Showing papers by "Brunel University London published in 1972"
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01 Jan 1972TL;DR: This short review of free radicals discusses certain types of free radical, such as nitroxyl-radicals and free radicals stabilized by steric or derealization features, which are stable enough to be crystallised and stored at temperatures above 0°.
Abstract: Free radicals are molecules or molecular fragments containing a single unpaired electron. In general, free radicals are reactive chemically, some (e.g. HO•) being extremely reactive. However, certain types of free radical, such as nitroxyl-radicals and free radicals stabilized by steric or derealization features, are much less reactive and a few (e.g. diphenyl picryl hydrazyl) are stable enough to be crystallised and stored at temperatures above 0°. Table 1 gives the general structures of free radicals that will be discussed in this short review.
1,643 citations
01 Jan 1972
TL;DR: In this paper, the isosteric heat of adsorption is calculated from the isotherms (over the temperature range −192° to −178°C) on representative materials and surface areas are calculated by means of the BET method and the new αs-method.
Abstract: Nitrogen adsorption isotherms were determined on a wide range of porous and nonporous silicas. Isosteric heats of adsorption were calculated from the isotherms (over the temperature range −192° to −178°C) on representative materials. Standard data for nitrogen adsorption at −196°C on nonporous hydroxylated silica are tabulated for the p p o range 0.001–0.90. The results indicate that certain high-area silicas are truly nonporous, but some grades of commercial Aerosil are porous. Surface areas are calculated from the isotherms by means of the BET method and the new αs-method. The latter is a graphical procedure in which the amount adsorbed is plotted against αs for the standard adsorption data, where αs is the ratio of the amount adsorbed (at the given p p 0 ) to the amount adsorbed at p p 0 = 0.4. Deviations of the a,-plots from linearity are explained in terms of micropore filling and capillary condensation. In the absence of micropore filling, the surface areas calculated from the slope of the αs-plots are in excellent agreement with the BET-areas. Enhanced isosteric heats and C values are associated with micropore filling; the isotherm is therefore distorted in the BET range and the BET-area is not valid. In certain cases, when micropore filling and monolayer coverage at low p p 0 are followed by multilayer formation and capillary condensation at higher p p 0 , a nearly linear αs-plot results, but again neither the BET-area nor the αs-area can provide a meaningful value of the internal surface area.
199 citations
TL;DR: In this article, the white layer is associated with scuffing of piston rings and cylinder liners in internal combustion engines and their properties suggest that it might be possible to take commercial advantage of friction hardened surfaces.
118 citations
TL;DR: Although the catalytic properties of gold are surpassed by those of the Group VIII metals, especially palladium and platinum, possible applications of gold in catalytic processes have been widely studied, more especially for oxidative dehydrogenation.
Abstract: Although the catalytic properties of gold are surpassed by those of the Group VIII metals, especially palladium and platinum, possible applications of gold in catalytic processes have been widely studied, more especially for oxidative dehydrogenation. Alloys and mixtures of gold with the platinum group metals are also receiving increased attention, and the developments outlined here indicate a number of potential uses.
66 citations
TL;DR: In sugar-fed A. gambiae females, light may affect flight activity directly or by changing the phase of the circadian rhythm; both responses depend on the phaseof the rhythm.
Abstract: 1. In sugar-fed A. gambiae females, light may affect flight activity directly or by changing the phase of the circadian rhythm; both responses depend on the phase of the rhythm. 2. The phase-response curve (1 h, 70 lux, signals given in the first cycle in DD following LD 12:12) shows a sharp swing, at about 3 h after normal light-off, from a maximum phase-delay to a maximum phase-advance, each of about 2 h. When signals are given at this time, phase re-setting is very variable; cyclical activity continues but the individuals are out of phase. 3. Phase shifting appears to be a function of the energy of the signal. A 5 min, 70 lux signal has no apparent effect. The effect of a 1 h signal increases with intensity, up to at least 500 lux, but does not appear to be significant below 10 lux. 4. Light normally inhibits flight activity, but there is a burst of activity at light-on (light-on response) if it occurs during the active half of the cycle following the initial activity peak. A vigorous light-on response occurs even at the lowest intensity used (0.3 lux).
50 citations
TL;DR: In this article, a method consisting of three conformal transformations is used for the numerical solution of a class of harmonic mixed boundary value problems containing boundary singularities, although less flexible than the well-known finite-difference and finite element techniques.
Abstract: A method consisting of three conformal transformations is used for the numerical solution of a class of harmonic mixed boundary value problems containing boundary singularities. The method, although less flexible than the well-known finite-difference and finite element techniques, is very well suited to problems of this class. In the present paper the transformations are described in detail and the merits and shortcomings of the method are discussed objectively.
48 citations
TL;DR: In this paper, the thermal behavior of CrOOH and CrO2 in different atmospheres has been studied by means of DTA, TG and X-ray diffraction, and the easy interconversion between the two substances at one atmosphere pressure is explained on the basis of the results obtained.
Abstract: The thermal behaviour of CrOOH and CrO2 in different atmospheres has been studied by means of DTA, TG and X-ray diffraction. The easy interconversion between the two substances at one atmosphere pressure is explained on the basis of the results obtained.
35 citations
TL;DR: The X-ray diffraction pattern of liquid Ge recorded by diffractometer between 964 and 1130°C provided a temperature-independent interference function presenting a low first peak with a shoulder on its high angle side as discussed by the authors.
Abstract: The X-ray diffraction pattern of liquid Ge recorded by diffractometer between 964 and 1130°C provided a temperature-independent interference function presenting a low first peak with a shoulder on its high angle side The corresponding rdf shows 71 first neighbors at 28 A, all order being lost beyond about 6 A The experimental interference function cannot be fitted to one derived from a hard-sphere model, but is in good agreement with a ball and spoke model based upon a white tin-like short-range order
34 citations
TL;DR: In this paper, the authors developed a theory for the initial slope for dilute suspensions of long thin cylinders with various electrical properties and showed that the inital slope depends not only on the particle-size distribution and electric field strength, but also on the nature of electrical properties of the molecules.
Abstract: The decay curves of electro-optic transients are generally used to measure the rotary diffusion contants (D) of colloidal particles in suspension or biopolymers in solution. Analysis usually is made by plotting the logarithm of the decay as a function of time, t. Curvature in such plots the presence of several values of D and hence a distribution of particle sizes. In this case, analysis of the complete curve to yield the distribution is severely restricted by lack of precision in the experimental data. The most reliable and reproducible part of the decay curve is the initial (t =0)slope.
This paper develops theory for the initial slope for dilute suspensions of long thin cylinders with various electrical properties. It is shown that the inital slope depends not only on the particle-size distribution and electric field strength, but also on the nature of the electrical properties of the molecules. Futhermore, for an identical polydisperse system. transient electric birefringence, electric optical rotation, and electric dichroism on the one hand, and transient electric light scattering on the other, yield different initial slopes and hence apperently different values for D. These important conclusions have not been appereciated in previous studies and indicate the need for caution when comparing data from different experimental methods. I11ustrative calculations are presented for a polydisperse system of long thin cylinders having a normal distribution in lengths, but of uniform diameter. Some preliminary measurement support the theory.
31 citations
29 citations
TL;DR: Some of the problems associated with the use of computer graphics and some applications in the physical and biological sciences are outlined.
Abstract: This article outlines some of the problems associated with the use of computer graphics and describes some applications in the physical and biological sciences.
01 Jan 1972
TL;DR: In this article, the results of Sard kernel theorems are used to obtain sharp bounds for the interpolation errors in each element, in this case a triangle, and these bounds can be used to produce bounds in the Sobolev norm for the finite element solution of the elliptic boundary value problem.
Abstract: Publisher Summary This chapter discusses the methods for obtaining bounds on the errors in finite element solutions to two-dimensional elliptic boundary value problems defined on simply connected polygonal regions. It also discusses the techniques of Birkhoff, Schultz, and Varga for obtaining bounds when using rectangular elements, and those of Zlamal and Bramble and Zlamal with triangular elements. The results of Sard are used, and it is through the use of the Sard kernel theorems that one obtains sharp bounds for the interpolation errors in each element, in this case a triangle. The forms of the bounds are similar to those of Zlamal and Bramble and Zlamal that contain unknown constants, but the corresponding constants are computable. The bounds can be used to produce bounds in the Sobolev norm for the finite element solution of the elliptic boundary value problem as in Bramble and Zlamal. The results are then applied to the problems containing boundary singularities by augmenting the spaces of trial functions with singular functions having the form of the dominant part of the singularity. It is shown that the approach using triangular rather than rectangular elements has the advantage that it is significantly simpler to implement computationally.
01 Jan 1972
TL;DR: In this paper, it is shown how the most accurate value indicated by a set of measurements can be found and how its accuracy can be estimated, and how the closer these limits the more accurate the measurement.
Abstract: Although physics is an exact science, the pointer readings of the physicist’s instruments do not give the exact values of the quantities measured. All measurements in physics and in science generally are inaccurate in some degree, so that what is sometimes called the “accurate” value or the “actual” value of a physical quantity, such as a length, a time interval or a temperature, cannot be found. However, it seems reasonable to assume that the “accurate” value exists, and we shall be concerned to estimate limits between which this value lies. The closer these limits the more accurate the measurement. In short, as the “accurate” value is denied us, we shall endeavour to show how the “most accurate” value indicated by a set of measurements can be found, and how its accuracy can be estimated.
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TL;DR: In this article, the authors consider some aspects of dropping out and wastage of students from craft courses and propose a method to reduce the number of dropped out students from these courses.
Abstract: This article considers some aspects of dropping out and wastage of students from craft courses.
TL;DR: In this paper, electrical conduction properties of thin co-evaporated films of SiOx (70%)-B2O3 (30%) are described and a region of differential negative resistance in the current-voltage characteristic is observed for films of thickness approximately 3 × 10−5 cm.
Abstract: Some electrical conduction properties of thin co-evaporated films of SiOx(70%)-B2O3 (30%) are described and a region of differential negative resistance in the current-voltage characteristic is observed for films of thickness approximately 3 × 10−5 cm. The films, deposited between aluminium electrodes, show evidence of electroforming and of electron emission into a vacuum when a bias voltage is applied. At voltages of order 30 V at which the negative resistance occurs, electron emission into a vacuum increases very rapidly with voltage. The emission current density is of order 10−5 Acm−2 and since the circulating current through the sandwich is lower than in similar films of SiO, the transmission efficiency is high.
On decrit quelques proprietes electriques des couches minces de SiOx (70%)-B2O3 (30%), preparees par co-evaporation dans le vide et on observe une region de resistance negative dans la caracteristique courant-tension pour les couches d'une epaisseur de 3 × 10−5 cm environ. Les couches, formees entre les electrodes d'aluminium, montrent l'evidence de formation electrique et de l'emission d'electrons lorsqu'on applique une tension directe a la couche dans le vide. Pour les tensions appliquees d'environ 30 V ou on observe la resistance negative, l'emission electronique dans le vide s'augmente rapidement avec la tension. La densite du courant d'emission est environ 10−5 Acm−2 et puisque le courant qui circulent dans cette couche est plus petit que celui qui est dans les pareilles couches de SiO, l'efficacite de transmission est grande.
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TL;DR: The Gaussian law of error as mentioned in this paper states that measurements of a given quantity which are subject to accidental errors are distributed normally about the mean of the observations, which is known as the Gaussian Law of Error.
Abstract: The function \(y = \frac{1} {{\sigma \surd \left( {2\pi } \right)}}{e^{ - {{\left( {x - \bar x} \right)}^2}/2{\sigma ^2}}}\) which defines a normal frequency distribution is often called the Gaussian law of error. This “law” states that measurements of a given quantity which are subject to accidental errors are distributed normally about the mean of the observations. More precisely, the law infers that any set of measurements of a given quantity may be regarded as a sample taken from a very large population—the aggregate of all the observations that could be made if the instruments and time allowed—and that this population is normal.
TL;DR: The theoretical problems of state reduction are discussed and how a rapid solution may be obtained based on the determination and use of a closure function associated with each maximal or prime compatible set is demonstrated.
Abstract: As part of a large research programme on the use of computer aids for logic circuit design, it was required to provide an algorithm for the reduction of sequential circuit state tables (completely or incompletely specified). This paper discusses the theoretical problems of state reduction and demonstrates how a rapid solution may be obtained based on the determination and use of a closure function associated with each maximal or prime compatible set. The use of several heuristics ensures that a near-minimal solution to the subsequent closed-cover problem is always obtained, rather than the absolute minimum that is theoretically possible but computationally impracticable. This is in keeping with the overall design philosophy of producing a viable engineering design rather than the theoretical optimum usually dictated by switching theory. The algorithm has been programmed and the paper further discusses the data structures used and problems encountered in its implementation.
TL;DR: In this article, the temperature dependence of the stability of various zinc borates has been investigated in the range 600 to 900°C using X-ray powder diffraction techniques.
Abstract: The temperature dependence of the stability of various zinc borates has been investigated in the range 600 to 900°C using X-ray powder diffraction techniques. At low temperatures, 640 to 700°C, the only zinc borates detected areα-3ZnO.B2O3 and ZnO.2B2O3; super-position of these patterns accounts for the pattern ofα-ZnO.B2O3 reported previously. At temperatures of 71°C and above, ZnO.2B2O3 decomposes to 4ZnO.3B2O3 and boric oxide. However, this decomposition is very sluggish and ZnO.2B2O3 may be detected at temperatures as high as 900°C.
TL;DR: In this paper, the electrical properties of evaporated manganese/silicon oxide cermet thin films were described and the variation of the electrical resistivity with starting composition (0 to 60% by weight) was explained in terms of the formation of silicides Mn 3 Si and Mn 5 Si 3.
TL;DR: In this paper, the pore structure of alumina aerogels was examined by adsorption of nitrogen and carbon tetrachloride, and it was shown that both the micropore volume and the external surface area diminished with temperature increase from 200 to 900 °C.
TL;DR: In this article, the effects of temperature, solvent, concentration and the nature of L on the linkage isomer ratio of trans-(L2PtHX) complexes were studied.
TL;DR: In this article, the authors used light scattering for determining the geometric parameters of macromolecular solutes in solution, which has serious theoretical and experimental limitations when the solutions partly absorb the incident light.
01 Sep 1972
TL;DR: Two methods of incoporating regenerative braking in slip-power-recovery systems are described: one requires a separate source of direct current and is called the `separately excited' regenerative brake; the second employs direct current from the d.u.c. link in the system and iscalled the self-excited system.
Abstract: The basic theory of solid-state slip-power-recovery systems is now well established. One of the disadvantages of such systems has been the lack of a suitable method of regenerative braking: this is frequently an important requirement for many applications. The paper describes two methods of incoporating regenerative braking in slip-power-recovery systems. One method requires a separate source of direct current and is called the `separately excited' regenerative brake; the second method employs direct current from the d.c. link in the system and is called the self-excited system. Both systems are shown to be practicable and capable of operation down to speeds of at least 0.05 p.u.
TL;DR: In this article, phase equilibria in the system CaO-ZnO-B2O3 were investigated at 850°C using X-ray powder diffraction techniques.
Abstract: Phase equilibria in the system CaO-ZnO-B2O3 were investigated at 850°C using X-ray powder diffraction techniques. The binary phases reported previously were confirmed, but no ternary phases were found. Solid solution effects were investigated for the binary phases by comparison of the patterns, whereas for calcium oxide and zinc oxide, accurate lattice parameters were compared. No solid solutions were detected.
TL;DR: In this paper, the authors define a new structural parameter of polymers, and show how this may be employed, for which only the structure and density are needed, is the relative number of network bonds per unit volume; it is written Ner (basis 1 cm3), and it is used in conjunction with the average connectivity or connection number CN of the network atoms, where the word connection also refers only to network bonds.
Abstract: In this paper we define a new structural parameter of polymers, and show how this may be employed. This parameter, for which only the structure and the density are needed, is the relative number of network bonds per unit volume; it is written Ner (basis 1 cm3), and it is used in conjunction with the average connectivity or connection number CN of the network atoms, where the word connection also refers only to network bonds. The relationship of these two numbers makes it possible to present a unified picture of all polymers, organic and inorganic, including such highly condensed networks as diamond. By plotting Ner against CN, the region in which inorganic and organic polymers occur can be seen. This is called the polymer zone, and it is evident that carbon polymers occupy only a small part of it. From this graph, an arbitrary measure of bond packing efficiency can be deduced for carbon polymers. In addition, the process of graphitization and carbonization can be followed graphically within the polymer zone. Ner is also related to certain physical properties such as bulk modulus, hardness, and cubical coefficient of expansion, which depend (among other things) on the tightness of bond arrangement. The resultant correlations can be used to predict the value of Ner required to achieve given values of these properties for the more rigid structures.
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TL;DR: In this article, the authors studied the response to incremental stress and temperature changes in germanium single crystals oriented for double slip, in the range 470 to 700° C, and found that the stress sensitivity of the compressive creep rate is numerically close to 3 at low strains, but increases appreciably with deformation.
Abstract: Isothermal creep, as well as the response to incremental stress and temperature changes, were studied in germanium single crystals oriented for double slip, in the range 470 to 700° C. The stress-sensitivity of the compressive creep rate δ In έ/δ In σ is numerically close to 3 at low strains, but increases appreciably with deformation. This effect, and a similar strain dependence of the activation energy as determined by thermal cycling, are explained in terms of the curvature of the creep curves on the basis of Boltzmann's superposition principle. The Peierls barrier seems to be an important obstacle to dislocation movement at relatively low temperatures, when S-shaped creep curves are observed. Other barriers, with greater heights, seem to become increasingly effective above about 550° C. Although dislocation loops, and the formation and break-up of dipoles were observed by TEM, recovery mechanisms involving self-diffusion did not appear to make a substantial contribution to the creep within the range of temperatures used.
01 Jan 1972
TL;DR: In this article, the elastic and plastic components of the deflection of joints formed from machined surfaces are calculated, which is necessary before the more complicated aspects of quantitive joint analysis, such as the distribution of pressure from a bolt.
Abstract: One of the biggest impediments to the complete computer simulation of Machine Tool Structures is the inability to calculate values of stiffness and damping for the joints. This paper sets out a method whereby the elastic and plastic components of the deflection of joints formed from machined surfaces may be calculated. This knowledge is necessary before the more complicated aspects of quantitive joint analysis, for example, the distribution of pressure from a bolt, may be carried out.