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Porous Si 3 N 4 ceramics were fabricated by liquid-phase sintering with a Yb 2 O 3 sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb 2 O 3 additive content. Fibrous β-Si 3 N 4 grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si 3 N 4 grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.

Porous ZrO2 ceramics were fabricated by adding Zr(OH)4 hard agglomerates to ZrO2 powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO2 powder were poor. The addition of Zr(OH)4 increased the strength and fracture toughness of the porous ZrO2 ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO2 ceramics increased with the incorporation of Zr(OH)4. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO2 powder were nonuniform due to the ZrO2 soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO2 ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)4, because the bimodal particle size distribution confined the formation of ZrO2 soft agglomerates due to a space constraint and an internal friction between the Zr(OH)4 hard agglomerates during compaction. As Zr(OH)4 decomposed into ZrO2 grains during heating, the Zr(OH)4 hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)4 hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO2 ceramics.

Porous ZrO2 ceramics were fabricated by compacting a fine ZrO2 powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO2 specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO2 grains. It was revealed that the ZrO2 agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO2 ceramics.

The mechanical properties of Al2O3-based porous ceramics fabricated from pure Al2O3 powder and the mixtures with Al(OH)3 were investigated. The fracture strength of the porous Al2O3 specimens sintered from the mixture was substantially higher than that of the pure Al2O3 sintered specimens because of strong grain bonding that resulted from the fine Al2O3 grains produced by the decomposition of Al(OH)3. However, the elastic modulus of the porous Al2O3 specimens did not increase with the incorporation of Al(OH)3, so that the strain to failure of the porous Al2O3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)3 particles. Fracture toughness also increased with the addition of Al(OH)3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.

The exciton states in V-shaped GaAs–Ga 1− x Al x As quantum wires (V-QWRs) are investigated by a coordinate transformation method. The exciton binding energies for the first three exciton transitions are calculated. It is found that the exciton binding energy decreases with increasing the index of exciton states. The exciton binding energy increases with the decrease in the dimension of V-QWRs, to a peak value at a small width of V-QWRs and then decreases. The dependence of exciton binding energy on the curvatures of V-QWRs is different for different exciton transitions.

Bulk porous Al 2 O 3 support was fabricated using a mixture of fine α-Al 2 O 3 powder and Al(OH) 3 particles, followed by pressureless sintering at temperatures between 1100°C and 1300°C. Al 2 O 3 support with high surface area was obtained, due to the presence of transitional Al 2 O 3 phases that were produced by the decomposition of Al(OH) 3 even after sintering. The Al 2 O 3 support exhibited superior mechanical properties and high strain to failure, compared with those fabricated by traditional methods. The strain to failure increased with the amount of Al(OH) 3 in the mixture, but decreased with increasing sintering temperature. A conceptual model for the microstructural evolution and reinforcing mechanisms, based on transmission electron microscopy (TEM) observations, is proposed. It reveals that the interface bonding of the Al 2 O 3 boundaries formed at the initial nucleation stage during θ- to α-Al 2 O 3 transformation is stronger than that formed by subsequent grain growth. The interface bonding between the Al 2 O 3 grains, which came from the decomposition of Al(OH) 3 , is stronger than that between the original Al 2 O 3 grains in the starting mixture.

Al 2 O 3 -based porous ceramics with high surface areas were fabricated by adding Al(OH) 3 to the starting powder, followed by pressureless sintering at temperatures >1100°C. Three types of starting powders were used in the present study: pure α-Al 2 O 3 , α-Al 2 O 3 + Al(OH) 3 , and α-Al 2 O 3 + ZrO 2 + Al(OH) 3 . The addition of Al(OH) 3 considerably increased the surface area of the porous Al 2 O 3 , and the addition of ZrO 2 further increased the surface area; a surface area as high as 36.39 m 2 /g was obtained, and the high surface area was retained at higher temperature. The pore size distribution of the specimens with high surface area was bimodal, with one peak at ∼100 nm and the other, which contributed most of the surface area, at ∼10 nm. X-ray analysis showed that in the sample with the fine pores and high surface area, there was θ-Al 2 O 3 phase produced by the decomposition of Al(OH) 3 , presumably because the phase transformation of θ-Al 2 O 3 to α-Al 2 O 3 was incomplete after low-temperature sintering. Moreover, the porous Al 2 O 3 with high surface area retained superior mechanical properties, attributed to the good sinterability of the fine α-Al 2 O 3 powder used in the present study. The sintered specimens could be large and designed to any shape, because pressureless sintering was used for fabrication. The present approach provides a new way of fabricating porous Al 2 O 3 ceramics that could be widely used as catalyst supports in industry, especially for high-temperature catalysis.

A unique porous ceramic with complex pore structure was synthesized by the freeze-dry process. A water-based ceramic slurry was frozen while controlling the growth direction of ice, and sublimation of the ice were generated by drying it at a reduced pressure. By sintering this green body, a porous ceramic with complex pore structure was obtained, where macroscopically aligned open pores exceeding 10 μm in size contained minute pores of about 0.1 μm in their internal walls. Wide control of the porosity was possible by changing the concentration of the starting slurry. The pore size distribution as well as the microstructure were substantially affected by the freezing and sintering temperatures. Optimization of the synthesis conditions was investigated in order to obtain the desired pore structure.

High-surface-area alumina with aligned macro- and microscopic pores was synthesized by a single-step processing using freeze-drying. A water-based slurry of Al2O3 and Al(OH)3 mixture was frozen while controlling the freezing direction. The frozen slurry was dried under reduced pressure and columnar pores were generated after sublimation of ice in the green body. By sintering this green body, we obtained porous alumina with a complex pore structure, where aligned macroscopic open pores contained 10nm sized pores on their internal walls. These minute pores were formed during thermal decomposition of Al(OH)3, which consequently resulted in a high surface area.

The effect of the addition of Y2O3-Al2O3 additive on the phase formation and densification process of in situ SiC-BN composite has been investigated. The in situ reaction started at 1450°C despite the addition of Y2O3-Al2O3 additive. However, when Y2O3-Al2O3 was added the reaction was much faster and completed after hot pressing at 1700°C. In addition, dense material with well-crystallized SiC and BN grains could be obtained. It was found that denitrification occurred during the reaction process for both specimens with and without Y2O3-Al2O3 additive. However, denitrification occurred continuously at temperatures higher than 1700°C for the specimen without Y2O3-Al2O3 additive and eventually resulted in decrease of nitrogen content. The above results were explained based on the following mechanism, that is, the Y2O3-Al2O3 additive formed a Y-Al-Si-O liquid phase with SiO2 present in Si3N4 raw powder, enhancing diffusion of the elements of the reactants.

Previous works indicated that Al particle surfaces could be modified by fine γ-Al2O3 grains, which can be used as a hydrogen-generation material, where the fine γ-Al2O3 grains were produced by the decomposition of the Al(OH)3 phase in the mixture. In this work, commercially available γ-Al2O3 powder was directly used as the modification agent and mixed with Al powder and heat treated at an elevated temperature. It was found that the modified Al powder produced by directly using γ-Al2O3 has an obvious shorter time for complete hydrogen generation than that using Al(OH)3-produced γ-Al2O3. Microstructure analyses revealed that directly using γ-Al2O3 powder has a better coverage of fine γ-Al2O3 grains on Al particle surfaces than the Al(OH)3-produced γ-Al2O3. This implies that a uniform distribution of modification agents on Al particle surfaces is an important factor for the Al–water reaction dynamics.

The subband structures and exciton and impurity states in V-shaped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ quantum wires (V-QWRs) are investigated by a coordinate transformation method with a variational procedure. The results show that the subband gaps are proportional to the curvature of V-shaped boundaries. Some ``forbidden'' transitions between electrons and heavy holes are found, due to lack of the inversion symmetry for V-QWRs. The exciton binding energies are different for different exciton transitions depending on the localization of exciton states. The theoretical exciton peaks are in good agreement with the recent experimental photoluminescence excitation (PLE) spectra [Phys. Rev. Lett. 78, 1580 (1997)]. The results also show that there is an asymmetrical distribution of impurity binding energy along the direction normal to the V-shaped boundaries. The impurity position corresponding to maximum binding energy deviates from the center, depending on the dimension and curvature of V-QWRs. The variation in impurity binding energy with impurity position is discussed.

The shallow acceptor impurity states in V-shaped GaAs-Ga1 - x Alx As quantum wires (V-QWRs) are investigated by a coordinate transformation method. An asymmetrical distribution of impurity binding energy along the direction normal to the V-shaped boundaries is found. The impurity position corresponding to the maximum binding energy deviates from the centre of V-QWRs. The variations in impurity binding energy with the dimension and curvatures of V-QWRs are discussed.

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science,Shanghai 200083, China~Received 11 February 1999!Persistent currents and charge stiffness in one-dimensional~1D! correlated disordered rings are investigatedby a transfer-matrix method. It is found that, under certain conditions the electronic states do not feel theexistence of the disorder and the ring recovers the ordered case. If the occupied energy is just at the unscatteredstate energy level, the persistent current is not reduced and shows the behavior of sudden jumps regardless ofthe disorder, contrary to the general theorem where the current is obviously reduced and becomes a smoothfunction of flux, passing through zero. For other occupied energy, the persistent current is scattered anddecreased as expected. The results indicate that a quantum interference effect can be observed in 1D correlateddisordered rings. Further, the results provide evidence that the realization of delocalized states in the experi-ments is easily obtained by observing only the persistent current of finite length rings, in contrast to the 1Ddisordered chain where the infinite length is needed.I. INTRODUCTION

The compressive creep behavior of monolithic mullite and a composite made of mullite reinforced by 40 vol% SiC fiber were investigated at temperatures from 1100 to 1200°C and under stresses from 5 to 55 MPa in air with a loading direction parallel and perpendicular to the fiber direction. For both situations the composite exhibits better creep resistance than monolithic mullite, although there is a creep anisotropy. The improvement in creep resistance when the fibers are parallel to the loading directions is due to the shedding of the applied stress on the SiC fibers, and the improvement in creep resistance when the fibers are perpendicular to the loading direction occurs because the fibers inhibit the lateral deformation of the mullite matrix along the fibers. The improvement mechanisms of the composites were confirmed further by their creep-recovery study, which indicated that the two types of composite specimens exhibit both an apparent creep-recovery behavior on load removal, due to the relaxation of the residual stress state between the mullite matrix and the SiC fibers after unloading. ©

Flexure creep and creep-recovery behavior were investigated for monolithic Al2O3 and 10-vol%-SiC-particle-reinforced Al2O3-matrix composites in an air atmosphere at temperatures of 1160°-1400°C. Two types of SiC particles were used: one has an average size of 2.7 µm and has an amount of SiO2 impurities per unit surface area that is one order of magnitude higher than the other, which has an average size of 0.6 µm. Compared to the creep behavior of monolithic Al2O3, the strain rate of the composites with the 0.6 µm SiC particles (denoted here as S-10) did not decrease; the composites with the 2.7 µm SiC particles (denoted here as L-10) exhibited excellent creep resistance. This difference was related to the microstructural features and the oxidation behavior of the composites: the Al2O3 grains in S-10 were mainly equiaxed, only ∼10% of the Al2O3 grains were elongated, and most of the SiC particles that resided at the grain boundaries or at triple-grain junctions were oxidized during creep, whereas the Al2O3 grains in L-10 were mostly irregularly shaped and elongated and most of the SiC particles were entrapped in the Al2O3 matrix grains, which prevented the oxidation of the SiC particles. These different microstructural features were associated with different amounts of SiO2 impurity content per unit surface area on the SiC particle surfaces. In addition, the monolithic Al2O3 showed no anelastic recovery when the load was removed; however, the composites exhibited significant anelastic recovery, especially for L-10. This phenomenon was attributed to the elongated grain morphology.

The optical absorption spectra associated with transitions from the first valence subband to donor impurities are calculated in a three-dimensional corner under an applied electric field. It is found that there are two peaks for the absorption spectra in the corner, and the width of the absorption spectra increases with the increase in the electric field strength. The comparison between the optical absorption spectra in the corner and in a cubic quantum box shows that there is one absorption peak in the cubic quantum box, and the width of the absorption spectra in the equivalent quantum box is larger than that in the corner.

We study shallow-donor impurity states and the optical absorption spectra associated with transitions from the n = 1 valence subband to the donor impurities in a corner under an applied electric field. It is found that the energy contours for the impurity binding in the corner become closed when a suitable electric field is applied; this is similar to the impurity-state behaviour in quantum wires. When the electric field strength increases, the centre of the contours moves towards the corner and the centre impurity binding energy increases. The optical absorption spectra associated with donors in the corner are also similar to those of quantum wires when the electric field is applied, and the width of the absorption spectra increases with the increase in the electric field strength.

The fracture mechanical properties of SiC particle reinforced Al2O3 matrix composites were investigated at room temperature and T=1200°C. The results showed that the fracture strength and toughness of composites are enhanced, compared with those of monolithic Al2O3, especially at elevated temperature, and the strength and toughness of composites increase with the increase in the content of SiC particles. It was found that the improvement in fracture mechanical properties of composites is due to the fracture mode transformation from intergranular in monolithic Al2O3 to transgranular in the composites, and the fracture mode change might result mainly from the pinning effect of SiC particles on the Al2O3 boundaries. The study on flexure creep behavior of monolithic Al2O3 and the composites with 20 vol% SiC particles at T=1260°C indicated that the composites also exhibited some improvement in their creep resistance, compared to the creep behavior of monolithic Al2O3, due to the pinning effect of SiC particles at Al2O3 grain boundaries on grain boundary sliding during creeping.

We study the electronic and impurity states in a three-dimensional corner under an applied electric field. It is found that the electronic states in the corner are discrete in all three directions, similar to the electronic state behaviour in a quantum dot, and the eigenenergies of the discrete electronic levels increase with the electric field strength. The impurity states in the three-dimensional corner have similar features as those in the quantum dots, and their maximum binding energy increases, but the impurity position corresponding to the peak binding energy moves towards the corner, as the electric field strength increases. The results also show that the binding energy of the ground impurity state at the corner point in the absence of electric field is equal to that of the fourth impurity excited state in the bulk.

With the use of a variational method to solve the effective-mass equation, we have studied the electronic and hydrogenic impurity states in a corner under an applied electric field. The electron energy levels and the impurity binding energies are calculated. Our results show that, with the increasing strength of the electric field, the electron energy levels increase, and the impurity binding energy in the ground state increases at first, to a peak value, then decreases to a value which is determined by the impurity position in the corner. The dependence of the impurity binding energy on the applied electric field and impurity position is discussed in detail.

The effect of temperature on the reaction of γ-Al2O3-modified Al powders with distilled water was investigated. It was found that by increasing the temperature up to 40°C, the hydrogen generation speed can be enhanced one to two orders of magnitude relative to that at room temperature (18°C). X-ray analyses and transmission electron microscopy observations revealed that the reaction by-product at 40°C is bayerite (Al(OH)3), which is chemically neutral. The present results imply that slightly increasing the temperature is an effective way to get the target hydrogen generation speed.

Metal Al particle surfaces were modified by fine γ-Al2O3 grains using a mixture of Al and Al(OH)3 powders, which was pressureless sintered at a temperature of 600°C in vacuum. All the constituents on original Al particle surfaces were transformed into γ-Al2O3 phase after sintering. No dense passive oxide layers on Al particle surfaces were formed even after the modified Al particles were exposed to oxidation environment. The modified Al particles could continuously react with pure water and generate hydrogen at room temperature. The present finding implies that metal Al could become hydrogen-generation material by surface modification.

We calculate variationally the binding energies of a hydrogenic impurity and a Wannier exciton in an arbitrary corner of well material surrounded by the barrier material. The results show that the binding energy of ground impurity states in the corner can be comparable with that of highly excited impurity states in the bulk when the corner structure becomes small. However, the behaviour of the exciton in the corner is not the same as that of impurity; the binding energy of the exciton varies with the corner structure insensitively. The dependences of the impurity and exciton binding energies on the dielectric mismatch between the well material and barrier material are also discussed.

We calculate the optical absorption spectra associated with transitions from the n = 1 valence subband to the donor level in a dielectric quantum well. It is found that the impurity binding energies evaluated from photoluminescence experiments are lower than the practical binding energies, because of the effect of the image potential on the exciton binding energy in the dielectric quantum well. The broadening of absorption spectra due to the impurity distribution and the fluctuation in well width is also discussed.

The flexure creep behaviour of monolithic Al 2 O 3 and 10 vol% SiC-particle reinforced Al 2 O 3 matrix composites was investigated in air atmosphere at 1160 to 1400 °C and under a stress of 40 to 125 MPa. Two kinds of SiC particles with different particle sizes and oxygen contents were used in the composites, one having an average size of 0.6 μm with 1.7 vol% SiO 2 impurities and the other of average size 2.7 μm with 3.4 vol% SiO 2 impurities. Compared with the creep behaviour of monolithic Al 2 O 3 the strain rate of the composites with 0.6 μm SiC particles did not decrease; however, the composites with 2.7 μm SiC particles exhibited excellent creep resistance. Microstructure analysis showed that the Al 2 O 3 grains in the composites with 0.6 μm SiC particles were mainly equiaxed with most of the SiC particles lying at the grain boundaries or triplegrain junctions, whereas the grain features of the composites with 2.7 μm SiC particles were irregular and elongated and most of the SiC particles were entrapped into Al 2 O 3 matrix grains. It was revealed that the entrapment of 2.7 μm SiC particles into Al 2 O 3 matrix grains was related to the high SiO 2 impurity content on SiC particle surfaces, and the change of grain morphology and the good high-temperature oxidation resistance were responsible for the creep resistance increase of the composites with 2.7 μm SiC particles.

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO2 glass. Because of the crystallization of amorphous SiO2 glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.

A ZrB 2 -SiC composite was prepared from a mixture of zirconium, silicon, and B 4 C via reactive hot pressing. The three-point bending strength was 506 ± 43 MPa, and the fracture toughness was 4.0 MPa.m 1/2 . The microstructure of the composite was observed via scanning electron microscopy; the in-situ-formed ZrB 2 and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B 4 C will diffuse into the Zr and Si sites and form ZrB 2 and SiC in situ, respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders.

In this work, 800 ppm of Zr4+ dopants were added to Al2O3-5 vol% SiC particle composite. Zr4+ doping led to a weak Al2O3 grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr4+-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr4+ and Si4+ ions at the Al2O3 grain boundary.

We calculate the effects of image potential due to dielectric mismatch on the density of impurity states and optical absorption spectra associated with impurities in GaAs/AlAs quantum wells. It is found that the effects of image potential on the density of impurity states and optical absorption spectra are significant, especially when the width of quantum well becomes small. The effects of impurity-ion image potential on the density of impurity states are much larger than those of electron image potential, but the effects of the impurity ion and electron image potentials on the optical absorption spectra are of the same order and opposite. The results also indicate that the effects of image potential on acceptors are comparable with those on donors.

We calculate the subband structures and inter-subband transitions in a corrugated GaAs-AlAs interface under normally applied electric fields. It is found that the first energy minigap of the system can be tuned linearly using the normally applied electric fields, and an anisotropy in the inter-subband optical absorptions exists for a linearly polarized incident light wave. The dependence of the inter-subband transition spectra on the interface structures and external electric fields is discussed.

Cyclotron resonance of a magnetopolaron in a rectangular harmonic quantum wire with magnetic field normal to the quantum wire is investigated theoretically. The results we obtained show that, the Landau levels in the quantum wire are raised relative to those in two-dimensional quantum well due to the addition of a parabolic potential. The results also show that, the cyclotron frequency and cyclotron mass of 1D polaron are very different from those of 2D polaron, especially for weak magnetic field region

We calculate the impurity binding energies in the vicinity of two orthogonal surfaces using the variational method. Our results indicate that the binding energies of ground impurity states tend to the value of the third impurity excited states in the bulk when the impurity approaches the corner. The variations in impurity binding energy with the dielectric mismatch between barrier materials and well material are also discussed.

We calculate the optical absorption spectra associated with an impurity in quasi-one-dimensional GaAs/AlAs quantum well wires with periodic lateral surface structures. The results indicate that there is an energy-forbidden region for optical transitions from acceptors to conduction minibands, and no forbidden region for optical transitions from valence minibands to donors. The dependence of optical absorption spectra on the interface amplitude is discussed.

The electronic and impurity states in the corner of well material surrounded by barrier material are investigated using the variational method. The results show that the binding energies of impurity ground states tend to the value of the third impurity excited states in the bulk when the impurity approaches the corner. The dependence of the ground-state electronic level and impurity binding energy on the dielectric mismatch between the well material and barrier material is also discussed.

We calculate the binding energies of shallow donors and acceptors in a spherical GaAs-${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum dot for both a finite barrier and an infinitely high barrier using the variational approach, including the spatial variation of dielectric screening. The results show that when the spatial variation of dielectric screening is considered, the impurity binding energies increase noticeably, especially when the radius of the quantum dot is small. The results also show that the effects of spatial variation of dielectric screening on acceptors are larger than those on donors. The dielectric mismatch in this structure is also discussed.

The effects of the image potential due to dielectric mismatch on the impurity states in GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells are investigated using the variational method. The results show that the effects of the image potential on the impurity binding energies in GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells are important, especially when the width of quantum well becomes small. The comparison with the results in the GaAs quantum well with an infinitely high confining potential and the dependence of the effects of the image potential on the composition x are discussed.

We calculate the effects of dielectric mismatch on shallow donors and acceptors in a spherical GaAs-Al1-xGaxAs quantum dot for both a finite barrier and an infinitely high barrier using the variational approach. The results have shown that the addition of dielectric mismatch enhances impurity binding energies considerably, especially when the radius of the quantum dot is small. The results also showed that the effects of dielectric mismatch on donors are comparable with those on acceptors, and the corresponding effects for an infinitely high barrier are larger than for a finite barrier.

We calculate energy miniband structures and densities of impurity states in quasi-one-dimensional GaAs/AlAs quantum well wires with periodic lateral surface structures. By a coordinate transformation, the structured interfaces of the wires are transformed into planar interfaces so that the boundary conditions of the electronic wavefunctions can be satisfied exactly on the interfaces. The new optical transition spectral structures associated with minibands and impurities are discussed.

The effects of image potential due to dielectric mismatch on electronic and shallow donor impurity states in quasi‐one‐dimensional GaAs‐Ga1−xAlxAs quantum‐well wires with rectangular cross section for both finite barrier and infinitely high barrier are investigated. The results have shown that, when the image potential is included, the variations in electronic energy level and impurity binding energy are considerable, especially when the cross‐section dimensions of the quantum wire become small. The results also showed that the effects of the impurity ion image potential on impurity binding energy are much larger than those of electron image potential.

We calculate hydrogenic impurity binding energy in quasi-one-dimensional GaAs/AlAs quantum wires with periodical lateral surface structure by making use of a variational approach. By a coordinate transformation, the structured interfaces of the wires are transformed into planar ones so that the boundary conditions of the electronic wave functions can be satisfied exactly on the interfaces. The dependence of the binding energies on the positions of the impurities and on the lateral surface structures is studied.

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.