Al-Cu-Fe QUASICRYSTALLINE PHASE FORMATION BY MECHANICAL ALLOYING

Al-Cu-Fe alloys were prepared from elemental powders in a high-energy planetary ball mill. A sequence of solid state reactions resulting in quasicrystal (QC) phase formation takes place during heating of the as-milled powder. These reactions were studied by both differential scanning calorimetry and x-ray diffraction methods. Mechanically alloyed powders were consolidated by cold and hot pressing, as well as by explosive compaction. After annealing at sufficiently high temperatures, the consolidated samples are single-phase QC, except the ones consolidated by explosion. The high reactivity of the as-milled alloys causes the appearance of high porosity of the consolidated samples after the annealing.     

Surface acoustic wave gas sensor of the sorption type sensitive to the thermal properties of gases

The basic principles of a new surface acoustic wave (SAW) gas sensor are described. Being essentially a sensor of the sorption type, the proposed device possesses certain features of the thermometric SAW sensors and is not only sensitive to the vapors of volatile substances, but capable of detecting gases by their thermal properties as well. In contrast to the known thermometric SAW sensors, the proposed sensor is characterized by high temperature stability and fast response. A variant of the sensor based on a LiNbO₃ SAW delay line is described and some results of the test for detecting propane-butane mixtures are presented.     

A Nonlinear Filtering Scheme for Multistage Statistically Uncertain Systems

A filtering scheme for multistage systems under uncertain probability distributions of perturbations is studied. A recurrent scheme for constructing point state estimates is designed and its relation with guaranteed filtering theory is determined. This scheme covers the case in which both uncertain and random perturbations are present.     

Pulse-Frequency Characteristics in Bifurcation Problems

Generation of self-oscillations under unstable stationary states of nonlinear control systems, which may contain delay of complicated nature (the Andronov–Hopf bifurcation), and approximate design of self-oscillating states and their asymptotes are studied with the help of pulse-frequency characteristics of the linear component of a system.     

New phenomena in barbotage and sedimentation in magnetic fluids

New multivelocity model was deduced for a dispersive magnetizable medium: magnetic fluid - gas bubbles or solid particles. Resulting equations may be used for discribing the processes of concentration, separation and barbotage. The characteristics of deduced set were examined. The problem was solved concerning floatation and sedimentation of particles or babbles in a magnetic fluid in a solenoidal magnetic field.     

Tunable circular patch antennas

A method to control the resonant or operating frequencies of circular patch antennas has been investigated experimentally and theoretically. It consists of the placement of passive metallic or tuning posts at approximate locations within the input region of the antenna. Comparison of measured and analytical results seems to establish the validity of a theoretical model proposed to determine the input performance of such circular patch antennas.     

Fuzzy sets based knowledge systems and knowledge elicitation

Fuzzy sets are adequate forms of knowledge representation when the information is uncertain due to vagueness and imprecision. Knowledge structures using fuzzy sets are similar to those implemented in non-fuzzy systems. Classical knowledge elicitation methods can be used in combination with techniques to develop membership functions. The fuzzy set representation has several advantages, including flexibility in expressing uncertain knowledge during elicitation, representation of the knowledge and its uncertainty as a unique entity, easy interfacing with classical systems, and a more robust system in ill-defined domains. These advantages result in increased system reliability.     

Quantitative determination of non-defect unstable structures in PVC through selective nucleophilic substitution with C6H5SNa

Summary The nucleophilic substitution with C₆H₅SNa, at -30°C, is compared for five PVC samples with various contents of isotactic triads. The conversion curves consist of a very fast stage followed by a steady one. All the samples are found to behave in the same way except for the content of the structures involved in the fast period. This content is estimated by extrapolating the straight lines of the steady stages to zero time. The obtained values are found to be a linear function of the content of isotactic triads. The results, together with some earlier ones, allow for the content of the labile non-defect structures in PVC to be determined.