Kinetics of Slag Hydration in the Presence of Calcium Hydroxide

The reaction between ground granulated blast furnace slag (GGBFS) having an empirical formula of C_7.88S_7.39M₃A and calcium hydroxide (CH) was investigated. The kinetics of the reaction were explored. Thermogravimetric analysis was used to monitor the consumption of CH from which rate constants were determined. Based on Knudsen's kinetic model, activation energies of 14.5, 17.9, and 22.6 kJ/mol were determined for three different mass ratios of slag and CH reacted over a temperature range of 15° to 50°C and hydrated for a period of time from 0 to 32 days. A comparison of the kinetics of the slag/CH reaction was made with slag/portland cement hydration. The basic features of both appear similar.     

Contributions of NIST/NBS Researchers to the Crystallography of Construction Materials

For more than 100 years, the primary theme underlying the NBS/NIST staff contribution to the crystallography of building materials has been the development of an improved understanding of concrete materials performance. Over that time period, portland cement concrete has become one of the most important of our construction materials for roads, buildings, and other large municipal structures. At the beginning of the 20th century our understanding of portland cement composition, performance, use in concrete, and how the concrete performs in harsh environments was lacking. The efforts of NIST have served to advance construction materials science and technology through the combined efforts of experimental, field study, and theoretical computational materials science. One major achievement in the late 1920s, derived from studies on phase equilibria in cement clinker, allows calculation of potential cement clinker composition. Known as the Bogue calculation, this continues to be an essential tool in cement plant process control to this day. Additionally, contributions of NIST scientists to our knowledge of the chemistry and nature of cement hydration products have been crucial in our understanding of cement hydration and concrete durability. Today, computational materials science is a rapidly developing discipline, and NIST is developing tools incorporating predictive models aided by empirical studies. Examples include a computer-integrated knowledge system for prediction and optimization of performance and life-cycle cost of high performance concrete and the Virtual Cement and Concrete Testing Laboratory. Understanding the relationships between material and performance properties has not been confined only to portland cements. One of the longest running experiments at NIST, the stone test wall, has stood for over 50 years as one of the world’s largest single collections of building stone, and is invaluable for studying weathering effects associated with stone mineralogy and texture. Standards development has also been promoted through participation on ASTM subcommittees on stone, cement, and concrete. The Cement and Concrete Reference Laboratory, established in 1929, continues to provide testing and training for outside laboratories and maintains a historical record of test data on construction materials.     

Analysis of reflector antenna systems with arbitrary feed arraysusing primary field superposition

In contrast to secondary pattern superposition, where the fields reflected from the main reflector arising from each element are superimposed in the far field of the reflector, the approach presented here sums the primary fields at the reflector surface before the physical optics radiation integral is performed. The method allows each feed array element to have arbitrary position, orientation, pattern, and excitation (magnitude and phase). In addition, it is inherently efficient because evaluation of only one time-consuming radiation integral is required, rather than one per feed element as in secondary superposition. The method allows for accurate calculation of the power radiated from the feed, permitting the reflector gain and spillover efficiency to be determined within the context of a single computer program. The accuracies and characteristics of this method are demonstrated with several examples     

Prediction of rain effects on earth-space communication links operating in the 10 to 35 GHZ frequency range

This paper reviews the effects of precipitation on earth-space communication links operating the 10 to 35 GHz frequency range. Emphasis is on the quantitative prediction of rain attenuation and depolarization. Discussions centre on the models developed at Virginia Tech. Comments on other models are included as well as literature references to key works. Also included is the system level modelling for dual polarized communication systems with techniques for calculating antenna and propagation medium effects. Simple models for the calculation of average annual attenuation and cross-polarization discrimination (XPD) are presented. Calculation of worst month statistics are also presented.     

A numerical study of the Jacobson-Oksman algorithm

The Jacobson-Oksman algorithm is a method for finding the unconstrained minimum of homogeneous functions. The optimal solution is obtained by solving a set of (n + 2) linear equation by a method based on Householder updating. The method has some very attractive features including robustness and rapid convergence, and it does not require accurate one dimensional searches. However, to date, little if any use has been recorded in the literature.We have studied the properties of the algorithm and also compared the method with the Fletcher-Reeves and Davidon-Fletcher-Powell methods. We have found the Jacobson-Oksman method to be superior to the former and comparable to the latter on general nonlinear functions. Therefore we recommend the Jacobson-Oksman algorithm as a competitive alternative to the celebrated class of quasi-Newton methods for unconstrained minimization.     

Cellular-Phone and Hearing-Aid Interaction: An Antenna Solution

With the introduction of digital cellular phones, hearing-aid users have experienced a severe buzzing noise caused by the interaction between digital cellular phones and hearing aids. The cellular-phone industry, the hearing-aid industry, and consumers have been seeking a solution for the interference issue. Efforts reported in the literature have focused on measurements, modeling, and evaluation of interference and RF emission, but not on methods to solve the problem. In this paper, we focus on the causes of the interference and an understanding of the problem. We also present a method to reduce near-field electromagnetic energy around a cellular phone, mitigating the interference between cellular phones and hearing aids. The theoretical investigation of both the radiation mechanisms and fundamental limits on antennas suggested that a low-g antenna, such as an ultra-wideband antenna, could reduce the near-field intensity. Simulations and measurements were performed at 900 and 1880 MHz, using both low- and high-Q test antennas mounted on a mock cellular phone. The results showed that the peak electric and magnetic near-field strengths of the low-g test antenna were lower than those of a high-Q test antenna by at least 5 dBV/m and 4 dBA/m, respectively. The improvement in the near-field performance for the low-g antenna was without any sacrifice in far-field performance. Furthermore, in the presence of a human head, the simulation results showed that the radiation efficiency of the low-Q test antenna was better than that of the high-Q test antenna.     

Millimeter wave attenuation prediction using a piecewise uniform rain rate model

A piecewise uniform rain rate distribution model is introduced as a quasi-physical model of real rain along earth-space millimeter wave propagation paths. It permits calculation of the total attenuation from specific attenuation in a simple fashion. The model predications are verified by comparison with direct attenuation measurements for several frequencies, elevation angles, and locations. Also, coupled with the Rice-- Holmberg rain rate model, attenuation statistics are predicated from rainfall accumulation data.     

ATS-6 satellite 20 GHz propagation measurements at low elevation angles

The 20 GHz downlink from the ATS-6 satellite was monitored almost continuously for low elevation angles (below 9°). The authors measured the copolarised and crosspolarised signals received, as well as the local rain rate. Data are presented for clear-weather scintillations, rain attenuation, depolarisation and other effects.     

Toward Stretchable Self‐Powered Sensors Based on the Thermoelectric Response of PEDOT:PSS/Polyurethane Blends

The development of new flexible and stretchable sensors addresses the demands of upcoming application fields like internet‐of‐things, soft robotics, and health/structure monitoring. However, finding a reliable and robust power source to operate these devices, particularly in off‐the‐grid, maintenance‐free applications, still poses a great challenge. The exploitation of ubiquitous temperature gradients, as the source of energy, can become a practical solution, since the recent discovery of the outstanding thermoelectric properties of a conductive polymer, poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS). Unfortunately the use of PEDOT:PSS is currently constrained by its brittleness and limited processability. Herein, PEDOT:PSS is blended with a commercial elastomeric polyurethane (Lycra), to obtain tough and processable self‐standing films. A remarkable strain‐at‐break of ≈700% is achieved for blends with 90 wt% Lycra, after ethylene glycol treatment, without affecting the Seebeck voltage. For the first time the viability of these novel blends as stretchable self‐powered sensors is demonstrated.