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Characteristic features of the rheological behavior of dielectric layered-structure suspensions of hydrated metal oxides in an electric field

In this paper suspensions containing hydrated oxides of magnesium and iron were objects of rheological studies. The different content of the “stoichiometric” water in fillers is attributable to the conditions of their precipitation and aging. According to the results of TGA - DTA “super-stoichiometric” water of all the samples was removed in the temperature range 130 - 150°C, indicating its strong bond with the surface. Filler ER activity is determined by the change in the shear stress of the ER-suspensions containing them under the external electric field. The important role in the manifestation of the ER effect is played not only by the quantity of “super-stoichiometric” water in the interlayer space of the disperse phase particles, but also by the steric conditions.


Introduction
The uprising of the rheological indices of electrorheological fluids (ERF) under the action of an external electric field depends on the properties of a disperse phase whose role may be played by both water-containing and water-free compounds [1][2][3][4][5][6][7]. Some researchers [8][9][10] attribute the manifestation of the electrorheological (ER) effect to the presence of the adsorbed activator (the substance capable of forming hydrogen bonds such as hydroxyl-carboxyl-or amino-containing compounds) on the surface of the disperse phase particles. The most common activator is water entering into the composition of the water-containing disperse phase. The preparation of such disperse phase (ERF filler) is possible by means of both water adsorption in water-free compounds, more often oxide ones, and by chemical precipitation of hydrated compounds from aqueous solutions. In this case, the energy state of H 2 O molecules and the form of their bonding in the hydrated filler may differ substantially and differently influence the ERF fluidity (viscosity, yield point, shear modulus, etc.). Water-containing ERF fillers, silicas (silica gel, aerosil, diatomite), metal oxides activated by H 2 O adsorption have been studied most extensively. Occurring in this case is the filling of the surface of the particles with water and formation of polymolecular H 2 O layers bound by the H bonds. The presence of film polarization ensures the transfer of charge carriers only along the particle surface. In an ERF based on particles with adsorbed water, the weakening or complete degeneration of the ER effect in time is possible especially on heating because of the irreversible desorption of weakly bound H 2 O molecules.  In fillers based on hydrated compounds obtained by chemical precipitation, in addition to the structural and crystalline-hydrate H 2 O there may also exists other O m H n groups capable of being preserved up to relatively high temperatures (200-250°C). They can potentially be used for creating thermally stable ERFs. The relationship between the nature, structure of O m H n groups in such type of fillers, and their rheological activity in an electric field is scarcely discussed in scientific publications. In the last 30 years, scientists of various countries have carried out numerous rheological investigations of ERF based on a water-containing disperse phase [1, 2, 4, 5 and 8]. In this connection, of particular interest is the investigation of the fillers of the ERFs, which are based on lamellar hydrated oxides of polyvalence metals, which, according to [11][12][13], consist of positively charged alternating two-dimensional metal-hydroxide layers, in the interlayer space of which the number of water molecules needed for ensuring the electrorheological (ER) activity of suspensions can be localized. The form and type of bonding as well as the quantity of H 2 O molecules depend in the first place on the metal cation nature, structure of lamellar compounds, and on the conditions in which they were prepared. Moreover, in chemical compounds, the O m H n groups, which represent the interlayer water, can possess specific mobility, which exerts its influence on proton transfer and, correspondingly, on the magnitude of the conduction current.
Earlier [14,15] we have studied the ER activity of the modifications of hydrated aluminum oxides (boehmite, pseudoboehmite, AlOOH, bayerity, Al(OH) 3 ), obtained by chemical precipitation under different conditions and differing in the type and quantity of H 2 O molecules in them. It was shown [14] that fillers based on boehmite, Al 2 O 3 ·H 2 O, bayerite, Al 2 O 3 ·3H 2 O, and containing only structural H 2 O in the form of hydroxyl groups have a low ER activity. At the same time, the ERF based on pseudoboehmite which contains "super-stoichiometric" or unstructured water manifest a considerable ER effect. Such water molecules whose quantity is 0.25-0.8 mole per mole of Al 2 O 3 are mainly localized in the interlayer space of the structure. It is shown that they are bound by strong hydrogen bridges in the boehmite lattice, are somewhat deformed, possess thermal stability, and are efficient activators of the disperse phase particles. The appearance of the proton conductivity in this case can be explained by the dissociation of H 2 O molecules in an electric field with formation of protons and OH groups with strong hydrogen bonds. This ensures the appearance of strong structural bridges from disperse phase particles in an ERF.
Taking into account the role of the interlayer H 2 O molecules in the initiation of the ER effect, as it is displayed in [14,15], it is of scientific and practical interest to reveal the dependence of the ER effect on the quantity of active O m H n groups entering into the composition of lamellar hydrated compounds obtained by chemical precipitation. This will allow one to obtain new experimental data needed for elucidating the nature of proton-containing groups, their role in proton processes in a disperse phase for purposeful creation of electrically active and thermally stable fillers for ERFs.
The aim of the present work is investigation of the influence of the composition and structure of chemically precipitated hydrated magnesium and iron oxides on the ER activity of suspensions based on them.

Experimental
Electrosensitive particles of the filler of an ERF based on hydrated magnesium and iron oxides were obtained by the method of chemical precipitation in the metal salt-base-water system. As metalcontaining reagents we used magnesium and iron chlorides. The precipitation of hydrate magnesium and iron oxides was effectuated in an alkaline medium, after which the sediment was kept for 6-8 hours under the liquid phase formed as a result of the chemical precipitation. Thereafter, it was separated from the liquid phase by filtering, washed and dried at a temperature of 20-30°C and then at 90°C up to a constant mass. The different content of the "stoichiometric" water in fillers is attributable to the conditions of their precipitation and aging. For the purposes of comparison we obtained hydrated aluminum oxide, the electrorheological properties of which were studied earlier in [14].
The compounds obtained were identified by the results of x-ray phase analysis with the use of a 08 Advance diffractometer of Bruker AXS Company (Germany). The content of magnesium and iron on conversion into the mass of oxide, as well as the quantity of water were determined by the well-known methods. The dispersity of sediments was determined by a scanning electronic microscope "Jeol JSM-5610LV". The IR spectra of the compounds were obtained with the aid of a NEXUS IR-Fourier spectrometer of THERMO NICOLET Company. The thermal stability of the compounds was evaluated by the results of a differential-thermal analysis recorded by a thermoanalytical system TGA/DSC-1/1600 HF (METTLER TOLEDO Instruments, Switzerland).
The ER activity of the synthesized hydrated metal oxides was evaluated by the change in the shear stress of ERFs containing 10.0 wt.% of the filler in an dielectric oil under the action of electric field of strength from 0 to 3 kV/mm at the shear rate γ = 17.24 s -1 . The shear stress of ERF was measured by "Rheotest 2.1" rotational viscometer with an axially cylindrical cell at room temperature.

Results and discussion
The influence of the conditions of precipitation of hydrated metal oxides and their composition are presented in Table 1. According to the x-ray-phase analysis, the synthesized samples of fillers Nos.1-3 are crystalline hydrated magnesium oxides. They have a typical lamellar structure of brucite in which each Mg 2+ ion is located between six OH groups and is bound with three OH ions of one layer and three OH ions of another one. Sample No. 4 is iron oxyhydroxide having the structure of goethite, which is a transitional one from a chain structure to a lamellar one. Its base is the densest hexagonal packing of O 2ions, since the H + protons are located between pairs of O 2ions not occupying particular space.
It has been established from the results of electronic-microscopic investigations that the synthesized samples of fillers are highly dispersed ones. Figure 1  Proceeding from the stoichiometric composition of the indicated compounds, in particular, of Mg(OH) 2 (MgO·H 2 O), FeOOH(Fe 2 O 3 ·H 2 O), in them, corresponding to 1 mole of metal oxide there is 1 mole of chemically bound or structural water, which in the form of OH groups participates in the construction of the crystal lattice. In addition to this type of water, the samples also contain "superstoichiometric" water. As shown in Table 1, its quantity for magnesium oxide depends on the precipitation conditions, in particular, on the concentration of the magnesium salt solution. Just as in the case of pseudoboehmite, the molecules of "superstoichiometric" H 2 O can be both localized in the interlayer space of the lamellar structure of the compounds and adsorbed on the surface of crystals with which they are strongly bound by hydrogen bonds.
For the "superstoichiometric" H 2 O molecules of brucite and goethite just as of pseudoboehmite, typical is the relatively high thermal stability, which is seen from the thermograms presented in Figure 2. The curves of heating the tested compounds have two endothermal peaks. The first, small peak is fixed in the region of T=100-250°C for brucite and T=100-230°C for goethite. The second, deep peak is located in the region of higher temperatures.   As is seen from Figure 2  of pseudoboehmite. This may mean that due to its small radius the Al 3+ cation (0.51Å) is capable of very closely approaching the unshared pair of water molecules and attract them by its strong electrostatic field.
It was shown earlier [14,15] that the energy state of H 2 O molecules determined by steric conditions, polarizing action of the cation field, and by other factors is closely connected with the manifestation of the ER effect.
The dependences of the shear stress (τ) and current density (j) of ERF based on the studied hydrated magnesium, iron, and aluminum oxides on the electric field strength (E) are presented in Figures 4 and  5.   Figure 2) and their thermal stability make it possible to assume the presence of strong hydrogen bonds of H 2 O molecules and the possibility of their protolytic dissociation in an electric field. It seems that by virtue of the specific features of the crystal structure of goethite, in which there are twinned layers of the densest packing of O 2ions, the transfer of the proton over a great distance to oxygen atoms can be hampered. As a result, a continuous chain of hydroxyl groups that interacts via the hydrogen bonding and ensures the transfer of the charge by the proton mechanism is not formed. In the structure of brucite, the Mg 2+ cations occupy all octahedron voids between two layers of OH groups located according to the principle of the densest hexagonal packing, which ensures additional paths for the charge transfer in the interlayer space. In this case, for the transfer of the proton by the relay-race mechanism a small amount of H 2 O is required, which is localized in the interlayer space. The increase in the shear stress in the ERF based on pseudoboehmite is larger than in the ERFs based on brucite and goethite, which contain 0.28-0.35 moles of "superstoichiometric" water at an electric field strength higher than 2 kV/mm. This is explained by the fact that, according to the IR spectra, stronger polarization is typical of the "superstoichiometric" water molecules in pseudoboehmite. It is determined by the donor-acceptor properties of aluminum and by the characteristic features of the structure of boehmite, in which the charge transfer may take place via the continuous network of the H bonds in the bulk of a substance, which plays the decisive role in the manifestation of the ER effect.

Conclusions
Thus, as follows from experimental data, despite the lamellar structure of all the ERF fillers studied, in the interlayer space of which approximately the same quantity of H 2 O molecules is localized that form strong H bonds the ER activity of ERF differs substantially. This fact makes it possible to assume that the important role in the manifestation of the ER effect is played not only by the quantity of "superstoichiometric" water in the interlayer space of the disperse phase particles, but also by the steric conditions, i.e., by the characteristic features of the lamellar structure of fillers.