Titanium dissolution

The discussion in Section 4.2.1 suggested that titanium dissolution may be driven by the reduction of H, the reduction of Oj, or the reduction of oxidizing agents. When titanium is polished in the presence of copper ions, copper ion reduction leads to the oxidation of the titanium. A galvanic couple is formed between the Cu/Cu and the Ti/Ti systems such that the copper ions are reduced and plated onto the titanium surface. At the same time, titanium is dissolved into solution. Thus, copper replaces the titanium on the surface. The reactions governing such a process are ... 

Schematic representation of titanium dissolution during CMP. In the absence of copper ions (a), O2 reduction drives the titanium dissolution reaction, and the dissolution rate is low. With coRter ions present in the slurry (b), the reduction of the cop r ions drives the titanium dissolution reaction, and the dissolution rate increases.

Given the assumptions made, the titanium dissolution current should theoretically increase by a factor of 85. However, the Cu ions plate onto the titanium surface as they oxidize the titanium. The plated copper blocks further dissolution of titanium until the copper is removed by polishing. Thus, while iji may increase by a factor of 85 theoretically, in practice the finite copper polish rate limits iV,. 

At the high current densities suggested by Figure 4.44b, titanium dissolution and copper oxidation may be controlled by concentration polarization rather than Tafel kinetics. Under concentration polarization, the current does not increase proportional to... 

Note that in the absence of the Ti/Cu couple. Figure 4.44a predicts the titanium dissolution current density to be 89 pA/cm This dissolution current density translates to a dissolution rate of only 3 nm/min, which is lower than the observed polish rate of 62 nm/min. It is possible that in the absence of copper ions, most of the mechanically abraded titanium is swept away from the wafer surface without dissolving The abraded material either falls into the pad as undissolved titanium or Ti02 or is adsorbed onto the abrasive particles. If this is the case, the titanium dissolution rate will be lower than the polish rate, since not all of the abraded material dissolves. 

FIGURE 22.23 Thickness of passive oxide films on metallic iron in 0.15 kmol m 3 phosphoric acid solution and on metallic titanium in 0.1 kmol m-3 sulfuric acid solution as a function of electrode potential [30-32] L = film thickness, iFe = iron dissolution current, iTi — titanium dissolution current, and ia = anodic total current. 

However, the active dissolution of titanium depends markedly on temperature in acid solution. At lower temperatures, the picture is not so clear. It is necessary to have a quantitative measure of the rate of the hydrogen reaction and the titanium dissolution reaction. The complete set of current-potential and impedance-potential data has been tested against the theory given above. The best strategy seems to be to fit to a single electrode reaction and then to look for deviations from the expected behaviour for a perfect redox reaction. A convenient way of doing this is to represent the electrochemical data as a standard rate constant-potential curve in conjunction with a double layer capacity-potential curve [21]. 

The corrosion resistance of TiN can be presented by anodic polarization curves. The polarization result of TiNo.gr sintered body determined at room temperature is shown in Figure 11.3.1 [11]. Since its electrode potential directly after immersion in dilute sulfuric acid is positive (4-0.016 V), it is not soluble for the dilute sulfuric acid. The current density increases rapidly with increasing the voltage, and then decreases reversibly from 0.1 to 0.5 V, showing passivation of TiN. This behavior resembles to anodic polarization curve of titanium. This would be due to substoichiometric composition TiN with excess amount of titanium. Dissolution of TiN is possible in hot fluoric nitric acid solution alone. 

PIZZINI It seems that in the case of Ti02 the oxygen evolution takes place under illumination without any titanium dissolution. Apparently water molecules adsorbed at the surface could be involved in the primary reaction step, and we, in fact, observed basification of the titanium oxide comportment. Could not well be this the case of ZnO, where, due to the basification (local) of the solution, dissolution of the crystal ZnO takes place instead of the anodic dissolution of Zn ... 

The transition from titanium dissolution (Oi) to oxidation of divalent titanium (O2) was evaluated, and by means of calculating the charges at tstart nd the current efficiencies were estimated. The current efficiency... 

Titanium(lV) fluoride dihydrate [60927-06-2] TiF 2H20, crystals can be prepared by the action of aqueous HF on titanium metal. The solution is carefully evaporated to obtain the crystals. Neutral solutions when heated slowly hydroly2e and form titanium(lV) oxyfluoride [13537-16-17, TiOF2 (6). Upon dissolution in hydrogen fluoride, TiF forms hexafluorotitanic acid [17439-11-17, ll]TiF. ... 

The cake produced by the digestion is extracted with cold water and possibly with some diluted acids from the subsequent processes. During the cake dissolution it is necessary to maintain the temperature close to 65°C, the temperature of iron sulfate maximum solubiUty. To prevent the reoxidation of the Fe " ions during processing, a small amount of Ti " is prepared in the system by the Ti reduction. The titanium extract, a solution of titanium oxo-sulfate, iron sulfate, and sulfuric acid, is filtered off. Coagulation agents are usually added to the extract to faciUtate the separation of insoluble sludge. 

Titanium tetrachloride is completely miscible with chlorine. The dissolution obeys Henry s law, ie, the mole fraction of chlorine ia a solutioa of titanium tetrachloride is proportional to the chlorine partial pressure ia the vapor phase. The heat of solutioa is 16.7 kJ/mol (3.99 kcal/mol). The appareat maximum solubiUties of chlorine at 15.45 kPa (116 mm Hg) total pressure foUow. 

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

The analytical chemistry of titanium has been reviewed (179—181). Titanium ores can be dissolved by fusion with potassium pyrosulfate, followed by dissolution of the cooled melt in dilute sulfuric acid. For some ores, even if all of the titanium is dissolved, a small amount of residue may still remain. If a hiU analysis is required, the residue may be treated by moistening with sulfuric and hydrofluoric acids and evaporating, to remove siUca, and then fused in a sodium carbonate—borate mixture. Alternatively, fusion in sodium carbonate—borate mixture can be used for ores and a boiling mixture of concentrated sulfuric acid and ammonium sulfate for titanium dioxide pigments. For trace-element deterrninations, the preferred method is dissolution in a mixture of hydrofluoric and hydrochloric acids. 

TYZOR TPT and the tetraethyl titanate, TYZOR ET [3087-36-3], have also been prepared by direct electrochemical synthesis. The reaction involves anode dissolution of titanium in the presence of the appropriate alcohol and a conductive admixture (3). 

The examples already discussed lead to the conclusion that any reaction of a metal with its environment must be regarded as a corrosion process irrespective of the extent of the reaction or of the rates of the initial and subsequent stages of the reaction. It is not illogical, therefore, to regard passivity, in which the reaction product forms a very thin protective film that controls rate of the reaction at an acceptable level, as a limiting case of a corrosion reaction. Thus both the rapid dissolution of active titanium in 40% H2SO4 and the slow dissolution of passive titanium in that acid must be... 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

Both titanium and boron can be added as grain refiners to ensure small grain size and hence high surface area grain boundaries. This reduces the risk of preferential attack at grain boundaries and promotes more uniform dissolution. 

Nanoparticles of the semicondnctor titanium dioxide have also been spread as mono-layers [164]. Nanoparticles of TiOi were formed by the arrested hydrolysis of titanium iso-propoxide. A very small amount of water was mixed with a chloroform/isopropanol solution of titanium isopropoxide with the surfactant hexadecyltrimethylammonium bromide (CTAB) and a catalyst. The particles produced were 1.8-2.2 nm in diameter. The stabilized particles were spread as monolayers. Successive cycles of II-A isotherms exhibited smaller areas for the initial pressnre rise, attributed to dissolution of excess surfactant into the subphase. And BAM observation showed the solid state of the films at 50 mN m was featureless and bright collapse then appeared as a series of stripes across the image. The area per particle determined from the isotherms decreased when sols were subjected to a heat treatment prior to spreading. This effect was believed to arise from a modification to the particle surface that made surfactant adsorption less favorable. 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

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