Nanocrystalline Powder

Las but not least, sample preparation is also an important issue. If we want to examine nanocrystalline powder samples. The grain size must be just a few nanometers, the layer, formed by these nanocrystals must be as thin as possible (to minimize dynamic difiraction), continuous and self-supporting. In many cases not all these requirements are fulfilled simultaneously. The nanocrystalline material to be studied is frequently present on a thin supporting carbon layer. In such cases peak decomposition can not yield an acceptable fit unless the presence of the amorphous material (in the form of a few diffuse rings) is taken explicitly into account in the model to be fitted. The size of the background is also affected by scattering in such a carbon support. 

Nanosized metal sulfide powders of Ag2S, CuS, FeS, Ga2S3. In2S3, MnS, NiS, and ZnS were synthesized for use as gas- and liquid-phase mercury sorbents. An aqueous-based synthesis method using the surfactant cetyltrimethylammonium bromide (CTAB) is described. The vapor- and aqueous-phase mercury-sorption characteristics of the nanocrystalline powders synthesized and of commercially produced Ag2S, AU2S. and AU2S3 are presented. 

The sensitivity of EPR to multiple coordination environments has been demonstrated in studies of Mn2+-doped CdS nanocrystals (63). In Mn2+ CdS nanocrystalline powders prepared by inverted micelle synthesis, four distinct resonances were observed and deconvoluted by varying experimental parameters including microwave power, microwave frequency, and temperature. The deconvoluted signals are shown in Fig. 18. Four distinct manganese species were detected through this experiment. A six line spectrum characteristic of isolated paramagnetic Mn2+ was observed at 300 K and below [multiline... 

The defective structure in nanocrystalline ceria based catalysts proved to have strong effect on the OSC. Mamontov et al. (2000) reported the neutron diffraction studies of the atomic structures of nanocrystalline powder of ceria and ceria-zirconia solid solution. They found that the concentration of vacancy-interstitial oxygen defects has a direct correlation with the OSC. This effect is stronger than the correlation of surface area with OSC. Zirconia reduces ceria and preserves oxygen defects to retard the degradation of ceria-zirconia in OSC. Yan et al. observed the strong correlation between OSC and the lattice strain in nanosized ceria-zirconia, which could be measured via XRD (Si et al., 2004 Figure 11). 

Dezanneau et al. reported Lai cMn03+a nanocrystalline powders prepared by an acylamide polymerization sol-gel method (Dezanneau et al., 2002a,b, 2003). The composition analysis revealed that for La/ Mn < 0.9 the Mna04 phase was present, while for La/Mn > 0.9, the high oxygen excess led to considerable vancancies on cationic sites. The Curie temperature remained constant at 295 K for the former case, while decreased Curie temperatures were observed for the latter case, due to the increasing amoimt of Mn vacancies. 

Cherivin et al. developed a non-alkoxide sol-gel method for the preparation of homogeneous nanocrystalline powders of Lo.85Sro.i5Mn03 (Cherivin et al., 2006). Calcination of the dried gels resulted in the crystallization of single-phase products at 700 °C with discrete particles free of hard agglomeration (Figure 57). 

Mechanochemical processing has been used to manufacture nanocrystalline powders of nitride and carbide ceramics. The majority of systems involve milling of the metal precursor with a source of carbon or nitrogen. The source of carbon or nitrogen has typically taken the form of the element itself. However, a variety of other reagents have also been used. For example, Zhang et al. reported the synthesis of titanium nitride by milling titanium metal with pyrazine in a benzene solution. 

Various kinds of low-temperature processing have been used in materials science and technology. Among these, the cryochemical method is believed to be the first from the work of Landsberg and Campbell who dealt with the synthesis of nanocrystalline powders of W and W-Re alloys. This new synthesis technique attracted the attention of several groups who used this new method for the... 

Another recent class of magnetic materials attracting a lot of attention is molecular magnets. Nanocrystalline powders of new molecular magnets. 

The main approach to reach higher sensor sensitivity is based on maximization of the electron depletion layer compared with the semiconducting core this can be realized by decreasing the particle size down to the scale of the depletion layer thickness.6-8 Oxide semiconductors in the form of nanocrystalline powders,9-11 nanorods,12 nanowires,13-15 nanotubes,1617 and nanobelts18 with a high surface area/ volume ratio have been studied intensively as highly sensitive materials. 

WPPM applications have been reported in several recent publications concerning nanocrystalline powders and heavily deformed ceramic and metallic... 

Carbon monoxide (CO). Between 80 and 200 °C, tungsten hexacarbonyl forms. From 1000°C upward, bulk tungsten is carburized W + 2COWC + CO2 resp. 2W + 2CO W2C + CO2. Nanocrystalline powders start to carburize already at about 550 °C. 

An alternative method for producing ceria-based nanocrystalline powders is that of urea-combustion [133]. In this case, the combustion synthesis of (Ce02)o.92 (Ln203)o.o4(CaO)o.o4 systems, where Ln = Y, Gd or Sm is described, but no investigation into their use in fuel cells was reported. 

In general, these methods are used for the production of nanocrystalline powders which may be further compacted via techniques such as hot-pressing [157, 158] or magnetic pulsed compaction [159, 160]. In addition, other types of nanoionic material maybe prepared, such as nanometer-thin films, using techniques including molecular beam epitaxy [161], pulsed laser deposition [162] or spin-coating methods [163]. Novel structures, such as core-shell [164—166] and multi-layered [167, 168] (so-called onion structures) materials, may also be produced in this way. 

Lithium Niobate Lithium niobate (LiNbOj) is widely used as a photonic material, and has a wide range of applications in lasers, nonlinear optics, optical communications, optical memories, and diffractive optics [25O[. It is not generally considered as an ionic conductor. However, since Li NMR measurements first revealed a rapid Li ion motion in nanocrystalline powders prepared by ball-milling [251], the material has attracted considerable interest and has proved to be a good model system [ 169]. As stated earlier, it is now known that the ball-milled material has a considerable amorphous content, as shown by EXAFS [169,252] and HRTEM [169],... 

X Alloys from amorphous or PM Al-Mg nanocrystalline powders Vaporquenched alloys... 

As mentioned briefly in Sect. 4, another way of stabilizing the tetragonal structure at room temperature is the formation of nanocrystalline powders or nanograined sintered specimens. To obtain powders of dense PSZ compacts at room temperature, the material has to contain crystals or grains below a certain critical size, which... 

The surface activity of nanoparticles is dramatically different from that of macroparticles. Macroparticles are loosely those in excess of 100 nm, although this borderline is somewhat arbitrary. Nanocrystalline powders can have enormous surface areas. When one considers the fact that a 3-nm diameter particle has about 50% of its atoms on the surface, these types of particles can react as nearly stoichiometric reagents. 

The morphologies of calcined Ti02-starch samples prepared in cyclohexane were observed by SEM and AFM (Fig. 4). The formation of particles with a diameter of 20 nm was observed on the surface of Ti02 samples using SEM. This observation is consistent with the crystallite size calculated by the Scherer equation (Table 2). Our preparation is of high interest since nanocrystalline powders are obtained at low temperature in short time compared to the... 

Stability of nanocrystalline powder samples, synthesized in Sr-Ba-Fe-Co-0 system by the mechanochemical activation route in Ha atmosphere is investigated. Samples of various compositions were used to establish the main reasons of samples destruction. XRD, EPR and XPS methods were employed to characterize the samples before and after treatment in the Ha atmosphere (600 °C, 1 h). Essential content of cobalt in a sample was found to degrade its stability, whereas an insertion of additional amount of FeaOj at the end of the mechanochemical activation enhances stability. 

We have reported a novel formation of nanosized boehmite powder in flaky morphology by a modified sol-gel route. " The sol-gel method is a well-known chemical synthesis route with high purity, high chemical homogeneity, lower calcination temperatures and good control of particle size. It is a versatile method not only to synthesize nanocrystalline powder but also nanostnictured... 

LUMINESCENCE OF Ho LU2O3 NANOCRYSTALLINE POWDERS AND SINTERED CERAMIC... 

In this paper translucent Yb , Ho LU2O3 ceramics were fabricated at 1850°C for 3 h in flowing H2 atmosphere using co-precipitated nanocrystalline powders as starting materials. The luminescent properties of the nanocrystalline powders and sintered ceramic were investigated. 

Lui.896Ybo 1H00.004O3 nanocrystalline powders were synthesized by co-precipitation method, which was described in detail before The as-prepared powders were pressed under 30 MPa into disk with 12-mm diameter, and then isostatically cold pressed under 200 MPa pressure. Finally, the disks were sintered at 1850°C for 3 h in flowing H2 atmosphere. Therefore, the translucent ceramics were obtained. 

Luminescence of Yb, Ho + Lu203 Nanocrystalline Powders and Sintered Ceramic... 

Fig. 3 Upconversion emission spectra of (a) sintered ceramic (b) and nanocrystalline powders Lui 896Ybo,rHoo.(XM03. Inset details of upconversion emissions in the range of 350-510 nm.

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