A modified route, based on calcined MgO, Ta2O5, BaCO3, and ZrO2, was developed and used for preparation of barium magnesium tantalate (BMT) and barium zirconate (BZ)-doped BMT ceramics (BZ-BMT). The dielectric constant for BMT ceramics sintered at 1600 °C for 2 h was 24.9 at 7.28 GHz. The average long-range ordering parameter for undoped BMT ceramics was 0.81 ± 0.03 and the coresponding average quality factor and resonant frequench product (Qd · fo) was 147,000 ± 3800 GHz. A significant level of B-site long-range cation disorder was introduced as a result of BZ doping of BMT ceramics. The average ordering parameters for 3 and 4 mol% BZ-doped BMT were found to be 0.69 ± 0.06 and 0.49 ± 0.13, respectively. The decrease in ordering parameters did not lead to a dramatic decrease in the corresponding average quality factors of 3 mol% BZ-BMT (Qd · fo = 127,000 ±3800 GHz) and 4 mol% BZ-BMT (Qd · fo = 139,000 ± 4200 GHz). The results suggest that the B-site cation ordering is not a primary factor that influences the observed microwave loss in BMT ceramics. The influence of atomic level point defects, induced from raw material impurities, processing, etc., may be more important in controlling the quality factor of BMT ceramics.

The influence of the remnant magnetization of the soft magnetic particulates, used as a dispersed phase, on the redispersibility of magnetorheological (MR) fluids is discussed. Calculations of the magnetic interaction energy showed that for 33-vol% MR fluids based on particles of iron (∼6 μm), manganese zinc ferrite (∼2.3 μm), and nickel zinc ferrite (∼2.1 μm), the ratios of the magnetic interaction energy to the thermal energy were 161,000, 6400, and 3900, respectively. These calculations showed that even the seemingly small levels of remnant magnetization, associated with particulates employed in MR fluids, introduced significant dipole–dipole interparticle interactions. It is proposed that this interaction causes most MR fluids to show a tendency for “cake formation,” which makes it difficult to redisperse these fluids. Our modeling presented here also suggests practical strategies to enhance the redispersibility of MR fluids.

Scientists and engineers are most familiar with single-crystal or polycrystalline field-responsive or “smart” materials with responses typically occurring while the materials remain in the solid state. This issue of MRS Bulletin focuses on another class of field-responsive materials that exhibits a rapid, reversible, and tunable transition from a liquidlike, free-flowing state to a solidlike state upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior in response to an externally applied electric or magnetic field and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids, respectively. They are often described as Bingham plastics, and exhibit a strong field-dependent shear modulus and a yield stress that must be overcome to initiate gross material deformation or flow. Prototypical ER fluids consist of linear dielectric particles (such as silica, titania, and zeolites) dispersed in nonconductive liquids such as silicone oils. Homogeneous liquid-crystalline (LC) polymerbased ER fluids have also been recently reported. MR fluids are based on ferromagnetic or ferrimagnetic, magnetically nonlinear particles (e.g., iron, nickel, cobalt, and ceramic ferrites) dispersed in organic or “aqueous liquids. Unlike ER and MR fluids, ferrofluids (or magnetic fluids), which are stable dispersions of nanosized superparamagnetic particulates (~5–10 nm) of such materials as iron oxide, do not develop a yield stress on application of a magnetic field. Applications of ferrofluids are primarily in the area of sealing devices (see Rosensweig for more information). Since ferrofluids are well-known and have been extensively discussed elsewhere in the literature, they will not be treated in detail here.

This article focuses on the synthesis and processing of novel magnetorheological (MR) fluids. The process for preparing MR fluids typically involves introducing magnetic particles into base liquid under low shear conditions. This is followed by ball milling in the fluid with zirconia (ZrO2) grinding media for about 24 h. High-purity carbonyi iron (Fe) powders have been used for the synthesis of ironbased MR fluids while the ferrite-based MR fluids used magnetic manganesezinc ferrite and nickel-zinc ferrite powders.

Typical volume fractions of the magnetic phase that lead to MR fluids with respectable yield stresses tend to be about 0.3–0.5. Higher volume fractions, in principle, can lead to higher strength MR fluids. However, higher volume fractions tend to cause a significant, and often undesirable, increase in the “off-state” viscosity of the MR fluids. The rationale for selection and the role of different components of MR fluids are briefly discussed in the following sections.

Magnetorheological (MR) fluids represent an exciting class ofsmart materials for use in active vibration control and other applications. This paper discusses some of the fundamental materials science concepts that define the scientific basis for designing MR fluids. Preliminary experimental data and observations concerning the synthesis as well as the rheological behavior of MR fluids based on carbonyl iron and iron oxide particulates are presented and discussed.

Several sol-gel syntheses make use of “additives” in the sol-preparation step. These additives can react with metal alkoxides causing important structural changes. In order to demonstrate the nature of the reactions involved in the molecular modification process, a model system based on Ta(OC2H5)5, acetic acid and acetylacetone (as modifiers), was studied using FTIR spectroscopy. At room temperature, both modifiers react with the ethoxide to cause formation of new organotantalum precursors. The acetic acid modification was observed from the characteristic symmetric and asymmetric vibrations of the COO groups. The average difference in wave numbers (δv = vasymm – vsymm) between the two of 140 cm−1, observed for different ratios of ac/Ta, signifies a bridging nature of the acetate group. For ac/Ta ≥ 1, formation of ethyl acetate, probably as a result of the reaction between ethanol and excess acetic acid, was also observed using FTIR. Results of the acetylacetone modification showed the enol form of acetylacetone which has its v(C–O) and v(C–C) vibrations centered around 1625 cm−1, split into two bands at 1581 and 1518 cm−1, respectively, indicative of chelation of the acetylacetanate ligand to Ta. In the case of acetylacetone modification, the formation of ethanol was confirmed by FTIR analysis of the liquid fraction distilled at ∼80 °C.

Controlled chemical polymerization of tantalum ethoxide in the presence of glacial acetic acid (HOAc/Alk. = 16) and solubilized lithium acetate (Li/Ta = 1.00, H2O/Alk. = 55.55) was used for the preparation of an amorphous gel precursor to LiTaO3. Although additional investigations are required, the results suggest that successful formation of amorphous gel network, as opposed to that of colloidal tantalum (hydrous) oxide, may be due to the generation of a new organotantalum precursor via a structural modification reaction between the tantalum ethoxide and glacial acetic acid. The evolution of LiTaO3 ceramics from pre-ceramic gels was investigated using thermal analysis, electron microscopy, and x-ray diffraction. The results indicate that after the completion of gel pyrolysis (200–400 °C) and crystallization (Tc = 590 °C), ultrafine (average particle size 100–300 nm), single phase, crystalline (a = 5.243, c = 13.812 Å) LiTaO3 powders can be prepared at low processing temperatures.

Constant pressure molecular dynamics (MD) simulations have been used to simulate amorphous and crystalline forms of BaTiO3 and TiO2. Simulation results for pure RaTiO3 and TiO2 glasses show fourfold titanium coordination with a Ti–O bond distance of 1.8 Å, consistent with experimental evidence for the structure of titania doped glasses for all optical switching and ultra low expansivity (ULE) TiO2–SiO2 glasses. Radial distribution function data for crystalline, liquid, and amorphous forms of BaTiO3 were also obtained. The displacement polarization and its contribution to susceptibilities have been calculated by application of an electric field to the simulation cell. The calculated ionic dielectric constants (Ki) for simulated NaCI (crystal), TiO2 glass, and TiO2 (crystal) were 3.34, 5.96, and 19.4 as compared to the experimental values of ≍3.34, 3–10, and 78, respectively. The calculated cubic nonlinear susceptibility (ξ3) values for NaCI (crystal), TiO2 (glass), and TiO2 (crystal) were, respectively, 0.194, 2.175, and 4.68 (x 10−18m2V−2). The increase in ξ3 values is consistent with experimentally observed trends of the linear refractive indices. Improved agreement between experimental and calculated values of susceptibilities and dielectric constant was obtained for materials with higher ionicity.

Chemical synthesis of titanate and zirconate based multicomponent electronic ceramics using molecularly modified alkoxide precursors is discussed. Controlled chemical polymerization of the modified alkoxides was conducted to prepare multicomponent gels which on calcination at moderate temperatures (700 – 1000°C) formed high purity, ultrafine powders (e.g. BaTiO3, ZrTiO4, LiTaO3). In the sol-precipitation technique modified precursors were used to prepare ultrafine (< 100 nm) and crystalline powders of such ceramics as BaTiO3 and BaZrO3 at low temperatures (< 100°C), under highly alkaline (pH > 13) conditions.

Sol-gel processes for the synthesis of high purity, ultrafine BaTi4O9 and BaTiO3 powders are described. Hydrolysis of an alkoxide precursor derived from barium metal and titanium (IV) isopropoxide resulted in formation of powders consisting of ultrafine (0.2 to 1.0 μm) spherical particles. These amorphous precursor powders were converted to crystalline BaTi4O9 powders (particle size 2–3 μm) by heat treatment at 1100 C. The sol-gel process for the synthesis of BaTiOj powders utilized chemical polymerization between moisture insensitive and relatively inexpensive barium acetate and titanium (IV) isopropoxide in the presence of acetic acid. The gel to ceramic conversion was achieved by firing the gels at 1000 °C to obtain high purity, stoichiometric BaTiO3 powders. The powders were sintered to obtain relatively dense (90 % relative density) ceramic bodies.