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An investigation was performed to assess the state-of-the-art of recycling Portland cement concrete (PCC) airport pavements. Previous laboratory studies have shown that recycling of PCC pavements is technically and economically feasible. This has been demonstrated in airport reconstruction projects at Jacksonville International Airport (Florida), Love Field (Texas), and Coffeyville Municipal Airport (Kansas), where PCC was recycled into econocrete base and aggregate subbase, cement stabilized base, and part of the aggregate base course, respectively. Recycling of PCC for surface courses in airport pavement construction has not yet been performed, but this should also prove to be beneficial as has been experienced by the Iowa DOT and other state highway agencies who have recycled PCC for surface courses. Equipment for recycling PCC pavements is currently available in the construction industry; however, these, along with the technology of PCC recycling, could be improved substantially. Recommendations for specific improvements are therefore made. (Author)

The permeability and strength characteristics of six gradations of crushed limestone were evaluated in the laboratory to identify a gradation of crushed limestone that is stronger than the Tyndall gradation. Based on limited laboratory tests, pore pressure buildup did not seem to be a significant problem associated with the crushed limestone. The frictional strength criterion is not suitable for this crushed limestone because the friction angles obtained from undrained triaxial tests may not be valid within the applied range of confining pressures. With the assumption that strength at low confining pressure is more important than at high confining pressure, the cohesion criterion was selected for analyzing tests. Field traffic tests are needed to validate the assumption. (Author)

We present our first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. Both Cd0.8Mn0.2Te/Cd0.7Mn0.3Te and Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te systems are considered explicitly. Magnetic field splittings are calculated with and without the exchange interaction. We find that the exchange interaction dominates the magnetic effects in the wide-gap Cd0.8Mn0.2Te/Cd0.7Mn0.3Te system while the Landau level shift is also important in the Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te system. We present calculations of the superlattice band-gap variation with temperature and its derivative with magnetic field as a function of the superlattice layer thickness. Variation of the band offset in determining the values of the various quantities is examined.

The near-band-gap optical properties of superlattice are essential in determining the usefulness of these structures for application in infrared systems. In this paper we report on studies of a HgTe–CdTe superlattice. The optical characterization of the superlattice in the infrared was carried out by measuring its photoluminescence, transmission, and photoconductivity spectra. Results of these measurements as functions of temperature are presented, as are the theoretically calculated absorption spectra. We obtained good agreement between different measurement techniques and the theoretical model for the optical absorption and band gap.

The current status of HgTe–CdTe superlattices is reviewed. The properties predicted for the superlattices have been studied qualitatively in the cases where both theory and experiment exist. In particular, it has been found by both infrared absorption and photoluminescence that band gaps of the superlattices are substantially less than those of the alloy with the same average composition. Theoretical studies have concentrated on the factors (strain, band offsets, interdiffusion) that could result in substantial deviation of the properties of the superlattices from those predicted by the original simple theories. Some of the areas requiring further development are reviewed and discussed.

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k·p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k·p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.

We report the first study of the band structure of ZnSe-ZnTe superlattices. Our calculations are based on second order k⋅p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. Since the valence-band offset for this system is not well known, we have investigated the dependence of the superlattice band gap on the valence-band offset. Based on the assumption that the photoluminescence from the superlattice is due to an isoelectronic Te1-bound exciton in ZnSe, we have fitted the experimental photoluminescence data with k⋅p theory to obtain the best value of the valence-band offset. The value we find is 0.975±0.098 eV. Alternatively, assuming that the photoluminescence is due to band-to-band transitions we obtain a valence-band offset of 1.196±0.134 eV. We have also calculated the superlattice band gap as a function of the constituent-material-layer thicknesses for the first valence-band offset quoted. We expect these results to be important in gaining an understanding of the value of the band offset, and the nature of the photoluminescence from this system.

The effects of AlAs acoustical and optical phonons on the tunneling current in an ideal GaAs-AlAs-GaAs-AlAs-GaAs structure are discussed. The transfer Hamiltonian method was extended to inelastic tunneling current in a double-barrier structure. It is found that the current off resonance in the Tsu-Esaki model could be enhanced by orders of magnitude by inelastic tunneling due to the coupling of electrons to barrier optical phonons. The contribution to the current due to the deformation-potential coupling of electrons to barrier acoustical phonons is found to be much less important.

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k·p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k·p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.

We report the first study of the band structure of ZnSe-ZnTe superlattices. Our calculations are based on second order k⋅p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. Since the valence-band offset for this system is not well known, we have investigated the dependence of the superlattice band gap on the valence-band offset. Based on the assumption that the photoluminescence from the superlattice is due to an isoelectronic Te1-bound exciton in ZnSe, we have fitted the experimental photoluminescence data with k⋅p theory to obtain the best value of the valence-band offset. The value we find is 0.975±0.098 eV. Alternatively, assuming that the photoluminescence is due to band-to-band transitions we obtain a valence-band offset of 1.196±0.134 eV. We have also calculated the superlattice band gap as a function of the constituent-material-layer thicknesses for the first valence-band offset quoted. We expect these results to be important in gaining an understanding of the value of the band offset, and the nature of the photoluminescence from this system.

The near-band-gap optical properties of superlattice are essential in determining the usefulness of these structures for application in infrared systems. In this paper we report on studies of a HgTe–CdTe superlattice. The optical characterization of the superlattice in the infrared was carried out by measuring its photoluminescence, transmission, and photoconductivity spectra. Results of these measurements as functions of temperature are presented, as are the theoretically calculated absorption spectra. We obtained good agreement between different measurement techniques and the theoretical model for the optical absorption and band gap.

The effects of AlAs acoustical and optical phonons on the tunneling current in an ideal GaAs-AlAs-GaAs-AlAs-GaAs structure are discussed. The transfer Hamiltonian method was extended to inelastic tunneling current in a double-barrier structure. It is found that the current off resonance in the Tsu-Esaki model could be enhanced by orders of magnitude by inelastic tunneling due to the coupling of electrons to barrier optical phonons. The contribution to the current due to the deformation-potential coupling of electrons to barrier acoustical phonons is found to be much less important.

We present our first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. Both Cd0.8Mn0.2Te/Cd0.7Mn0.3Te and Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te systems are considered explicitly. Magnetic field splittings are calculated with and without the exchange interaction. We find that the exchange interaction dominates the magnetic effects in the wide-gap Cd0.8Mn0.2Te/Cd0.7Mn0.3Te system while the Landau level shift is also important in the Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te system. We present calculations of the superlattice band-gap variation with temperature and its derivative with magnetic field as a function of the superlattice layer thickness. Variation of the band offset in determining the values of the various quantities is examined.

The current status of HgTe–CdTe superlattices is reviewed. The properties predicted for the superlattices have been studied qualitatively in the cases where both theory and experiment exist. In particular, it has been found by both infrared absorption and photoluminescence that band gaps of the superlattices are substantially less than those of the alloy with the same average composition. Theoretical studies have concentrated on the factors (strain, band offsets, interdiffusion) that could result in substantial deviation of the properties of the superlattices from those predicted by the original simple theories. Some of the areas requiring further development are reviewed and discussed.

A multi-attribute decision model was developed for selecting the optimal mining method for thin coal seams. A total of 10 indicators encompassing economic, technical, and ergonomic criteria were selected for use in the evaluation, and their comprehensive weight vectors determined using Monte Carlo simulation. Alternatives were ranked by traditional AHP, in which qualitative and quantitative indices are calculated separately. According to the engineering verification, the Monte Carlo AHP method has more adaptability and precision than traditional AHP. The optimal mining method can be selected precisely using the MAHP technique. Using panel 43101 in Liangshuijing coal mine (Shaanxi Province, China), the ranking results and the certainty of the alternative mining methods for thin seam mining were calculated in terms of single and integral criteria., A multi-attribute decision model was developed for selecting the optimal mining method for thin coal seams. A total of 10 indicators encompassing economic, technical, and ergonomic criteria were selected for use in the evaluation, and their comprehensive weight vectors determined using Monte Carlo simulation. Alternatives were ranked by traditional AHP, in which qualitative and quantitative indices are calculated separately. According to the engineering verification, the Monte Carlo AHP method has more adaptability and precision than traditional AHP. The optimal mining method can be selected precisely using the MAHP technique. Using panel 43101 in Liangshuijing coal mine (Shaanxi Province, China), the ranking results and the certainty of the alternative mining methods for thin seam mining were calculated in terms of single and integral criteria.