**10.4. Solid oxide fuel cells**

**10.3.5. Apatite-mullite glass-ceramics**

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

**10.3.6. FAP-anorthite-diopside glass-ceramics**

ceramics was found by HILL et al [74].

**10.3.7. Apatite-wollastonite ceramics**

**10.3.8. Oxyapatite glass-ceramics**

mixed with sacrificial PMMA microbeads as the templates [75].

energy [54],[73].

Apatite-mullite glass-ceramics crystallize from the glass of generic composition SiO2-Al2O3- P2O5-CaO-CaF2 to form an osseoconductive apatite phase existing as spherulites within a mullite matrix. The formation of apatite spherulites is accompanied by a depletion zone, from which calcium, phosphate and fluorine is taken and added to the growing crystal. The depletion zone also inhibits the formation of further apatite crystals in the immediate vicinity due to the glass' compositional similarity with a mullite composition. As mullite begins to crystallize, there is interdependence between the growth of apatite and mullite. Spherulitic grain boundaries in partially devitrified apatite-mullite ceramics act as crack promoters, offering preferential paths to propagation due to the grain boundary interfacial surface

There is a considerable interest in oxyfluoride glasses and glass-ceramics for laser amplifiers and up-conversion processors. Fluoride-containing crystals have low phonon energies and apatite crystals, in particular, are good host phases to adsorb rare-earth ions. Fluoridecontaining glass-ceramics also often crystallize on a nanoscale, which is an added advantage, since optically transparent materials are required for the applications such as fiber amplifi‐ ers. The evidence of the nanoscale crystallization in an FAP-anorthite-diopside-based glass-

Wollastonite-hydroxyapatite ceramics was successfully prepared by a novel method, corre‐ sponding to the thermal treatment of a silicone embedding micro- and nanosized fillers in air. CaCO3 nanosized particles, providing CaO upon the decomposition, acted as "active" filler, whereas different commercially available or synthesized hydroxyapatite particles were used as "passive" filler. The homogeneous distribution of CaO, at a quasi-molecular level, fa‐ vored the reaction with silica derived from the polymer, at only 900°C, preventing extensive decomposition of hydroxyapatite. Open-celled porous ceramics suitable for scaffolds for bonetissue engineering applications were easily prepared from the filler-containing silicone resin

The crystallization of oxyapatite of the composition NaY9(SiO4)6O2 (P63/M, *a* = 9.334 Å and *c* = 6.759 Å, *c*:*a* = 1:0.7241, *V* = 509.97 Å, **Fig. 7**) from three different glasses from the SiO2-B2O3- Al2O3-Y2O3-CaO-Na2O-K2O-F glass-ceramics system with different F and B2O3 content after the Solid oxide fuel cell (SOFC) is an electrochemical energy-conversion device, which offers tremendous promise for delivering high electrical efficiency and significant environmental benefits in the terms of fuel flexibility (hydrocarbons and municipal waste) as well as clean and efficient (>70% with fuel regeneration) electric power generation. SOFC produces useful electricity by the reaction of fuel with an oxidant via the diffusion of oxide ions (or protons) through an ion-conducting solid-electrolyte layer [77].

**Fig. 8.** Schematic diagram of solid oxide fuel cell (SOFC) showing non-ion-conducting electrolyte (a) and proton-con‐ ducting electrolyte during its operation (b) [77].

SOFC is composed of a dense electrolyte layer that is sandwiched between two porous electrodes (i.e. cathode and anode) as shown in **Fig. 8**. SOFC can use either oxide ion (a) and/ or proton conduction through the electrolyte (b). Electrons generated through the oxidation of fuel on anode are accepted for the oxygen reduction on cathode, which completes the external circuit. The electricity is, thus, produced by the flow of electrons in the external circuit (from the anode to the cathode). Since the current is obtained via the diffusion of oxide ions (or protons) through a solid electrolyte, it becomes imperative to use high operating temperatures (~800 – 1000°C) for achieving high ionic conductivity (of ~0.1 S·cm−1) [77],[78].

The first fuel cell was invented in 1838 by an English scientist, WILLIAM GROVE. He named it "wet cell battery" or "Grove cell", which operated by reversing the electrolysis phenomena of water [77],[79]. The fuel cell, invented in 1839 by Grove, is an electrochemical device that converts the chemical energy of fuels directly into electricity and heat by electrochemically combining H2, CO/H2 or reformed hydrocarbons in fuel and an oxidant gas transported via an ion-conducting electrolyte. Direct combustion of fuels is eliminated here, which renders the fuel cells much higher conversion efficiencies compared to other conventional thermome‐ chanical methods. Moreover, with fuel cells, the power generation is virtually noise-free and can produce 0.9 times lower emissions of NOx and SOx per unit of power output compared to that of conventional technologies. Additionally, it is possible to use the fuel cells for com‐ bined heat and power (CHP or cogeneration) generation [77],[80].

The development of high-performance SOFC involves the material selection and operationrelated issues (of anode, cathode, electrolyte, sealant and interconnects). These challenges open up the myriad research opportunities for researchers in the field of SOFC. A list of various materials used in SOFC is presented in **Fig. 9**.


**Fig. 9.** Comprehensive list of various materials used in SOFC [77].

electricity by the reaction of fuel with an oxidant via the diffusion of oxide ions (or protons)

**Fig. 8.** Schematic diagram of solid oxide fuel cell (SOFC) showing non-ion-conducting electrolyte (a) and proton-con‐

SOFC is composed of a dense electrolyte layer that is sandwiched between two porous electrodes (i.e. cathode and anode) as shown in **Fig. 8**. SOFC can use either oxide ion (a) and/ or proton conduction through the electrolyte (b). Electrons generated through the oxidation of fuel on anode are accepted for the oxygen reduction on cathode, which completes the external circuit. The electricity is, thus, produced by the flow of electrons in the external circuit (from the anode to the cathode). Since the current is obtained via the diffusion of oxide ions (or protons) through a solid electrolyte, it becomes imperative to use high operating temperatures (~800 – 1000°C) for achieving high ionic conductivity (of ~0.1 S·cm−1) [77],[78].

The first fuel cell was invented in 1838 by an English scientist, WILLIAM GROVE. He named it "wet cell battery" or "Grove cell", which operated by reversing the electrolysis phenomena of water [77],[79]. The fuel cell, invented in 1839 by Grove, is an electrochemical device that converts the chemical energy of fuels directly into electricity and heat by electrochemically combining H2, CO/H2 or reformed hydrocarbons in fuel and an oxidant gas transported via an ion-conducting electrolyte. Direct combustion of fuels is eliminated here, which renders the fuel cells much higher conversion efficiencies compared to other conventional thermome‐ chanical methods. Moreover, with fuel cells, the power generation is virtually noise-free and can produce 0.9 times lower emissions of NOx and SOx per unit of power output compared to that of conventional technologies. Additionally, it is possible to use the fuel cells for com‐

The development of high-performance SOFC involves the material selection and operationrelated issues (of anode, cathode, electrolyte, sealant and interconnects). These challenges open up the myriad research opportunities for researchers in the field of SOFC. A list of various

bined heat and power (CHP or cogeneration) generation [77],[80].

materials used in SOFC is presented in **Fig. 9**.

through an ion-conducting solid-electrolyte layer [77].

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

ducting electrolyte during its operation (b) [77].

Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells. Lanthanum silicate with the composition of La10Si5.5Al0.5O26.75 was evaluated as an electrolyte with the electrode materials commonly used in SOFC, i.e. manganite, ferrite and cobaltite as cathode materials and NiO-CGO composite, chromium-manganite and Sr2MgMoO6 as anode materials by MARRERO-LÓPEZ et al [81]. This electrolyte has conductivity values higher than those of YSZ and comparable to most important solid electrolytes proposed for the intermedi‐ ate-temperature range, such as doped ceria and lanthanum gallate-based electrolytes. The chemical compatibility did not reveal appreciable bulk reactivity between silicate and many electrodes up to 1300°C [81]. Among several reported rare earth apatites, lanthanum sili‐ cates exhibit the ionic conductivity higher than their germinate counterparts [77].

On examining and modeling the probable conduction mechanism in apatite silicates, the atomistic simulation results suggest that the conduction in La9.33(SiO4)6O2 and La8Sr2(SiO4)6O2 takes place via interstitial and vacancy mechanism, respectively [82],[83]. The predicted pathway appears to be a complex nonlinear "sinusoidal-like" process for the interstitial oxygen migration along the c-axis (**Fig. 10**), while a direct linear pathway is predicted for oxygen migration via the vacancy mechanism [77].

ZENG et al [84] developed a model capable of prediction of oxygen ion conduction from relative Coulomb electronic interactions in oxyapatites and rationalized observed experimental trends reported in the literature. Two types of fundamental chemical property, i.e. the electronega‐ tivities and ionic radii of the constituents, control oxygen ionic conduction in oxyapatites. Those two properties were used to represent the relative charge densities and the distances between charged units, respectively, and then to formulate the relative Coulomb energy. It was found that this relative Coulomb energy is linearly correlated to the oxygen ionic

**Fig. 10.** Structural defect position and possible conduction mechanism along the c-axis representation of two adjacent unit-cells [77].

conductivity (in logarithmic form) in the oxyapatite systems. Doping a cation with large ionic radius and low electronegativity tends to increase the ionic conductivity of oxyapatite.

The c-axis-oriented apatite-type lanthanum silicate (La10Si6O27) ceramics was prepared by NAKAYAMA et al [85] via the sintering process under high magnetic field. The degree of orientation in the La10Si6O27 ceramics sintered at 1700°C was 48.1%. The conductivity of the caxis-oriented ceramics is about 0.5 orders of magnitude higher than that of non-oriented ceramics. Higher conductivity is caused by the orientation of oxide ions in the grains com‐ posing the ceramics, which are located along the c-axis and are responsible for the ionic conduction.
