogeneous precursor at room temperature does not ensure homogeneity throughout a reaction and many sol–gel routes have therefore been designed to control phase segregation during synthesis .
Hydrolytic Sol–gel synthesis
The precursors can be either aqueous solutions of inorganic metal salts or metal organic compounds (alkoxides, acetates or acetylacetonates). Among them, the most popular are metal alkoxides due to the following advantages: (1) they are soluble in polar solvents providing high homogeneity and (2) they can easily be converted to the corresponding oxide through the following reactions:
In the hydrolytic sol–gel approach, the following aspects have to be considered: (1) the chemical reactivity of the precursor (electronegativity of the metal atom, its ability to increase the coordination number, the steric hindrance of the organic group and its molecular structure), (2) the amount of added water and how the water is added during the hydrolysis step and (3) the polarity, dipole moment and acidity of the solvent used in the sol–gel process.
Finally, a crucial role in the sol–gel process concerns the removal of the solvent from the gel (drying step). Drying by evaporation under normal conditions gives rise to capillary pressure that causes shrinkage of the gel network due to the liquid located within the pores. The resulting dried gel denoted as “xerogel”, a word issued from the Greek word “xeros” and which means dry, is often reduced in volume by a factor of 5–10 compared to the original wet gel. However, when the gel is dried by a supercritical drying process, aerogels are produced. The drying step is performed inside an autoclave which allows overpassing the critical point (P C, T C) of the solvent. Under supercritical conditions, there is no interface between liquid and vapor, which avoids the formation of capillary pressure and results in relatively little shrinkage. Xerogels and aerogels are useful in the preparation of dense ceramics, but they are also interesting themselves, because their high porosity and surface area make them useful materials as catalytic substrates, filters and others .
Non-hydrolytic sol–gel synthesis
In the past 20 years, several non-hydrolytic routes to synthesize metal oxides and mixed metal oxides have been developed, involving reactions of suitable precursors (metal chlorides, alkoxides, acetylacetonates, etc.) with oxygen donors (metal alkoxides, ethers, alcohols, acetates, aldehydes, ketones, etc.). The main problem of the aqueous sol–gel approach is to control the hydrolysis and condensation rates. For most transition metal precursors, either organic or inorganic, these reactions are too fast, giving rise to reducing the control of the morphology and the crystal structure of the resulting solid product. These problems increase when two or more metal alkoxides are involved in the sol–gel process to yield a complex multi-metal oxide. Due to the different reactivity of the individual precursors, the control of composition and homogeneity of the final complex oxide is a big challenge.
Non-hydrolytic (or non-aqueous) sol–gel processes in organic solvents are able to overcome some of the major limitations of aqueous systems and thus represent a powerful and versatile alternative [30, 37, 38, 40, 41]. Sol–gel syntheses are considered as non-hydrolytic when the oxygen donor is not water, and when water is not generated in situ. The advantages are a direct consequence of the manifold role of the organic components in the reaction system (e.g., solvent, organic ligand of the precursor molecule, surfactants,or in situ formed organic condensation products). Non-aqueous synthesis routes allow the synthesis of oxidic and non-oxidic nanoparticles with uniform, yet complex crystal morphologies, crystallite sizes in the range of just a few nanometers and good dispersibility in organic solvents.
In the synthesis of oxidic compounds, the oxygen is provided either by the solvent (ether, alcohols, ketones or aldehydes) or by the organic constituent of the precursor (alkoxides or acetylacetonates). Furthermore, the organic solvent strongly determines the particle size and shape as well as the surface properties due to their distinct coordination properties.
The solvothermal approach offers several advantages over other conventional and non-conventional ceramic synthesis methods. Due to the fact that it is a wet synthesis procedure, processes like diffusion, adsorption, dispersion, reaction rate and crystallization are favored, compared to solid-state processes. From the standpoint of ceramic powder production, there are some processing steps which require high energy consumption like mixing, milling and calcination that are either not necessary or minimized in the solvothermal process. In addition, this approach has the ability to precipitate already crystallized powders directly from the solution. It allows a better control of the rate and uniformity of the nucleation, growth and aging steps which results in an improved control of size and morphology of the crystallites and significantly reduced aggregation levels .
Solvothermal synthesis is governed by two parameters: thermodynamic parameters and kinetic parameters.
The thermodynamic parameters involved in a solvothermal process are temperature and pressure. The solvents have different properties at above their boiling point (T b), especially at their critical point (T c). In terms of temperature, we can distinguish three different regions: superheated conditions (T b − 150 °C), hydro(solvo)themal conditions (150 − T c) and supercritical conditions (T > T c). The hydrothermal (solvothermal) reactions are mainly performed under mild temperature conditions (T < 400 °C). To explain how the physicochemical properties of the solvents can change by increasing the temperature and pressure, we will take water as an example.
The kinetic parameters are referred mainly to the reaction time and the nature of the precursors and the solvent. If the synthesis is performed in water, the pH and the ionic strength of the system have to be considered. The chemical composition of the reagents has to be adjusted to that of the target material. In addition, the concentration of the precursors and complexing agents seems to play a role in controlling the shape of the resulting nanocrystals. Various solvents showing distinct different chemical properties are used for solvothermal syntheses and include (1) polar protic solvents such as H2O, NH3, HF, HCl and HBr; (2) polar non-protic solvents like tetrahydrofuran and (3) non-polar solvents such as hexane, benzene, xylene and CO2.
Solvothermal solvents, critical temperature (T c) and pressure (P c) points and materials that can be synthesized
T c (°C)
P c (°C)
Materials that can be synthesized
Oxide, carbonate, silicate
Amide, imide, nitride
Oxide, hydrolysis sensitive compounds
Oxide, hydrolysis sensitive compounds
Oxide, sulfide, selenide
The physicochemical properties of the selected solvent play also an important role for orienting the polymorph of the final material. For example, the solvothermal synthesis of MnS can lead to metastable (β and γ) or stable (α) structural forms depending on the nature of the solvent. Using MnCl2 × 4H2O and SC(NH2)2 as reagents and either using water or ethylenediamine as solvents, the α polymorph of MnS is formed. However, with benzene as the solvent, the γ-phase is obtained, whereas with tetrahydrofuran the β-phase is obtained. The stabilization of different polymorphs can be attributed to the ability of the solvents to form stable and distinct complexes with the solved metal ions involved in the reaction process. With Mn ions, water and ethylenediamine form stable complexes such as, [Mn(H2O)6]2+ and [Mn(en)3]2+ which lead to the formation of the thermodynamically stable α-phase. However, for the metastable phases, a non-polar solvent like benzene is more appropriated to stabilize the γ-phase, whereas the polar solvent tetrahydrofuran is more suitable to obtain the metastable β-phase.
The type of materials synthesized hydrothermally are, in most cases, oxides, phosphates and silicates. The oxides can be simple oxides such as ZrO2, TiO2, SiO2, ZnO, Fe2O3 and Al2O3, or complex oxides such as BaTiO3, LiNbO3 and BaFe2O3 . The hydrothermal method can be used to prepare metastable compounds, such as tungstates (e.g., ZnWO4, Bi2WO6), vanadates (e.g., YVO4) and molybdates (e.g., CdMoO4, Bi2MO6, BaMo2O7)  and zeolites, for example Na n Al n Si96−n O192x16H2O, n < 27 (ZMS-5), and Na8Al8Si140O96x24H2O (modernite) , many of which may not be obtained when using classical synthesis reactions. The method is also suitable to prepare high purity monocrystals in large scale for industrial purposes, such as, quartz, ZnO, sapphire and ruby. Moreover, non-aqueous solvothermal synthesis can be also used to synthesize non-oxidic materials such as nitrides, imides, sulfides and telurides by choosing a suitable solvent (Table 3). For example, the ammonothermal method has considerably increased the research interest over the past 20 years, since it is one of the few techniques leading to group III bulk nitrides. In comparison to other techniques, the ammonothermal method allows crystal growth on native substrates and growth of the initial native substrate itself in high quality. Increasing commercial efforts are directed to the ammonothermal growth of GaN and AlN on native substrates. GaN and AlN are semiconductors with wide band gaps used as base for optoelectronic and electronic devices (e.g., light-emitting diodes (LEDs), high electron mobility transistors, lasers with high optical storage capacity) .
Summary and outlook
Since the discovery by early man that rocks could be modified to make tools, the world has demanded materials with increasingly complex functionality. Advanced materials are essential to address challenges in clean energy, national security and human well-being. Subsequently, one of the primary research goals has become the development of new materials to reach advanced functional properties, such as chemical reactivity, thermal stability, catalytic activity or optical, magnetic and electronic properties, to achieve better performances and novel applications. Naturally, this has to led to the improvement of the classical synthetic techniques and the development of new synthetic approaches.
Many inorganic materials, such as metal oxides or carbides, can be prepared fairly simply by mixing powdered reactants and heating them to give the desired product. While reaction conditions are relatively easy to achieve (furnace technology being well established), there are some drawbacks, such as the inhomogeneity of the starting materials and the difficulty to control particle size and morphology. Therefore, novel synthetic methods, many of them detailed in this chapter, have been developed in the past decades to prepare advanced ceramics as an alternative to the solid-state synthesis of materials to overcome the aforementioned limitations.
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