Low Alkali Content Research Articles

Na-ion transport in borate and germanate glasses and liquids:A23Na and11B NMR spin-lattice-relaxation study

The temperature dependence of the spin-lattice relaxation times (${\mathrm{T}}_{1}$ ) of $^{23}$ Na and $^{11}$ B nuclides has been studied in a number of Na-borate and -germanate glasses and liquids in order to understand the effects of the type of network, concentration of the modifying cation, and network former-modifier interaction on the transport of Na cations. Our results indicate that at low alkali contents the dynamics of both the network former and the modifier are strongly coupled to the glass transition and mixed alkali effect on spin-lattice relaxation is absent. This can be explained by the lack of any percolation pathway at low alkali concentrations for the long-range diffusion of the Na cations. Eventually, such continuous pathways for diffusion form with increasing Na concentration (${\mathrm{Na}}_{2}$ O\ensuremath{\geqslant}15\char21{}20 mol %) and a mixed alkali effect becomes observable, indicating decoupling of the dynamics of the network-former and -modifier ions. Simulation of the ${\mathrm{T}}_{1}$ data with a barrier height distribution model also indicates such a transition from homogeneous to percolative transport with increasing ${\mathrm{Na}}_{2}$ O content. These phenomena are less pronounced in germanates than in borates, indicating a stronger coupling between the Na ions and the network in the latter.

Kinetics of Alkaline Deacetylation of Poplar Wood

The kinetics of alkaline deacetylation of cottonwood (Populus deltoides) was experimentally studied. In order to minimize resistance to mass and heat transfer, wood meal (40/60 mesh) was employed. Wood samples were isothermally treated with alkali which was maintained at constant concentration. The temperature range was 50-90°C and the alkali concentration was up to 9.6 g l -1 . A simple expression relating acetyl content in wood, temperature and alkali concentration in the liquid phase was found. The rate equation was of second order with respect to the acetyl content and of order 1.35 for the alkali concentration. The rate constant of the equation followed the Arrhenius law. A total deacetylation rapidly took place at high temperature. even at low alkali concentration. At low temperatures the deacetylation was completed in a few minutes then the alkali concentration was the highest in the range studied.

Far-infrared spectra of alkali germanate glasses and correlation with electrical conductivity.

The infrared spectra of 0.20{ital R}{sub 2}O-0.80GeO{sub 2} ({ital R}=Li,Na,K,Rb,Cs) and {ital x}Rb{sub 2}O-(1{minus}{ital x})GeO{sub 2} (0{lt}{ital x}{le}0.27) glasses were measured in the reflectance mode and analyzed by the Kramers-Kronig technique to investigate the nature and composition dependence of metal ion sites in germanate glasses. The deconvolution of the far-infrared profiles showed that in glasses of low alkali content ({ital x}{le}0.075 for {ital R}=Rb) alkali ions occupy one type of sites ({ital M}), while for higher alkali contents two types of site ({ital L} and {ital H}) were found. The ion motion frequencies in these sites are in the order {nu}{sub {ital L}}{lt}{nu}{sub {ital M}}{lt}{nu}{sub {ital H}}, and increase with increasing alkali oxide content. Factor group analysis of the alkali motion modes in analogous crystalline germanate compounds showed that the {ital H} band in glass can be assigned to ion motion in sites similar to those in the crystal. The low-frequency band ({ital L}) was attributed to ion motion in {open_quote}{open_quote}secondary{close_quote}{close_quote} energetic sites, whose coordination numbers and charge density are correspondingly larger and smaller than their optimum values. The presence of {ital L} sites is the cause of the extra absorption exhibited by glasses at low far-infrared frequencies, as comparedmore » to the crystals of similar composition. {ital M}-type sites were shown to be the precursors of {ital H} sites, but for the organization of the latter a minimum alkali oxide content is required. The comparison of activation energies for conductivity calculated on the basis of the free-ion model with observed values suggests that long-range ion movement is probably facilitated along {ital M} and {ital H}-type sites. {copyright} {ital 1996 The American Physical Society.}« less

Fracture behavior of Al2O3/SiC-platelet composites

Mechanical behavior of hot-pressed SiC platelet reinforced alumina composites has been analyzed as a function of SiC platelet content for two different alumina matrix powders. Fracture toughness and flexural strength at temperatures ranging from 25 to 1200 °C, R-curve behavior, and thermal shock resistance have been determined. Small differences in the impurity content of the starting Al2O3 powders strongly determine the microstructure and the mechanical behavior of Al2O3/SiC-platelet composites. Low alkali content alumina led to composites with large matrix grain size which presented spontaneous microcracking. At high temperature, a high viscosity liquid phase is formed that shields cracks enhancing mechanical properties and R-curve behavior. A small amount of impurities reduced Al2O3 matrix grain size and avoided spontaneous microcracking. Enhanced fracture toughness (up to 30%) at room temperature, R-curve behavior, and thermal shock resistance were achieved for these materials.

Mechanisms of the influence of externally supplied NaCl on the expansion of mortars containing reactive aggregate

Translation from Proceedings of JSCE, 25, No. 502, 1994 This study aims at revealing the mechanisms of the influence of NaCl externally supplied on the expansion of mortars due to the alkali–silica reaction. In reactive aggregate-bearing mortars with relatively low alkali contents, the promotion of their expansions in 1 M NaCl solution at 38°C was caused by a rise of OH− ion concentration in the pore solution. The increase in OH− ion concentration in the pore solution in mortars was responsible for the intrusion of Cl− ions into mortars containing a reactive aggregate. In reactive aggregate-bearing mortars with a relatively large amount of added NaOH, at least a part of unusually large expansions of the mortars was caused by the delayed formation of ettringite during the immersion in NaCl solution.

Correlation between far infrared absorption and electrical conductivity of rubidium germanate glasses

Abstract The far infrared spectra of x Rb 2 O-(1 − x )GeO 2 (0 x ≤ 0.27) glasses show that at low alkali content ( x ≤ 0.075) Rb alkali ions occupy one type of sites (M), while for higher alkali contents two types of sites (L and H) exist, such that the corresponding ion motion frequencies are in the order ν L ν M ν H , and increase with increasing alkali oxide content. The H band arises from ion motion in sites similar to those in the crystal. The low frequency band (L) represents ion motion in ‘secondary’ sites having no crystalline analog. According to the free-ion model, the ionic conductivity should be primarily due to the Rb ions in M and H type sites. This model does not predict the composition dependence of the activation energy of dc conductivity correctly because it does not fully account for the strains in the glass network which make up a significant fraction of total activation energy for the Rb germanate system.

Thermal history effects on electrical relaxation and conductivity for potassium silicate glass with low alkali concentrations

Electrical response measurements from 10 Hz to 100 kHz between 120 and 540°C were made on potassium silicate glasses with alkali oxide contents of 2, 3, 5 and 10 mol%. Glasses with low alkali content were chosen to reduce the Coulombic interactions between alkali ions to the point that frozen structural effects from the silica network on the electrical response could be observed. Conductivity and electrical relaxation responses for both annealed and quenched glasses of the same composition were compared. Lower dc conductivity, σ dc , activation energies were measured for the quenched glass than for the annealed glasses. All of the glasses exhibited Arrhenius behavior in the log σ dc against 1 T plots. A sharp decrease in σ dc was observed for glasses containing K 2 O concentrations of 5 mol% or less. The frequency at which a maximum occurs, f max , in the modulus loss, M ″, plots, when plotted against 1 T , exhibited Arrhenius behavior for both annealed and quenched samples. The activation energies for these plots closely agreed with the σ dc activation energies. A sharp increase in activation energy was observed for σ dc and f max as the potassium oxide concentration decreased. Changes in the electrical response are attributed to structural effects due to different alkali concentrations.

Use of acidobasic indicators to detect interfacial reactivity during alkaline flooding

A novel technique has been developed to detect the depletion of the alkaline reagent that occurs during the displacement of an acidified oil by an aqueous alkaline solution in a consolidated model porous medium. An acidobasic indicator (thymolphthalein) was added to the displacing aqueous solution in order to monitor the progress of the chemical reaction. The initially blue alkaline solution turned colorless after prolonged contact with the acidified oil, indicating that the alkali in the vicinity of the interface became depleted as a direct result of interfacial chemical reaction. Under such conditions, the displacement front changes to a nonreactive one, very similar to a pure water displacement. The disappearance of color was particularly pronounced at low alkali concentrations and low flowrates.

Ionic transport properties in oxide glasses derived from atomic structure

The atomic environments of alkalis are affected by the presence of other alkalis, suggesting that they are juxtaposed in the glass structure. Indeed there is mounting evidence from local structure spectroscopies, e.g., X-ray absorption fine structure (XAFS) from alkalis and oxygen magic angle spinning nuclear magnetic resonance (MAS-NMR), pointing to alkalis being coordinated primarily to non-bridging oxygens and microsegregated in silicate glasses. Reconfiguration of oxygens must therefore accompany the nearest neighbour hopping of alkalis. Simple expressions for the conformational energy associated with this and for the electrostatic energy contributions to the total microscopic energy barrier, E a, facing the migrating alkali are presented. Cooperative effects manifest in the ac electrical conductivity, σ ac, are considered and it is argued that these come into play for high concentrations of alkali in the dc electrical conductivity, σ dc, and in the self diffusion, D, and that the macroscopic activation energy, W, is related to E a by W = E a/ β, where β is the Kohlrausch exponent which can be obtained from σ ac. It is proposed that since at low concentrations of alkali (i.e., below the percolation threshold) β appraoches unity, alkalis become decoupled and hopping to more distant sites will become more frequent in consequence, with E a approximating to the binding energy of the alkali. In mixed alkali glasses, similar considerations apply to each of the alkalis. The majority alkali will mainly engage in nearest neighbour hopping, largely unaffected by the presence other alkali, while the minority alkali, because it is largely surrounded by foreign alkalis, will be constrained to hop to more distant sites. By parameterising these expressions for E a for both single and mixed alkali glasses from XAFS and MAS-NMR results, one is able to predict W for σ dc and D, obtaining good agreement with literature values for sodium and potassium silicates. Simple hopping expressions are used to predict the pre-exponents for these glasses in terms of the alkali concentration, the hopping distance and the hopping attempt frequency, all which can be obtained from the glass composition, alkali XAFS and far IR absorption, respectively. Agreement with transport values is encouraging. From the pre-exponents and activation energies, one can therefore quantitatively predict σ dc and D at any temperature, including the pronounced minimum in the σ dc isotherms across a sodium-potassium silicate glass series.

The compositions of primary carbonate melts and their evolution through wallrock reaction in the mantle

We have experimentally determined the compositions of near-solidus melts from depleted natural lherzolite at pressures greater than 25 kbar. The melts are carbonatitic with low alkali contents and Ca/(Ca + Mg) ratios of 0.72–0.74. Primary carbonate melts from fertile mantle are more sodic with Ca/(Ca + Mg + Fe + Na) of 0.52 and Na/(Na + Ca + Mg + Fe) up to 0.15. The melt compositions are similar to many natural magnesio-carbonatites, but differ substantially from the more abundant calcio-carbonatites. Experimentally we find that calcio-carbonatites are produced by wallrock reaction of primary melts with harzburgite at pressures of < 25 kbar. At 15 kbar we have obtained a Ca/(Ca + Mg + Fe + Na) ratio of up to 0.87 and very low Na contents generated by this process. Values of Ca/(Ca + Mg + Fe + Na) up to 0.95 are possible at lower pressures. Low pressure wallrock reaction of primary carbonate melt with fertile lherzolite produces melts richer in Na 2CO 3, corresponding to possible parental magmas of natrocarbonatite. Wallrock reaction at low pressures transforms the bulk peridotite composition from that of a harzburgite or lherzolite to wehrlite. Examples of such carbonatite metasomatism are now widely documented. Our experiments show that the calcium content of olivine and the jadeite content of clinopyroxene may be used to constrain the Ca and Na contents respectively of the carbonatite melt responsible for metasomatism.

Preconcentration of trace metals from sea water with the chelating resin Chelamine

The complexing properties (capacity, pH effect, breakthrough curve) of the chelating resin Chelamine, containing a pentamine ligand, were investigated. The resin was used in a column procedure for the preconcentration of Cd(II), Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from deionized water and sea water and the recoveries were 93–105 and 91–102%, respectively. The absolute blanks varied from less than 0.6 ng for Cd to 11 ng for Cu, permitting the determination of the above six metals in oceanic water. The accuracy of the method was demonstrated by replicate analyses of the marine reference material CASS-2. The high selectivity of the resin leads to low concentration of alkali and alkaline earth metal ions in the acidic eluate.

Investigations of supplementary cementing materials for reducing alkali-aggregate reactions

The use of supplementary cementing materials (SCMs) in reducing expansion of concrete and mortar caused by alkali-aggregate reaction (AAR) was investigated by the Canada Centre for Mineral and Energy Technology (CANMET) under contract to Lafarge Canada. A total of 15 fly ashes, slags, condensed silica fumes and natural pozzolans were chosen, to give a wide range in physical-chemical features of SCMs commercially available in Canada. One reactive carbonatic and two reactive siliceous aggregates were used in combinations with these SCMs and a high-alkali normal Portland cement. The effectiveness of an SCM in reducing the deleterious expansion was compared with that of a low-alkali cement. The effective high-alkali cement replacement levels were also established for the effective SCMs. Most but not all of the SCMs were useful in suppressing the expansion. For those which were judged to be effective, the effective cement replacement levels were 15%, or possibly greater, for the condensed silica fumes; 20–30% for the natural pozzolans, 40–40% for the fly ashes; and 50–65% for the slags. The desirable characteristics of the SCMs for reducing the expansion were revealed by multiple linear regression analysis to be (1) high total of SiO 2 + Al 2O 3 + Fe 2O 3, (2) particles finer than 45 μm, and (3) low alkali content.

Threshold alkali contents for expansion of concretes containing British aggregates

this article investigates the threshold alkali contents for expansion of concretes containing certain british aggregates. the concretes investigated were all of constant mix proportions, with water/cement ratio of 0.5; aggregate/cement ratio of 4.6, and cement content of 400 kg/cubic metre. the aggregate consisted of a mixture of 2 parts coarse aggregate (20 - 5 mm) to one part fine aggregates (an unreactive limestone). the coarse aggregates were: silucian siltstone from north yorkshire; lower carboniferous silicified limestone from lancashire; and lower cambrian greywacke (from gwynedd). the portland cement used had a low alkali content (0.324% na2o equivalent). appropriate additions of sodium hydroxide were made to the mix water to produce concretes with various required alkali contents in the range 3.0 to 7.0 kg/cubic metre na2o equivalent. the prisms were made in accordance with the relevant british standard and cured in moist air for 7 days. gauge markers were fixed to the prisms which were then wrapped in absorbent towelling soaked with 200 ml distilled water. these were then sealed within 3 polythene bags and stored at 38 plus or minus 2 degrees c in a thermostatically controlled room. at monthly intervals they were removed from this environment and allowed to cool to 20 plus or minus 3 degrees c for 24 hours before being measured. the strains were recorded over a 12 month period. the results indicate that all 3 of the coarse aggregates studied gave rise to expansion of the concrete prisms to an extent which depended on the alkali contents of the mixes. aar (alkali aggregate reaction) may be induced in concretes made from certain british aggregates at alkali contents as low as 3.5-4.0 kg/cubic metre expressed in terms of na2o equivalent. a discussion of this paper by dw hobbs is published in cement and concrete research (1993), 23(2) pp495-9.

29Si MAS‐NMR Study of the Short‐Range Order in Lithium Borosilicate Glasses

29Si MAS‐NMR measurements have been made on a series of lithium borosilicate glasses of general composition RLi2O.B2O3·KSiO2. At low alkali contents (R &lt; 1), the 29Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the 29Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the 11B NMR studies of these same glasses, the 29Si NMR data are quantitatively best‐fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R= 0. Models of sharing developed from the 11B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K) or variable (dependent on K) minimum R0, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.

Raman spectra of MO 1/2[sbnd]TeO 2 (M = Li, Na, K, Rb, Cs and Tl) glasses

The structure of MO 1/2 TeO 2 (M = Li, Na, K, Rb, Cs and Tl) binary glasses has been studied by means of Raman spectroscopy. The glasses having low alkali content have a continuous network constructed by sharing corners of TeO 4 trigonal bipyramids (tbp's) and TeO 3 + 1 polyhedra having one non-bridging oxygen atom (NBO). In the glasses containing 20–30 mol% alkali oxide, TeO 3 trigonal pyramids (tp's) having NBOs are formed in the continuous network. When alkali content exceeds 30 mol%, isolated structural units, such as Te 2 O 2− 5 ion, coexist in the continuous network. The fraction of TeO 4 tbp's decreases with increasing alkali content. The glasses, which contain nearly 50 mol% alkali oxide, are composed of a continuous network constituted by TeO 3 + 1 polyhedra and TeO 3 tp's, and of isolated structural units, such as Te 2 O 2− 5 and TeO 2− 3 ions. The structure of thallium tellurite glasses having less than 30 mol% TlO 1/2 is similar to that of alkali tellurite glasses containing equal amounts of MO 1/2 . The fraction of TeO 3 tp's having NBOs in the thallium tellurite glasses, when TlO 1/2 content is equal to or higher than 40 mol%, is larger than that in the corresponding alkali tellurite glasses. In the 66TlO 1/2 ·34TeO 2 glass, most of tellurium atoms are in a form of isolated TeO 2− 3 ion. A new hypothesis is also given for a mechanism for the basic structural changes in the tellurite glasses.

Internal Friction of Mixed Alkali Aluminogermanate Glasses

The internal friction of alkali aluminogermanate glasses containing no nonbridging oxygen ions was measured by the free torsional vibration method. The internal friction as a function of temperature revealed two peaks in mixed-alkali glasses with low alkali contents. Those peaks increased rapidly in magnitude and came close to overlapping with increasing alkali content. The results indicate that both peaks can be assigned as mixed-alkali peaks, and that the mixed-alkali peak was closely related to the low-temperature peak in single alkali glasses. The following explanation is proposed. (1) There are two kinds of mechanism responsible for the mixed-alkali peak. (2) The mixed-alkali peak is essentially caused by the individual and duplex movement of two species of alkali ions which are affected by the interaction between different kinds of alkali ions. (3) When two mixed-alkali peaks overlap, there is little difference between the two mechanisms since the two species of alkali ions move with almost the same activation energy. (4) The rapid increase in magnitude of the mixed-alkali peak with increasing alkali content is probably attributed to a considerable increase in the number of mobile alkali ions which contribute to the mechanism of the mixed-alkali peak, and at the same time is also attributed to the increase in energy consumption during alkali ion migration.

Sugarcane bagasse pith dry pretreatment for single cell protein production

The effect of spraying a solution of sodium hydroxide onto sugarcane bagasse pith, in such a low volume that no free liquid was present, was evaluated with respect to Single Cell Protein production (SCP), and biological degradability. This type of processing will be referred to as a dry pretreatment and it is compared with a wet pretreatment. In-vitro digestibility and microbial growth were obtained with a mixed culture of Cellulomonas flavigena and Xanthomonas sp. Maximum digestibility was 76% using dry pretreatment with a NaOH dosage of 0·1 g per g pith, a temperature of 50°C and moisture content of 80% (based on oven dry weight). A digestibility value of 71% was obtained for wet conditions, with 0·2 g NaOH per g pith, a temperature of 92°C, and a solid: liquid ratio of 1:10. Biomass production was higher in the dry pretreatment than the wet pretreatment, when washing of the treated pith was omitted before fermentation. The spraying was carried out using a rotary system, which permits a better surface area contact between the substrate and the alkali. In this way the NaOH was efficiently used allowing relatively low alkali concentrations. Furthermore, by eliminating the washing stage, the solubilized carbohydrate fraction is retained.

Alkali Extraction of Starch from Corn Flour

AbstractThe alkali steeping process was tested for extraction of starch from degerminated yellow corn flour. The effect of three process parameters, alkali concentration (0.1 and 0.4%), steep temperature (25 and 55°C) and steep time (30 and 90min) were evaluated for their effect on yield, protein and sodium contents of starch. Two‐level full factorial design was used to investigate the process. The experimental results indicated that starch yield increased with increasing steep temperature and at low alkali concentration. All main effects and interactions were found to have significant effect on protein content in the extracted starch. The sodium content of the starch was effected by the three main effects and the three two‐factor interaction effects. Conditions were identified that simultaneously produced a high starch yield, low protein and sodium contents were 0.1% alkali steep at 55°C for 30 min.

The glass transition temperature of lithium-alkali borates

Glass transition temperatures, T g, were measured for mixed lithium-alkali borate glasses over wide ranges of composition in samples prepared from both oxide and carbonate starting materials. Raman spectroscopy was used to compare samples prepared from different sets of starting materials. Differences in T g were observed as a function of alkali. At low alkali content LiNa borates had the greatest T g and LiCs borates the least, while at high alkali contents the trends were reversed. Differences were also seen between samples prepared from carbonates when compared with oxides for the LiRb and LiCs systems with the carbonate-based glasses having higher T g. No such differences were found in the LiNa case. The trends of the data were explained in terms of models for the atomic arrangements. At low alkali contents, the dependence of T g on the fraction of four-coordinated borons was invoked while at high alkali contents differences between orthoborate units — tetrahedral in the cases of Rb and Cs and trigonal in the sodium and lithium systems — were used. The T g trends were also compared with density and glass formability observations.

Borate glass structure by Raman and infrared spectroscopies

Raman and infrared spectroscopies have been employed to probe the continuous evolution of borate glass structures as a function of the nature and concentration of alkali oxide modifier. At relatively low alkali contents, the glasses contain covalent networks consisting of interconnected units bearing BO − 4 tetrahedra. Further addition of alkali oxide causes the progressive depolymerization of the network as a result of the formation of non-bridging oxygen atoms. Eventually, complete network disruption into small and highly charged borate units results in “ionic” glasses. The nature of the alkali cation determines the structure of these glasses by affecting several high- temperature isomerization or disproportionation equilibria.