Results indicated that the nanomontmorillonite formed an intercalated structure and complete exfoliation was not observed under the experimental conditions used. Important properties of fiber-forming polymers may be improved using montmorillonite as a filler [ 43 ]. In general, clay mineral nm showed flame—retardant effects as assessed by a reduction in the peak heat release rate for various thermoplastic polymers including polystyrene, polyamide-6, polypropylene, polyamide, poly methyl methacrylate , polyethylene, and ethylene vinyl acetate EVA.
Other improvement observed includes storage modulus, stiffness, and heat distortion temperature, and the reduction in water absorption relative to virgin nylon Electrospinning is the technique successfully used for the production of a variety of polymer nanofibers.
The properties of polymer nanofiber produced through electrospinning are influenced by melt viscosity, surface tension, dielectric permeability, electric field strength, solvent properties to evaporate, polymer molecular weight, and concentration. Interest in using the clay was observed to obtain the flame-retardant properties in cotton fiber [ 44 ]. The two types of local clay samples study to evaluate the flame-retardant effects on bleached cotton fabric. Aqueous water dispersion was applied to bleached cotton fabric.
Flame retardancy was improved as indicated by the ease of ignition and the char length of burnt cotton fabric. The growing variety of montmorillonite utilization is perhaps indicated by the use of bentonite in the study to form a part of deep geological repository for spent nuclear fuel.
Pure homo ionic Ca-montmorillonite may be considered bentonite-similar system to obtain information on natural bentonite behavior. Water-saturated structure and porosity of Ca-montmorillonite were studied using X-ray diffraction, small angle X-ray scattering, nuclear magnetic resonance, transmission electron microscopy, and ion exclusion. The obtained results indicate multiple porosity for the bentonite structure [ 45 ].
Multiple porosity model supports two different groups of water present in bentonite: one present in the interlamellar space and the other found in the volume between clay stacks. Synthetic mica-montmorillonite SMM shows Bronsted acidity. The evaluation of SMM for adsorption energies using ammonia and pyridine showed the acid strength. The composition of SMM platelets influences the acidity [ 46 ]. Interesting results were obtained in the study explaining the Bronsted acidity in relation to the platelet structure. Bronsted and Lewis acid catalytic sites in montmorillonite provided the useful applications [ 47 ].
The exchangeability of interlayer cations, through ion exchange, helps in altering the acidic nature. Modified montmorillonite types known as montmorillonite-K produced by the calcinations of montmorillonite were found as efficient catalysts. Sodium montmorillonite NaMMT is indicated to accelerate the curing of urea-formaldehyde resin. In acid-curing conditions, urea-formaldehyde resin was used as adhesive for plywood and wood particleboard. Cross-linking of urea-formaldehyde in the presence of NaMMT produces plywood with improved water resistance. An accelerating effect on urea-formaldehyde curing was observed in differential scanning calorimetry results.
Dry internal bond strength of wood particleboard increases with small additions of NaMMT [ 48 ]. Green chemistry is a demanding approach in organic synthesis, where the release of hazardous gases and liquids is undesired. Environment damage and ecological balance are required to be least affected. Montmorillonite is a solid acid used in organic synthesis. Natural and modified clays, including montmorillonite, received significant interest as catalyst Section 7. The use of montmorillonite as a greener catalyst in organic synthesis is reviewed [ 49 ]. Good mining practices have shown bentonite as environmentally not hazardous provided dust abatement mask used.
Bentonites demonstrate good performance as sealant and absorbent and used as barrier for landfill and toxic waste repository. Montmorillonite particles, depending upon the size range, may come in contact with living species. The health and safety concern related to montmorillonite particles received interest for study.
The nature and distribution of inorganic contaminants, such as metals and metalloids like arsenic, iron, and lead, in clay-bearing rocks, may introduce the environmental concerns. These environmental factors may influence the use of clays in natural and industrial applications [ 50 ]. Information obtained on environmental effects for industrial minerals, including various clay types, sand, gravel, and crushed stone may not be applicable to montmorillonite composition, and studies will be more useful that is clearly based on any montmorillonite structure.
Information on occupational exposure to bentonite dust in mines, processing plants, and user safety is limited [ 5 ]. Bentonite comprising montmorillonite as major fraction, and kaoline have not shown local or systematic adverse effects in cosmetics. However, these were indicated to cause reduced toxicity toward aquatic organisms. Particle size and the chemical structure of montmorillonite are two obviously different aspects. The increased concerns in the toxicity of airborne fine 0. Epidemiological studies indicated an increase in morbidity, and mortality was associated with the rise in airborne particles, particularly in ultrafine size range.
The decreasing potencies of quartz, kaolinite, and montmorillonite to introduce lung damage were related to their known relative active surface areas and surface chemistry. Therefore, handling of ultrafine particles requires more vigilant control to abate health effects. Natural clay particles are smaller than 0. These nanoparticles can enter human body, reaching vital organs through blood circulation.
The possibility is therefore indicated for tissue damage. Nanosized particles coming in contact with the tissue may introduce toxicity and health concerns; however, particles incorporated in the bulk materials or polymer products will not be exhibiting such effects. Therefore, there is clear partition line in risk assessment for two types of particle composition: ultrafine, and agglomerated and bulk material. The properties of nanosized particles are different from the bulk material of the same composition.
Beneficial effects of montmorillonite are known in the form of a voluminous product used effectively and discussed in Section 7. It is important that there are several standards practiced, mainly in industrialized regions, addressing the health and safety risks at work places. Therefore, it is unlikely that the future would eliminate montmorillonite uses. Clay minerals have long been benefiting the human and society. Montmorillonite, an important clay type, has received growing interest in utilization as an additive in polymer and products for enhanced effects.
Bentonite is an important source of montmorillonite in nature. The basic molecular structure comprises silica tetrahedron and aluminum octahedral. Chemical composition, ionic substitution, layer structure, and particle size of natural clay minerals are important to introduce the functional properties and effects in the application of montmorillonite.
Sheet structure was used to classify the clay minerals, and chemical composition was used for nomenclature. Important applications of montmorillonite include uses additive for food, health, and stamina, for antibacterial activity, improved polymer performance; as sorbent for nonionic, anionic, and cationic dyes; and as green chemistry catalyst in organic synthesis, and so on. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.
Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Mansoor Zoveidavianpoor. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists.
Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Abstract Clay mineral is an important material available in nature. Keywords clay montmorillonite bentonite water cationic exchange. Introduction Historically, clay mineral has long benefitted human life and civilization.
This group has relatively larger member minerals and sometimes considered as separate group not as part of clays. Montmorillonite or smectite 0.
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Illite 1. Vermiculite 1. Chlorite Variable. Bentonite Bentonite is an important rock of clay found in nature.
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Montmorillonite structure The physical structure of montmorillonite particle is generally perceivable in sheets and layers. Crystal system Monoclinic 3. Hardness 1—2 on Mohs scale, soft, possess fine-grained occurrence 4. Fracture Irregular, uneven 5. Cleavage Perfect 6. Luster Earthy, dull 7. Transparency Translucent 8. Color White, buff, yellow, green, rarely pale pink to red presence of high valance Mn produces pink to red coloration. Montmorillonite in nanoclay Montmorillonite is the basic raw material used in producing nanoclay.
Functional properties 5. Cation exchange property Cation exchange capacity is a property of soil introduced by clay and organic matters. Electrical conductivity Clay particles are the porous materials. Heat resistance Montmorillonite is a good heat insulator, and heat-resistant effects are obtainable using it as an additive in any substance. Water sorption Water sorption is an important characteristic of natural clay particles. The montmorillonite platelets can be negatively charged when tetrahedral substitution of Si by Al in two tetrahedral sheets, or octahedral substitution of Al by Mg in central octahedral sheet.
Functional utilization The addition of montmorillonite in material, polymer, and products may result in significant enhancement in the required performance. Resistant to nausea and diarrhea Dietary toxins, bacterial toxins, and metabolic toxins can be absorbed by clay to resist nausea, vomiting, and diarrhea. Supportive to health and growth The use of calcium montmorillonite Nova Sil clay type in human diet can diminish health- harming effects of aflatoxin-contaminated food.
Resistance to tooth decay Another important obtainable effect is the resistance to tooth decay. Intercalated form, where a polymer or a substrate molecule can be between platelets. Drug delivery system Adsorption and swelling characteristics of montmorillonite are useful in drug delivery systems.
Adsorption of dyestuff Effluent loading, from dyeing industries and textile-processing units, to natural environment is a serious concern. Adsorption of toxic heavy metals An important application of adsorption properties of montmorillonite is seen in the removal of toxic heavy metals from aqueous solution. Montmorillonite in biopolymer Biopolymer modification using montmorillonite as nanofiller is found to improve the thermo- mechanical properties. Effects in fiber-forming polymer Important properties of fiber-forming polymers may be improved using montmorillonite as a filler [ 43 ].
Flame-retardant finishing of cotton fiber Interest in using the clay was observed to obtain the flame-retardant properties in cotton fiber [ 44 ]. Geological repository for spent nuclear fuel The growing variety of montmorillonite utilization is perhaps indicated by the use of bentonite in the study to form a part of deep geological repository for spent nuclear fuel. Environment concern Green chemistry is a demanding approach in organic synthesis, where the release of hazardous gases and liquids is undesired.
Conclusion Clay minerals have long been benefiting the human and society. Conflict of interest The author declares no conflict of interest in publishing this chapter. More Print chapter. In this work we study a complex interplay of crystal structure, surface termination, extrinsic and intrinsic defects as well as electronic structure for BiTeI The results are based on experiments on cleaved single crystals by several complementary surface science methods as well as on ab initio DFT and model calculations.
In accordance with previous findings [ 7 ] the investigated BiTeI surfaces consist of I- and Te-terminated domains. The boundaries between these domains are not affected by the presence of step edges, which confirms stacking faults in the bulk crystal to be the origin for the simultaneous appearance of different terminations.
An analysis of the atomic structure reveals that the two terminations feature different characteristic defect types in the surface and subsurface layers. We show that the electronic structure of BiTeI is prone to time-dependent modifications, even under ultra-high vaccuum conditions, causing energy shifts in the band structure on the order of several meV. Similar effects are also observed after room temperature deposition of Cs adatoms leading to the conclusion that adsorption of residual gas atoms gives rise to the time-dependence of the surface electronic structure.
Interestingly, the adatoms accumulate on I-terminated surface areas and therefore do not affect electronic states of the Te-terminated domains. We reconcile this peculiar preferential adsorption behaviour by a calculation of adsorption energies and diffusion lengths that turn out to differ considerably for the two terminations. The used experimental setup is designed for a comprehensive analysis of the geometric and electronic properties of solid surfaces, both in real space and in reciprocal space.
BiTeI single crystals were cleaved at room temperature along the direction with a rod glued on top of the samples at pressures below mbar. STM tips were electrochemically etched from a polycrystalline tungsten wire, prepared on a noble metal surface before scanning BiTeI Additionally, it enables a fast data acquisition which is quite important for samples exhibiting aging processes.
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In order to obtain such maps we started at decreasing the voltage gradually each 26 scan lines by to V. The energy resolution was approximately 25 meV. XPS measurements were done with Al excitation Both synthesis and growth of BiTeI were performed in the same evacuated quartz ampoule. Since elementary iodine is very volatile, it is more convenient to prepare first. The excess of iodine was taken in order to compensate its loss during evacuation.
On the other hand, the residue of iodine may be easily distillated to the cold end of the ampoule. According to the published data, BiTeI melts congruently at [ 37 ]. Therefore Bi, Te and taken in proportion 2 : 3 : 1 were heated to a temperature higher than the melting point. Crystal growth was done by a modified Bridgman method with rotating heat field [ 38 ]. After pulling the ampoules through the vertical temperature gradient with a rate of the furnace was switched off. The calculations were performed within the density functional formalism as implemented in the VASP code [ 39 , 40 ].
We used the all-electron projector augmented wave [ 41 , 42 ] basis sets with the generalized gradient approximation of Perdew, Burke and Ernzerhof [ 43 ] to the exchange correlation potential. Relativistic effects, including spin-orbit coupling, were fully taken into account.
When modeling the Cs diffusion on the BiTeI surfaces, the spin—orbit interaction was neglected. Experimental lattice parameters [ 45 ] of BiTeI were used in the calculations. The surface of BiTeI has been reported to suffer from stacking faults leading to mixed surface terminations [ 7 ].
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Electronic band structure of the single Te-terminated surface was calculated using the slab with I-terminated side passivated by a hydrogen monolayer. The atomic positions for the first two surface layers of the free surfaces of the slab as well as position for passivating hydrogen layer and for Cs adatom at different adsorption sites were optimized. On the individual terraces one can discern an additional mottled texture resulting from two types of surface areas that are about 10— nm wide and differ in contrast.
This observation indicates that the two surface areas correspond to two chemically different terminations [ 44 ]. We will show later that one type of surface area is terminated by an I layer and the other one is terminated by a Te layer. Therefore, we will from now on use the terms I termination and Te termination to distinguish between the two different types of surface areas.
It is important to note that both surface areas cover step edges without any apparent influence of the latter see for example the position marked with an arrow in figure 1 b. The diffraction image in figure 1 c represents a superposition of the signals from both terminations because the electron beam spot by far exceeds the length scale on which the two surface terminations vary. Based on figure 1 c we determine the in-plane lattice constant to 4. In the following we discuss the two different terminations of BiTeI surface on the atomic scale. Defects within the surface layer are marked with circles in figures 2 a and c.
Those defects with dark appearance correspond to vacancies or antisites, whereas those with bright appearance correspond to antisites or adatoms in on-top position. In addition to these surface defects, both terminations exhibit several on-surface and subsurface defects which are marked by black squares, labeled A—D figure 2 a and E—H figure 2 b , and presented at higher magnification in figures 2 b and d , respectively. The total densities of defects visible in these topographic images scanned surface area each amount to approximately 50 and 30 for the I- and Te-teminated surface, respectively.
Panel A in figure 2 b shows a triangular cluster that consists of six adatoms. It is centred in hollow positions of the surface I layer. For example, the horizontal line in the bottom part of figure 2 a indicates such a material transport during the scan. The atomic species forming these clusters is presently unknown, but the most plausible origin are Te atoms that remain on the surface after the cleaving and arrange in small clusters.
Figure 2. Four different types of defects are identified and labeled by A, B, C, and D.
Additional defects in the first layer of both terminations are highlighted by black circles in a and c , where a dark bright appearance corresponds to either an antisite or a vacancy either an antisite or an adatom in on-top position. For the characterization of the remaining seven subsurface defects we follow the procedure developed by Jiang et al in [ 30 ]. The crucial assumption is that those atoms which are connected with the defect by a straight sequence of the bonds are most strongly affected.
Due to the threefold symmetric crystal structure, the surface corrugation resulting from subsurface defects must also exhibit a threefold symmetry. Because of the above-mentioned straight bond, however, the lateral size of the defect structure grows linearly as the depth of the defect site increases.
We start with the simplest defect of the I-terminated surface which is shown in panel B of figure 2 b. It appears as a roughly triangular depression dark which covers three iodine sites. According to the model of Jiang et al [ 30 ] the defect must be located in the second layer Bi which affects the three neighbouring surface atoms. This defect occurs only rarely indicating a low defect density in the Bi layer. Panel C also shows a triangular feature but in this case the depressions are separated by three iodine surface atoms which maintain their original corrugation.
According to its larger lateral size we attributed this feature to a defect in the third layer Te , possibly a vacancy. Panel D presents another defect which is characterized by three triangular clusters of iodine atoms triangles in panel D with an reduced apparent height. The defects in panel E and F exhibit a regular threefold symmetry and are probably caused by antisites or a vacancy.
Interestingly, the defects in panels G and H are asymmetric marked by an arrow and thus do not obey the threefold symmetry of the lattice. One possible origin for this could be a vacancy in the second layer Bi which was filled by an atom of the third layer I leaving behind a vacancy on this position. In this scenario the combination of second and third layer features breaks threefold symmetry.
Indeed, close inspection reveals that defect in panel H corresponds to a defect as shown in panel G with an adsorbate at the open end. We speculate that the high electronegative charge of iodine in the second layer leads to the pinning of adsorbates and adatoms resulting from the fracturing process. Interestingly, no defects below the first triple layer were observed. The clusters on the I termination and the bright adsorbates on the Te termination are also visible in the topographic image in figure 3 a taken at low temperature.
At the domain boundaries between two terminations we find an increased DOS indicated by arrows. This observation points to an edge effect on the electronic charge distribution.
Figure 3. The distributions of clusters figure 2 b A on the I termination and of adsorbates figure 2 d F on the Te termination are visible. Arrows in b indicate an increased density of states at the domain boundaries between both terminations. Due to potential changes in the near-surface layers of BiTeI and related compounds strongly spin—orbit coupled surface states split off from the bulk valence and conduction band edges [ 6 , 7 , 9 , 12 , 13 , 15 ].
The potential change is negative for the Te-terminated surface and positive for the I-terminated surface. Figure 4 shows the calculated electronic structure of BiTeI slab. The simulated slab is terminated by Te layer on one side and by I layer on the opposite side. Hence, the calculated band structure in figure 4 a can be regarded as a superposition of the surface electronic structures corresponding to the two different terminations.
Bands denoted by red blue markers correspond to electronic states that are predominantly localized near the I-terminated Te-terminated surface. The two most prominent Rashba-split features are an electron-like surface band on the Te termination with a band minimum at eV and a hole-like surface band on the I termination with a band maximum at 0.
In general, we find that the negative at the Te-terminated side of the slab leads to a downward shift of energies of the electronic states trapped in then near-surface potential while the positive at the I-terminated side of the slab produces an upward shift. As a net result, the surface states arising in the system with mixed termination overlap the gap. Figure 4. Panel a shows the result for a BiTeI slab that incorporates an I-terminated surface at one side of the slab and a Te-terminated surface at the opposite side.http://derivid.route1.com/ui-1-oso-cuento-para-aprender-a-leer.php
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