Polymorphism in Gd2Ge2O7 ceramics: Structural,vibrational, and optical features

In this work, Gd₂Ge₂O₇ polymorphs were obtained by solid-state reactions at 1100–1300 °C. Structural and vibrational features were investigated by X-ray diffraction and Raman spectroscopy. For the triclinic (space group P1) polymorph, all the predicted phonons were discerned in perfect agreement with the group theory calculations, while for the tetragonal polymorph (space group P4₁2₁2), 53 bands of the 81 predicted modes could be identified and characterized. The Gd³⁺ 4f-4f electronic transitions were investigated by diffuse reflectance spectroscopy in the range 200–340 nm. By applying the Kubelka-Munk function, it was possible to determine the bandgap values for all ceramics studied. The tetragonal polymorph exhibited higher bandgap values (5.88 eV) than the triclinic one (5.59 eV), which are both more energetic than other pyrochlore polymorphs reported in the literature. The results indicate that the presence of polymorphism in Gd₂Ge₂O₇ ceramics can be used to produce tailor-made materials since their crystal structures have a strong influence on their optical properties. Consequently, these properties could be used to tuning the optical properties of Gd-containing materials to sensitize and transfer energy to other luminescent lanthanide ions, aiming for innovative applications.     

Evaluation of particle recovery from microalgae

Coccoliths are micro-structured biomineral particles found in cell protective covering layers of coccolithophore species. They are mainly composed of CaCO₃ and their individual crystal entities are arranged in such a way that they construct complex and unique structures. This complexity is found down to the individual particle level and appears to have promising properties to offer. This study focuses on the essential step prior to any kind of implementation, which is the recovery of the material. It summarizes cleaning protocols found in literature, compares them for the first time for the same freshly cultivated material and addresses challenges that still need to be overcome. Further, it highlight the advantages and disadvantages of the best cleaning protocols, suggests optimizations with promising results and uses size distribution measurements to analyse the recovery efficiency. To that end, further characterization techniques, new for coccoliths, are introduced and used to improve our current knowledge of the particles behaviour.     

Surface modifications of activated carbon by gamma irradiation

Four commercial activated carbons with different chemical and textural characteristics were modified by gamma irradiation under five different conditions: irradiated in absence of water, in presence of ultrapure water, in ultrapure water at pH = 1.0 and 1000 mg L⁻¹ Cl⁻, in ultrapure water at pH = 7.5 and 1000 mg L⁻¹ Br⁻, and in ultrapure water at pH = 12.5 and 1000 mg L⁻¹ NO₃⁻. Changes in surface chemistry were studied by X-ray photoelectron spectroscopy; pH of point of zero charge, total acidic groups and total basic groups, which were determined by assessment with HCl and NaOH; and textural changes were determined by obtaining the corresponding adsorption isotherms of N₂ and CO₂. Outcomes show that the activated carbon surface chemistry can be modified by gamma irradiation and that the changes depend on the irradiation conditions. Modifications in the sp² hybridization of the surface carbons suggest that the irradiated carbons undergo graphitization. Measurements of structural parameters indicate that the irradiation treatment does not modify the textural properties of the carbons. Finally, studies of pristine and irradiated activated carbons using diffuse reflectance spectroscopy with the Kubelka–Munk function revealed a reduction in band gap energy in the irradiated carbons associated with an increase in sp² hybridization of the carbon atoms.     

Tunable morphological evolution of in situ gold catalysts mediated silicon nanoneedles

Achieving tunable growth of high quality Silicon (Si) nanoneedles (NNs) is challenging. We report the optimized morphology of in situ gold (Au) catalysts assisted SiNNs grown via very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The vapor-liquid-solid (VLS) mechanism mediated morphological evolution is tuned using hydrogen (H₂) and silane (SiH₄) gas flow. Au-coated Si(100) substrates are treated using H₂ plasma to create in situ Au nanoparticles (NPs) with high catalytic activity. FESEM images manifested the existence of mono-dispersed Au NPs and high yield of SiNNs having diameter ranging from 80 to 140 nm and lengths up to 2.31±0.3 µm. Furthermore, these NNs gradually became thinner to form sharp tips of diameter as small as 4 nm. XRD pattern confirmed the diamond (cubic) crystalline phases of SiNNs. HRTEM images revealed the occurrence of Au NPs at the crystalline SiNNs tips. Raman spectrum of as-grown SiNNs exhibited the TO phonon mode accompanied by a red-shift (~23.59 cm⁻¹). Synthesized SiNNs displayed extremely low reflectance (~8%) at short wavelengths (λ<700 nm), indicating excellent antireflection properties. Our controllable and optimized growth method may constitute a basis to achieve high quality SiNN arrays, which are beneficial for various applications.     

The Comparison of Reflectance based on Different Terrain Correction

Selecting the Xinglong Mountain which locates in the southeast of Lanzhou city，GanSu province as an example.Using the Landsat8 satellite image as the data source，the Cosine method，C and the modified Minnaert methods were used for each band in the study area.Comparing with the results of the field measurement reflectance and the statistical characteristics of image，the result showed that the cosine method has a perfect correction in the visible and short wave infrared wavelength，the C correction has a serious over\|correct，however.In the Near Infrared Wavelength，the better result obtained by C correction，and the cosine method has over\|correct otherwise.Comparing with the correct effect of whole bands，the modified Minnaert method has an ideal correct effect.The comparison of before and after correction we found that there is a smaller difference for three methods in the smaller slope，and the over correct is mostly in the south.What’s more，with the increase of the slope，the over correct is more obviously，but，there is a little over correct which used the modified Minnaert method in the whole area，it’s more suitable in large slope and the complicated area.     

Periodic nano/micro-hole array silicon solar cell

In this study, we applied a metal catalyst etching method to fabricate a nano/microhole array on a Si substrate for application in solar cells. In addition, the surface of an undesigned area was etched because of the attachment of metal nanoparticles that is dissociated in a solution. The nano/microhole array exhibited low specular reflectance (<1%) without antireflection coating because of its rough surface. The solar spectrum related total reflection was approximately 9%. A fabricated solar cell with a 40-μm hole spacing exhibited an efficiency of 9.02%. Comparing to the solar cell made by polished Si, the external quantum efficiency for solar cell with 30 s etching time was increased by 16.7%.     

The effect of rear surface polishing to the performance of thin crystalline silicon solar cells

As the thickness of crystalline silicon solar cells decreases, light loss cannot be avoided due to the absorption limit in long wavelength light. Internal rear side reflection can be enhanced by polishing the rear surface. The rear polishing processes are performed before the texturing and before and after doping the emitter layer to optimize the solar cell fabrication process sequences. All cells made by rear surface polishing showed improved light trapping in long wavelength region (900-1100 nm) compared to that in the conventional cells. However, silicon solar cells fabricated by rear polishing before and after doping have similar (35.5 mA/cm²) or lower (35.26 mA/cm²) short circuit current density compared to the cells produced by the conventional process (35.59 mA/cm²) due to pore damage to the anti-reflection layer and the surface of the emitter layer during rear polishing. This surface damage was effectively prevented adapting the rear surface polishing before the front surface texturing, which led to increasing the current density from 35.59 to 36.29 mA/cm².     

Direct and indirect uncertainties in the prediction of tilted irradiance for solar engineering applications

Global radiation measured on fixed-tilt, south-facing planes (40° and vertical) and a 2-axis tracker at NREL’s Solar Radiation Research Lab. in Golden, CO is compared to predictions from ten transposition models, in combination with either optimal or suboptimal input data of horizontal irradiance. Suboptimal inputs are typically used in everyday engineering calculations, for which the necessary data are usually unavailable for the site under scrutiny, and must be estimated in some way. The performance of the transposition models is first evaluated for ideal conditions when optimal data are used. In this specific case, it is found that the Gueymard and Perez models provide the best estimates of global tilted irradiance under clear skies in particular.The performance of four direct/diffuse separation models is also evaluated. Their predictions of direct and diffuse radiation appear biased in most cases, with a model-dependent magnitude. Finally, the performance of the resulting combinations of separation and transposition models is analyzed in a variety of situations. When only global irradiance is known, the accuracy of the tilted irradiance predictions degrades significantly, and is mainly conditioned by the local performance of the direct/diffuse separation method. For the south-facing vertical surface, inaccuracies in the ground reflection calculations becomes another key factor and significantly increase the prediction error. The Reindl transposition algorithm appears to perform best in this case. When using suboptimal input data for the prediction of plane-of-array irradiance on a moderately tilted plane (40°S) or a 2-axis tracking plane, the Hay, Reindl and Skartveit models are less penalized than others and tend to perform better. It is concluded that further research should be conducted to improve the overall process of predicting irradiance on tilted planes in realistic situations where no local high-quality irradiance or albedo measurements are available.     

吸光性污染物对积雪光谱反射率的影响研究

积雪中存在的吸光性污染物对积雪反射率具有显著的降低作用,进而对能量平衡和气候变化有重要影响。但是,污染浓度变化如何影响积雪反射特征仍然缺乏定量描述和深入探讨。选择新疆富蕴作为典型干旱与半干旱积雪实验区,通过人工控制试验在自然积雪状态下生成不同浓度的污染雪样方,并对积雪及污染物自身的反射率进行测量。在实测数据基础上通过构建线性混合模型定量分析不同浓度条件下污染物对积雪反射率的影响力。研究结果表明:积雪反射率降低与污染浓度呈非线性关系,随着污染浓度增大,单位浓度影响力降低,在350~450 nm波段范围1 813 ppm浓度的单位影响力甚至是9 507 ppm浓度的1.5倍以上;同时发现除了污染浓度,积雪与污染物自身物理特性也是影响反射率变化的重要参数。