NASICON composition

Other ceramic ion conductors similar to /3-alumina were introduced by Goodenough et al in 1976 they belong to the so-called Nasicon family. Nasicon is a solid solution of the composition Na3Zr2Si2P012, exhibiting a Na+ conductivity of 0.2 S/cm at 300°C. Numerous Nasicon derivatives,... 

As well as the solid solution formula given above, there have been suggestions that compositions off the join may also give single-phase NASICON. Part of the problem in determining solid solution stoichiometries and limits in materials such as NASICON arises because of the... 

Figure 52. The free enthalpy of mixing of the fast ion conductor Nasicon (Nai+ ZiyPsyM/tiJ (y= x/3) obtained from emf measurements as a function of composition.272 The shape of the curves indicates the stability of a thermodynamic two phase system at T< 615 K. Reprinted from U. Warhus, J. Maier, and A. Rabenau, J. Solid St Chem., 72 (1988) 113-125. Copyright 1988 with permission from Elsevier.

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

FIGURE 12.12 Examples of FGM CMCs (left) the combination of NLM Nicalon SiC fiber reinforced zirconia (in white) and mullite (in black) matrices (right) a Nasicon matrix reinforced with mullite (Nextel) and SiC (NLMTM) fibers (see the sketch). (Reprinted from Colomban, R, Process for fabricating a ceramic matrix composite incorporating woven fibers and materials with different compositions and properties in the same composite, Mater. TechnoL, 10, 89, 1995. With permission.)... 

The measurement of local mechanical properties is an important step in understanding of the macroscopic behavior of multiphase materials. The indentation hardness test is probably the simplest method of measuring the mechanical properties of materials. Figure 12.2b shows the evolution of the microhardness as a function of the thermal treatment temperature of a Nasicon sample. The use of load-controlled depth-sensing hardness testers which operate in the (sub)micron range enables the study of each component of the composite more precisely. 

Mouchon, E., and Colomban, R, Microwave absorbent preparation, mechanical properties and RF/microwave conductivity of SiC (and/or mullite) fibres reinforced NASICON matrix composites, J. Mater. ScL, 31, 323, 1996. 

Transition metal (TM) ions can be readily incorporated particularly into phosphate glasses. Since TM ions give rise to characteristic spectra in the region of near IR to near UV, their characterization is relatively easy. An example is the study of a series of glasses having chemical compositions of NASICONs. NASICON is an acronym for sodium superionic conductors of the general formula A B2(P04)3, where B is generally a TM ion or elements like Zr, Ge, Sn, etc. (or their combination) and A is an alkali metal, whose number n in the formula is dependent on the... 

As in 3 alumina, the sodium in NASICON can be replaced with other ions [60]. The conductivity increases with increasing silicon content, and the maximum conductivity occurs at a composition of about Na3Zr2PiSi2Oi2 (x 2) [61, 62]. The conductivities of some NASICON materials are shown with the ranges of conductivities for YSZ and 3 alumina in Figure 13.5 [63-68]. 

Wang, L. and Kumar, R.V. (2005) A SO2 gas sensor based upon composite Nasicon/Sr-p-AhOj bielectrolyte. Mater. Res. Bull., 40, 1802-15. 

Magnetic susceptibility measurements of two of the synthesized compositions strongly suggest, that the majority oxidation state of vanadium in these NASICON phases is The respective measured values are 1.57 BM for NbVP30i2 and 2.13 BM for SbVP30i2. Additional measurements, including XPS are necessary to more exactly define the oxidation state of the vanadium. 

The composition of NbTio75Vo25P30i2 exhibits an XRD pattern characteristic of a NASICON structure (Table 1 and Figure 1) and has a surface area of 16.9 m /g. Its structural features lie intermediate between those of NbTiP30 2 and NbVP30i2 NASICONS, as might be expected. 

Maruyama et al.[57] of Tokyo Institute of Technology, Japan, reported COj sensing characteristics of galvanic cells composed of Na" conductor like NASICON or P-Al Oj, and NajCOg as an auxiliary electrode. The cell composition is expressed as follows. 

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