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Series of experiments were performed to simulate the invasion of formation sand into and the plugging process of gravel-pack at different viscosities and flowing rates of fluid. Two types of formation sands with the medium size of 0.10 mm and 0.16 mm and the quartz sand and ceramsite of 0.6—1.2 mm were used in the experiments. A new viscosity-velocity index (the product of fluid viscosity and velocity) was put forward to characterize the influencing mechanism and law of physical property and flow condition of formation fluid on gravel-pack plugging, and a new method to optimize the production rate of wells controlling sand production with gravel-packing was proposed. The results show that the permeability of formation sand invaded zone and final permeability of plugged gravel-pack have negative correlations with viscosity and flow velocity of fluid, the higher the flow velocity and viscosity, the lower the permeability of formation sand invaded zone and final permeability of plugged gravel-pack will be. The flow velocity and viscosity of fluid are key factors affecting plugging degree of the gravel zone. The viscosity-velocity index (v-v index) can reflect the flow characteristics of fluid very well and make it easier to analyze the plugging mechanism of gravel zone. For different combinations of fluid viscosity and flow velocity, if the v-v index is the same or close, their impact on the final gravel permeability would be the same or close. With the increase of the v-v index, the permeability of plugged gravel zone decreases first, then the reduction rate slows down till the permeability stabilizes. By optimizing production and increasing production step by step, the optimal working scheme for sand-control well can reduce the damage to gravel-pack zone permeability caused by sand-carrying fluid effectively, and increase well productivity and extend the sand control life. Key words: gravel-pack, sand retention experiment, plugging law, viscosity-velocity index, working scheme, optimization method, plugging simulation

Recently, the emerging inorganic perovskites CsPbX 3 (X = Br, I) have received great attention in the photovoltaic community due to their excellent photoelectric property and good thermal stability. However, most of the reported CsPbX 3 films are fabricated by the uncontrolled solution method, and it is challenging to generate pinhole-free Br-rich films due to the over-rapid liquid-phase reaction feature and the solubility limitation of bromide. Herein, we develop a sequential chemical vapor deposition (CVD) method to fabricate colorful, band-gap tunable, and stable CsPb(I x Br 1-x ) 3 films. First, air-stable CsPbBr 3 precursor films are fabricated via a Br 2 -assisted CVD process. Second, a series of CsPb(I x Br 1-x ) 3 (x = 0, 0.2, 0.33, 0.5, 0.67, 0.8, 1) high-quality films with consistent compositions are then obtained by another MAI/MABr-assisted CVD process. Experimental results show that all of the CsPb(I x Br 1-x ) 3 films have the same cubic structure and (100) preferred orientation. With the increase of MAI component, the colors of films darken gradually, the main diffraction peaks at (100) and (200) planes shift to small angle, their optical bandgaps monotonously decrease from 2.34 eV to 1.67 eV, the grains of films grow up, and the porous films become more compact. Strikingly, it is found that CsPb(I x Br 1-x ) 3 films become very stable in air ambient when x is less or equal to 0.5. Therefore, we first employ a cost-effective CVD technology to modulate the anion compositions of CsPbX 3 inorganic perovskite films, which shows a high potential for the photovoltaic applications such as highly efficient tandem perovskite solar cells (PSCs) and colorful smart photovoltaic windows.

Nowadays, inorganic perovskites such as CsPbI3 and CsPbBr3 with excellent photoelectric property and superior thermal stability have triggered strong interest in the photovoltaic community. However, all of the inorganic perovskite absorbers reported in literatures are fabricated by the complicated solution process. Meanwhile, CsPbI3 exists serious structural phase-transition; while stable CsPbBr3 suffers solubility limitation of bromide in conventional solution route. Herein, we develop a smart Br2-vapor-assisted chemical vapor deposition (CVD) method to realize the fast anion-exchange from CsPbI3 to CsPbBr3. With the substitution of small Br− anions, the undesirable phase-transition of CsPbI3 is heavily restrained and its structural stability is tremendously enhanced; and thus here we open a new avenue to solve the solubility issue of bromide. Consequently, CsPbBr3 all-inorganic perovskite solar cells (PSCs) with Carbon top electrode are first achieved by cost-effective CVD technology, showing a relatively high power conversion efficiency (PCE) of 5.38%. The charge transport mechanism of CsPbBr3 PSCs is correspondingly elucidated in-depth. We find that the interface of TiO2 and CsPbBr3 might be the root cause of hysteresis phenomenon, while another interface between Carbon and CsPbBr3 is not a rectifying contact but only increases the ohmic resistance of electrode. Moreover, our PSCs demonstrate a good long-term stability and give ∼90% of initial efficiencies after exposure to air for 21 days.

GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

The InGaN/GaN multiple quantum wells (MQWs) with truncated pyramid structure have been successfully epitaxially grown on the GaN hexagonal pyramids template through a simple and low-cost etch-regrown process. GaN hexagonal pyramids template was contained by a convenient photo-assisted chemical (PAC) etching method. The truncated pyramids are composed of { 10 1 ¯ 1 ¯ } and { 11 2 ¯ 2 } semi-polar facets as well as (0001) polar facet. It was observed that the InGaN/GaN MQWs substantially emitted broad spectrum with multiple peaks by room temperature photoluminescence (PL). The cathodoluminescence of MQWs red-shifts as the location moves from bottom to top on the facets due to the indium diffusion mechanism.

The formation sands of natural gas hydrate (NGH) reservoirs in the South China Sea are fine silty sands with a high content of clay, and it is difficult to be controlled effectively. We carried out a series of experiments to investigate the microcosmic retaining and plugging behavior of screen media with highly argillaceous fine sand samples, with median size of 10.13 μm. The screen sand-retention media and joint involved four types with slot width or accuracy of 20–60 μm. The screen media/joint sample was placed in the linear/radial flowing apparatus, and then the mixture of gas, water and sand were displaced flowing into the media to simulate the sand retaining process. The Microscopic imaging system was used to visually observe the dynamics of sand particle deposition and plugging inside the media as well as on its surface. The plugging permeability and the amount of the passed sand were also analyzed. According to the result, three microcosmic mechanisms of sand retaining and plugging were proposed, concerning fine sand invading and coarse sand bridge, partial sand block, and integrated sand bridge. The plugging permeability performance of screen media generally shows three stages of slow decreasing, rapid increasing and balance. At the balance stage of displacement, the plugging permeability of media sample mostly decreases more than 90%. During the whole test, the sand passing rate tends to slow down gradually, and the final retained sand ratio were more than 85%. Different configurations of sand retraining test show a little difference in plugging performance, depending on the factors of medium type, accuracy, sand characteristics, and flow conditions. Based on the effect of sand retaining and final flow ability observed in these experiments, the sand control in NGH reservoirs with highly argillaceous fine sands is practicable.

The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580″, can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 × 10 18 cm −3 has been obtained.

In this paper, we took a brief investigation on the properties of two-dimensional electron gas (2DEG) in AlGaN/InGaN/GaN heterostructures. Band profiles and the 2DEG distribution were calculated by self-consistently solving the Schrodinger and Poisson equations. Our calculated results indicate that the 2DEG concentration can be considerably increased with the incorporation of an InGaN layer. Quantum confinement, and thereby the 2DEG distribution are very sensitive to the strain of the incorporated InGaN layer. Hence one can design the 2DEG by incorporating proper InGaN layer into the conventional AlGaN/GaN heterostructures. These novel features are attributed to the strong polarization effect in AlGaN/InGaN/GaN heterostructures.

Pt/Au Schottky contacts were fabricated on modulation-doped Al0.22GaN0.78/GaN heterostructures. Some different surface treatments were used before the deposition of the metal contacts to prevent the formation of the native oxide layer on the samples’ surfaces. Comparing the barrier height and the ideal factor of the Schottky contacts, we found that the treatment of the boiling (NH4)2S solution can remove the native oxide layer effectively. Detailed X-ray photoelectron spectroscopy measurements were also made to investigate the chemical reactions on the surface of samples after different treatments.

The specific contact resistivity (ρ C ) and interfacial reaction between Au/Pt/Al/Ti metallic multi-layers and Si-doped n-type Al x Ga 1-x N (n-AlGaN) layers in modulation-doped Al 0.22 Ga 0.78 N/GaN heterostructures have been investigated. By means of the measurements based on the transmission line model, the ρ C as low as 1.6 × 10 -4 Ωcm 2 is obtained. Based on the X-ray diffraction analysis, it is found that N atoms in n-AlGaN layer diffuse out and a much amounts of N-vacancies are formed in n-AlGaN layer near the interface after the sample is annealed at temperatures higher than 500 °C. It induces the heavy n-type doped region in n-AlGaN near the interface, and thus leads to the decrease of the ρ C . With increasing the annealing temperature, more N atoms in n-AlGaN layer diffuse out and react with Ti atoms. Ti 2 N phase is formed at the interface after the sample is annealed at 800 °C. In this case, the ρ C further decreases.

Transmission electron microscopy (TEM), double crystal X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurement were applied to study the correlation between the microstructure and material properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. Corresponding to the density of the threading dislocation (TD) increasing approximately one order, the yellow luminescence (YL) intensity was strengthened from negligible to two orders higher than the band-edge emission intensity. The full width of half maximum (FWHM) of GaN (0002) peak of the XRD rocking curve was widened from 11 to 15 min, and in Raman spectra, the width of E 2 mode is broadened from 5 cm −1 to 7 cm −1 . A ‘zippers’ structure of GaN buffer layer was discovered by high-resolution electron microscope (HREM).

AlGaN epilayers were successfully grown using light radiation heating, metal–organic chemical vapor deposition (MOCVD). The optimized growth temperatures are between 950 and 1000, which are lower than those of normal RF-heating MOCVD. X-ray diffraction (XRD) spectra indicated that the films are of high crystalline quality. Photoluminescence excitation (PLE) spectrum shows an abrupt absorption edge of the AlGaN layer, which also indicates that the samples are of sound optical quality and with Al composition uniformity. The results indicate that the incorporation efficiency of Al is as high as that of Ga, which is beneficial for the improvement of the Al composition uniformity in the AlGaN films. A model of the influences of parasitic reaction on the Al composition in the AlGaN films is developed based on previous works. Light radiation may depress the parasitic reaction and thus improve the incorporation efficiency of Al into the AlGaN films and the Al composition uniformity of the AlGaN layers. Therefore, a high-quality growth of AlGaN may take place at relatively low temperatures in comparison with the growth in RF-heating MOCVD.

The deposition and crystallization of amorphous GaN buffer layers on Si(1 1 1) is firstly investigated by using an atomic force microscope (AFM), X-ray diffraction (XRD) and a high-resolution transmission electron microscope (HRTEM). The amorphous GaN layers were deposited by MOCVD at 300°C. The islanding process by annealing at higher temperatures has been revealed by AFM. It is found that the amorphous layers begin to be crystallized by solid-phase epitaxy at 500°C in MOCVD, and the full-developed islands formed at about 600°C. XRD data show that the GaN peak appeared only after an annealing at higher temperatures. Cross-sectional HRTEM micrographs of the buffer region of the samples with amorphous GaN buffer layers reveal that many domains exist in the GaN buffer layers and these domains misorientate each other with a small angle. The boundaries between domains locate near the bunched steps, and, the films on a terrace between steps have the same crystal orientation. The amorphous buffer layer deposited at 300°C and the buffer layer directly deposited at 550°C are used in the growth of GaN epilayers, respectively. The XRD results and photoluminescence at room temperature of such GaN epilayers show that using the amorphous GaN buffer layer lead to monocrystalline and a narrower full-width at half-maximum (FWHM) of near band emission (16 nm) than using the buffer layer directly deposited at 550°C (FWHM of 21 nm). These results indicate that the quality of GaN epilayer on Si(1 1 1) can be improved by using the amorphous GaN buffer layer.