• P Vishnu Kamath

Articles written in Bulletin of Materials Science

• Thermal behaviour of hydroxides, hydroxysalts and hydrotalcites

Mass spectrometric analysis of gases evolved during thermal decomposition of divalent metal hydroxides, hydroxysalts and hydrotalcites show that all these compounds undergo dehydration in the temperature range 30 &lt;T &lt; 220°C followed by decomposition at temperatures above 250°C. The latter step involves simultaneous deanation and dehydroxylation of the layers. Our observations conclusively prove that alternative mechanisms which envisage CO2 evolution due to deanation at lower temperatures proposed by Kanezaki to be wrong.

• Ageing behaviour of unary hydroxides in trivalent metal salt solutions: Formation of layered double hydroxide (LDH)-like phases

The hydroxides of Mg, Ni, Cu and Zn transform into layered double hydroxide (LDH)-like phases on ageing in solutions of Al or Cr salts. This reaction is similar to acid leaching and proceeds by a dissolution-reprecipitation mechanism offering a simple method of LDH synthesis, with implications for the accepted theories of formation of LDH minerals in the earth’s crust.

• Aging of trivalent metal hydroxide/oxide gels in divalent metal salt solutions: Mechanism of formation of layered double hydroxides (LDHs)

• Oxidative leaching of chromium from layered double hydroxides: Mechanistic studies

The layered double hydroxide (LDH) of Zn with Cr on treatment with a hypochlorite solution releases chromate ions as a result of oxidative leaching by a dissolution–reprecipitation mechanism. The residue is found to be 𝜀-Zn(OH)2. The LDH of Mg with Cr on the other hand is resistant to oxidative leaching. In contrast, a X-ray amorphous gel of the coprecipitated hydroxides of Mg and Cr yields chromate ions. These results suggest that the oxidation potential of Cr(III) in LDHs is determined by the nature of the divalent ion and the crystallinity of the phase while being unaffected by the nature of the intercalated anions.

• Temperature-induced control over phase selection among hydroxides of nickel

While at low (4°C) temperatures, addition of ammonia to aqueous metal nitrate solutions induces the precipitation of 𝛼-nickel hydroxide and at high (25–65°C) temperatures, 𝛽-Ni(OH)2 is formed. The crystallinity of the product improves at higher temperatures of precipitation and the product obtained at 65°C is devoid of various disorders such as stacking faults, turbostraticity and interstratification. This provides a simple and efficient alternative to the hydrothermal synthesis of crystalline 𝛽-Ni(OH)2. The temperature induced control over phase selection provides direct experimental evidence for the metastability of 𝛼-nickel hydroxide. Crystalline 𝛽-Ni(OH)2 is, however, a poor electrode material for alkaline secondary cells and exhibits a capacity of only 75 ± 10 mAh/g, against the theoretically expected 289 mAh/g.

• Thermal decomposition of Co–Al layered double hydroxide: Identification of precursor to oxide with spinel structure

The layered double hydroxide (LDH) of Co with Al decomposes to yield an oxide residue with the spinel structure below 250°C. The decomposition reaction is preceded by the formation of an intermediate hydroxide in which the metal hydroxide layers are regularly stacked about the 𝑐-crystallographic axis, but the layers themselves are aperiodic. Aperiodicity is modeled by locating randomly chosen Co2+ ions in tetrahedral sites in the interlayer region. This phase is characterized by a single strong basal reflection in its powder diffraction pattern. All other reflections are extinguished on account of

turbostratic disorder which destroys all ℎ𝑘𝑙 reflections and

layer aperiodicity, which destroys all two dimensional ℎ𝑘 reflections.

Given its topochemical relationship with the spinel structure, such an intermediate is a necessary precursor to spinel formation.

• Synthesis and structure refinement of layered double hydroxides of Co, Mg and Ni with Ga

Homogeneous precipitation by urea hydrolysis results in the formation of highly ordered layered double hydroxides of divalent metal ions (Co, Mg, Ni) and Ga. Structure refinement shows that these carbonate containing layered hydroxides crystallize with rhombohedral symmetry (space group 𝑅-3𝑚) in the structure of the 3𝑅1 polytype. An analysis of the structure shows that, coulombic attraction between the layer and interlayer remains invariant in different layered hydroxides, whereas the strength of hydrogen bonding varies. The Ni–Ga LDH has the weakest hydrogen bonding and Co–Ga, the strongest, as reflected by the layer–interlayer oxygen–oxygen distances. The poor polarity of the OH bond in the Ni–Ga hydroxide points to the greater covalency of the (𝑀2+}/𝑀′3+)-oxygen bond in this compound as opposed to the Co–Ga hydroxide. These observations are supported by IR spectra.

• Electrodeposition of dicalcium phosphate dihydrate coatings on stainless steel substrates

Cathodic reduction of an aqueous solution containing dissolved calcium and phosphate ions results in the deposition of micrometer thick CaHPO4.2H2O (dicalcium phosphate dihydrate) coatings on stainless steel substrates. The coating obtained at a low deposition current (8 mA cm-2) comprises lath-like crystallites oriented along 020. The 020 crystal planes are non-polar and have a low surface energy. At a high deposition current (12 mA cm-2), platelets oriented along 12$\bar{1}$ are deposited. CaHPO4.2H2O is an important precursor to the nucleation of hydroxyapatite, the inorganic component of bones. Differently oriented CaHPO4.2H2Ocoatings transform to hydroxyapatite with different kinetics, the transformation being more facile when the coating is oriented along 12$\bar{1}$. These observations have implications for the development of electrodeposited biocompatible coatings for metal endoprostheses for medical applications.

• Electronic spectra of anions intercalated in layered double hydroxides

Transition metal complexes intercalated in layered double hydroxides have a different electronic structure as compared to their free state owing to their confinement within the interlayer gallery. UV–Vis absorptions of the intercalated complex anions show a significant shift as compared to their free state. The ligand to metal charge transfer transitions of the ferricyanide anion show a red shift on intercalation. The ferrocyanide ion shows a significant blue shift of 𝑑–𝑑 bands due to the increased separation between 𝑡2g and 𝑒g levels on intercalation. MnO$^{-}_{4}$ ion shows a blue shift in its ligand to metal charge transfer transition since the non-bonding 𝑡1 level of oxygen from which the transition arises is stabilized.

• Interaction of pristine hydrotalcite-like layered double hydroxides with CO2: a thermogravimetric study

Metal oxides in general have surface acidic sites, but for exceptional circumstances, are not expected to mineralize CO2. Given their intrinsic basicity and an expandable interlayer gallery, the hydrotalcite-like layered double hydroxides (LDHs) are expected to be superior candidate materials for CO2 mineralization. However, the incorporation of Al3+ adversely impacts the ability of the metal hydroxide layer to interact with CO2 in the gas phase in comparison with the unitary Mg(OH)2. Thermogravimetric analysis shows that the decomposition reaction of the [Mg–Al–CO3] LDH is only marginally delayed in flowing CO2 in comparison with flowing N2, showing only an apparent marginal CO2 uptake. Al3+ ion severely attenuates the surface basicity of the LDHs, as the unitary Al(OH)3 is acidic in comparison with Mg(OH)2 and shows little or no interaction with CO2 in the gas phase.

• # Bulletin of Materials Science

Volume 43, 2020
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Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019