Hedvall effect

As a phase transition occurs, there is a change of some type in the lattice. The units (molecules, atoms, or ions) become more mobile. If the solid is reacting in some way and the temperature is at or very near that at which the solid undergoes a phase transition, there may be a rapid increase in the rate of the reaction with a slight increase in temperature because lattice reorganization enhances the ability of the solid to react. A similar situation can occur in cases in which two solids are reacting if the temperature is at or near the temperature corresponding to a phase transition in one of the solids. This phenomenon is sometimes referred to as the Hedvall effect. 

Boehmite Based. Catalysts. Hedvall (24) has discussed the formation of cobalt aluminate from CoO and AI2O3. He has shown that a relatively fast solid state reaction takes place when the alumina undergoes a phase change, viz. y-Al203 ->rt-A Og. This phenomenon is known as the Hedvall effect. 

Some bulk cobalt aluminate formation is expected to take place for the boehmite based catalyst, owing to a Hedvall effect (24).The spectrum of adsorbed pyridine on CoMo-124 B shows indeed a weaker 1612 cm l band, comparable with the intensity of this band for the MoCo-123 catalyst. This indicates that about 25 % of the cobalt ions has disappeared in the bulk of the alumina. (Figure lid). 

Another example of an effect of magnetic properties on reactivity was found in the oxidation of Fe304 to Fe203 [127], The Arrhenius plot of the rate data showed no significant change in slope well below and well above the Curie temperature (578°C). In the temperature range from 560 to 640°C, however, an anomalous deviation (a maximum and a minimum on log k), known as the Hedvall effect [128], was observed. No satisfactory explanation has yet been given. 

The Hedvall effect [77] ascribes enhanced reactivity within particular temperature intervals in reactions of solids as being due to increased ionic mobility during a condition of crystalline disorder or recrystallization. Gam and Habash [78] selected the reaction ... 

Increasing the mobility by making use of phase transitions in solids during synthesis the Hedvall effect as described in Section 5.2. 

These are primarily concerned with the degree of disorder in the solid. Factors which increase this disorder enhance the reactivity of the material. The existence of a brief period of enhanced reactivity during a solid i solid2 phase transition has been controversial and is referred to as the Hedvall effect (see References 17, 18, and references therein). An example in ceramic processing is the use of metastable anatase instead of the stable rutile form of titanium dioxide for the preparation of barium titanate. ... 

Ramachandran, V. S., and Sereda, P. J., Applications of Hedvall Effect in Cement Chemistiy, Nature, 233 134-135 (1971)... 

A1(0H)3 decomposes at 300 °C to form y-AlOOH, which decomposes about 400 °C to form y -AkOs in the atmosphere. According to our previous results, y-AlOOH and y-AkOs were transformed into a-Ak03 above 400 °C by the capsule HIP. In addition, Mg(OH)2 did not melt at 500 °C in the HIP capsule. Therefore, in the mixture of hydroxides, Mg(OH)2 reacts with the intermediate compound between AlOOH or y-AkOs and a-Ak03. Generally, intermediate compound is very active because the atoms need rearrangement while the transformation occurs. The reaction between Mg(OH)2 and the intermediate compound would be very rapidly proceeded. This seems to be an example of Hedvall effect [7]. The water from the hydroxides would affect the formation of the intermediate phase [4] and the reaction of the intermediate phase and Mg(OH)2, based on the solution-precipitation process. 

On the other hand, we consider that low CO2 pressure and high water vapor pressure cause the formation of hydroxide from carbonate. During this process, so-called Hedvall effect would accelerate the reaction. In order to solve this problem, the experiments are preferred to be performed in pure water vapor atmosphere. However, the vapor pressure of water was not precisely described in the literatures cited above. Chen et al. [8] generated the water vapor witii bubbling and supplied it to the horizontal tubular furnace using a carrier gas. This procedure seems to be generally used in this kind of experiments [9-11]. It seemed that the water vapor with less than 1 atm of partial pressure was supplied to the furnace. Therefore, we developed the apparatus for the experiments under I atm of pure water vapor. In this study, the thermal decomposition of CaC03 and... 

In this experiment, the partial pressure of CO2 in the silica tube was very low, compared to the Pcoz in air (4.0 X 10 atm), because file water vapor completely put the air out of the silica tube. In this atmosphere, carbonate is considered to be unstable. Hence, under 1 atm of superheated steam, hydroxide would be stable, compared to carbonate. It was concluded that CaCOs transformed into CaO via Ca(OH)2. During the transformation of carbonate into hydroxide, the crystal lattice is temporarily disordered to make it active. So-called Hedvall effect would occur to make the decomposition temperature of CaCOs lower. 

As described above, both MnC03 and CaC03 were transformed into y-Mn203 and CaO (via Ca(OH)2), respectively, at lower temperature in the superheated steam. In addition, die transformation of the reagents accelerates the solid state reaction, because the crystal lattice of each reagent was highly activated due to the transformation i.e. so-called the Hedvall effect [12]. 

Reactivity of carbonates in superheated steam under 1 atm was investigated by using a newly developed apparatus. The water vapor clearly affected the decomposition of CaC03 and MnC03, and the solid state reaction between MnCQs and CaCQ3. It was concluded that the reaction in superheated steam was promoted by the Hedvall effect, which was caused by the formation of intermediate phase such as hydroxide. 

R Alizadeh, J.J. Beauddn, V.S. Ramachandran and L. Raid, Applicability of the Hedvall effect to study... 

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