Assessment of accidental refinery wastewater discharge: a case study

Carbon dioxide (CO₂), one of the green house gases (GHGs) is well known for more than a century. Its emission from the combustion of fossil fuels in addition to other industrial sources is adversely affecting the climate on earth. Climate change is emerging as a risk all over the world that has generated public concern. Estimates have indicated that power production contributes to the tune of 70% of the total CO₂ released into the atmosphere from fossil fuel combustion worldwide. Capturing and securely storing CO₂ from the global combustion systems thus constitutes an important and achievable target. A legion of researchers have thus far developed absorbents to remove CO₂ from combustion facilities that are currently recognized globally as most effective. The cost of capturing CO₂ can be reduced by finding a low-cost solvent that can minimize energy requirements, equipment size, and corrosion. Monoethanolamine is being used for removing CO₂ from the exhaust streams and is a subject inculcated over a period of about last 80 odd years. Host of such other amines are being investigated and put into practice. However, commercializations of such operating plants for capturing CO₂ from power plants in the world are few and far between. On the other hand, aqueous ammonia is the other chemical solvent for capturing CO₂ that has proven experimentally to be more effective than amine-based processes. This communication aims at critically elucidating relative merits and demerits of ammonia and amine-based CO₂ capture options from the exhausts of coal fired thermal power plants (TPPs). It includes the life cycle CO₂ emissions for both the processes. Finally, it is estimated that a total emission of about 152 Mt CO₂-equivalent could occur after use of 100 Mt ammonium bicarbonate (NH₄HCO₃) as synthetic N-fertilizer that is about 50% of the total CO₂ captured (315 Mt) for producing the fertilizer, NH₄HCO₃. Clearly, this estimate demonstrates that the synthetic N-fertilizer, NH₄HCO₃, produced by NH₃ scrubbing of CO₂ from fossil fuel (e.g., coal) fired TPP could have a significant beneficial environmental impact so far as GHG emission is concerned.     

Atmospheric dispersion modeling of CO<Subscript>2</Subscript> emissions from a cement plant’s sources

The study aims to estimate a cement plant’s carbon dioxide (CO₂) concentrations from individual sources as well as combined emissions from all the sources. Four main CO₂ emission sources were considered: process from the calcination of limestone, the combustion of fossil fuel in the kilns, the power plant, and the dump trucks used for raw material transportation. An integrated modeling system comprised of the California PUFF and Weather Research and Forecasting was applied. The power plant and the stacks of three kilns were modeled as point sources, whereas the vehicular emissions were treated as a line source. In the first part of the study, modeling of the cement plant’s individual sources was carried out to predict CO₂ at each receptor of the domain. In the second part, the CO₂ concentrations of combined emissions from all of the plant’s sources were predicted. Individual modeling of each of the plant’s CO₂ emission sources showed that the highest CO₂ at each receptor of the domain resulted from the calcination process. In the case of combined modeling of all the cement plant’s sources, the predicted peak concentrations of CO₂ were 357.19 and 36.11 mg/m³ for one-hour and 24-hour averaging periods, respectively.     

A systematic technique for cost-effective CO<Subscript>2</Subscript> emission reduction in process plants

There has been growing interests to reduce the environmental impact caused by greenhouse gas emissions from process plants through various energy conservation strategies. CO₂ emissions are closely linked to energy generation, conversion, transmission and utilisation. Various studies on the design of energy-efficient processes, optimal mix of renewable energy and hybrid power system are driven to reduce reliance on fossil fuel as well as CO₂ emissions reduction. This paper presents a systematic technique in the form of graphical visualisation tool for cost-effective CO₂ emission reduction strategies in industry. The methodology is performed in four steps. The first step involves calculating the energy consumption of a process plant. This is followed by identification of potential strategies to reduce CO₂ emissions using the CO₂ management hierarchy as a guide. In the third step, the development of “Investment” versus “CO₂ Reduction” (ICO₂) plot is constructed to measure the optimal CO₂ emission reductions achieved from the implementation of possible CO₂ reduction strategies. The Systematic Hierarchical Approach for Resilient Process Screening (Wan Alwi and Manan in AIChE J 11:3981–3988, 2006) method is used in the fourth step via substitution or partial implementation of the various CO₂ reduction options in order to meet the cost-effective emission reduction within the desired investment limit or payback period (PP). An illustrative case study on a palm oil refinery plant has been used to demonstrate the implementation of the method in reduction of CO₂ emissions. The developed graphical tool provides an insight-based approach for systematic CO₂ emission reduction in the palm oil refinery considering both heat and power energy sources. Result shows that 31.2 % reduction in CO₂ emissions can be achieved with an investment of USD 38,212 and PP of 10 months based on the present energy prices in Malaysia.     

Simultaneous reduction of NO–smoke–CO<Subscript>2</Subscript> emission in a biodiesel engine using low-carbon biofuel and exhaust after-treatment system

The present work focuses on the simultaneous reduction of NO–smoke–CO₂ emission in a Karanja oil methyl ester (KOME)-fueled single-cylinder compression ignition engine by using low-carbon biofuel with exhaust after-treatment system. Replacement of KOME for diesel reduced smoke emission by 3% but resulted in increase of NO emission and CO₂ emission by 13 and 35% at 100% load condition. In order to reduce CO₂ emission, tests were conducted with a blend of KOME and orange seed oil (OSO), a low-carbon fuel on equal volume basis (50–50). At the same operating conditions, compared to KOME, 27% reduction in CO₂ emission and 5% reduction in smoke emission were observed. However, a slight increase in NO emission was observed. To achieve simultaneous reduction of NO–smoke–CO₂ emissions, three catalysts, namely monoethanolamine, zeolite and activated carbon, were selected for exhaust after-treatment system and tested with optimum KOME–OSO blend. KOME–OSO + zeolite showed a great potential in simultaneous reduction of NO–smoke–CO₂ emissions. NO, smoke and CO₂ emissions were simultaneously reduced by about 15% for each emission compared to diesel at 100% load condition. The effect of exhaust after-treatment system with KOME–OSO blend on combustion, performance and other emission parameters is discussed in detail in this study. Fourier transform infrared spectrometry analysis and testing were done to identify the absorbance characteristics of zeolite material.     

Co-production of power and urea from coal with CO<Subscript>2</Subscript> capture: performance assessment

Coal-based power plants are largest emitter of CO₂ as a single sector. To use fossil fuels (including coal), CO₂ capture and storage is a visible option. But large energy requirement for this process and risk associated with storage of CO₂ demand alternative solutions including recycling of captured CO₂. In this paper, a co-production of power and urea is proposed using coal with captured CO₂. Detailed ASPEN Plus^® model is developed for this plant. As shift reaction for producing H₂ has significant effect on output parameters, analysis is done for two different values of shift reaction, i.e., 90 and 95 % conversion. Plant consumes substantial auxiliary power (~19 % for the base case). Auxiliary power becomes a minimum for about 25 % captured CO₂ utilization for 95 % shift conversion. An economy factor is also defined to estimate the economic advantage of utilizing captured CO₂. Results show that economic advantage is obtained for CO₂ utilization beyond ~5 % for 95 % water gas shift reaction and it is beyond ~10 % for a 90 % shift reaction.     

Measurement of Ambient NH<Subscript>3</Subscript>, NO and NO<Subscript>2</Subscript> at an Urban Area of Kolkata,India

Mixing ratios of ambient NH₃, NO and NO₂ were measured in campaign mode at Kolkata a megacity of Indo-Gangetic plain of India to study the diurnal variation and mixing ratios of NH₃, NO and NO₂ during 24–27 February 2012. The present study has been carried out on campaign based measurement of mixing ratios of NH₃, NO and NO₂ for short period of time at Kolkata represent the indicative values over the region. The average mixing ratios of ambient NH₃, NO and NO₂ were recorded as 43.4 ± 7.0 ppb, 46.0 ± 8.7 ppb and 31.9 ± 5.5 ppb at Kolkata. In the present case, significant diurnal variation of NH₃, NO and NO₂ were recorded at Kolkata during study. Mixing ratio of ambient NH₃ reaches its maxima (78.9 ppb) at night and minimum during daytime. Result reveals that the ambient NH₃ mixing ratio is positively correlated with ambient NO (r ² = 0.395) and NO₂ (r ² = 0.404) mixing ratio and significant negatively correlated with ambient temperature (r ² = –0.669). Surface wind direction and wind speed analysis indicates that the local acitivities (livestock, drainage, agriculture, vehicles etc.,) may be the possible sources of ambient NH₃ at the observational site of Kolkata.     

Synthesis,crystal structure and luminescence in Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript>

Bluish green emitting phosphor, Ca₃Al₂O₆:Ce³⁺, is prepared by low-temperature combustion method. X-ray diffraction, photoluminescence, scanning electron microscopy techniques are used to characterize the synthesized phosphor. The most efficient bluish green (483 nm) emission is observed under the excitation by near UV light. The emission characteristics are credited to 5d → 4f type transitions in Ce³⁺. The luminescence properties of Eu²⁺ are predicted for the first time from those of Ce³⁺. Also, photoluminescence of Eu³⁺ is studied in the same host. The emission spectrum of Ca₃Al₂O₆:Eu³⁺ shows the peak at 592 (orange) and 614 nm (red) wavelengths. Ca₃Al₂O₆:Ce³⁺phosphor can be a potential blue phosphor for field emission display, solid-state lighting and LED.     

Structural and optical characterization of RE (Eu<Superscript>2+</Superscript>, Ce<Superscript>3+</Superscript>) doped SrMg<Subscript>2</Subscript>Al<Subscript>6</Subscript>Si<Subscript>9</Subscript>O<Subscript>30</Subscript> nanocrystalline phosphor

This article present the reports on optical study of Eu²⁺ and Ce³⁺ doped SrMg₂Al₆Si₉O₃₀ phosphors, which has been synthesized by combustion method at 550 °C. Here SrMg₂Al₆Si₉O₃₀:Eu²⁺ emission band observed at 425 nm by keeping the excitation wavelength constant at 342 nm, whereas SrMg₂Al₆Si₉O₃₀:Ce³⁺ ions shows the broad emission band at 383 nm, under 321 nm excitation wavelength, both the emission bands are assigned due to 5d–4f transition respectively. Further, phase purity, morphology and crystallite size are confirmed by XRD, SEM and TEM analysis. However, the TGA analysis is carried out to know the amount of weight lost during the thermal processing. The CIE coordinates of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor is observed at x?=?0.160, y?=?0.102 respectively, which may be used as a blue component for NUV-WLEDs. The critical distance of energy transfer between Ce³⁺ ions and host lattice is found to be 10.65 Å.     

Photocatalytic CO<Subscript>2</Subscript> conversion over Au/TiO<Subscript>2</Subscript> nanostructures for dynamic production of clean fuels in a monolith photoreactor

Global economic development intensifies the consumption of fossil fuels which results in increase of carbon dioxide (CO₂) concentration in the atmosphere. The technologies for carbon capture and utilization to produce cleaner fuels are of great significance. However, phototechnology provides one perspective for economical CO₂ conversion to cleaner fuels. In this study, CO₂ conversion with H₂ to selective fuels over Au/TiO₂ nanostructures using environment friendly continuous monolith photoreactor has been investigated. Crystalline nanoparticles of anatase TiO₂ were obtained in the Au-doped TiO₂ samples. The Au deposited over TiO₂ in metal state produced plasmonic resonance. CO₂ was efficiently converted to CO as the main product over Au/TiO₂ with a maximum yield rate of 4144 µmol g-catal.^?1 h^?1, 345 fold-higher than using un-doped TiO₂ catalyst. The significantly enhanced photoactivity of Au/TiO₂ catalyst was due to hindered charges recombination rate and Au metallic-interband transition. The photon energy in the UV range was high enough to excite the d-band electronic transition in the Au to produce CO, CH₄, and C₂H₆. The quantum efficiency over Au/TiO₂ catalyst for CO was considerably improved in the continuous monolith photoreactor. At higher space velocity, the yield rates of CO gradually reduced, but the initial rates of hydrocarbon yields increased. The stability of the recycled Au/TiO₂ catalyst was sustained in cyclic runs. Thus, Au-doped TiO₂ supported over monolith channels is promising for enhanced CO₂ photoreduction to high energy products. This provides pathway that phototechnology to be explored further for cleaner and economical fuels production.     

Yellow and warm white light emitting Zn doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> for near UV excitable phosphor converted WLED

Optical properties of Zn doped Y₂O₃ microsheets prepared by sol–gel combustion method have been investigated and their application in phosphor converted white LED has been examined. The formation of single phase, well crystalline cubic Y₂O₃ is confirmed from powder XRD results. Effective substitution of Zn in Y₂O₃ crystal lattice is inferred from shifting of diffraction peaks. SEM images have showed that undoped as well as Zn doped Y₂O₃ formed as microsheets. Doping of Zn enhanced the growth of the sheets and its length increased from 1.5 to 19 µm. Development of structural disorder in Y₂O₃ crystal structure after Zn doping and confirmation of the conserved cubic structure of Zn doped Y₂O₃ without any secondary phase have been revealed from micro-Raman spectra. The optical band gap of Y₂O₃ has been altered after Zn doping and it is found to be decreased from 5.6 to 5.22 eV as increasing Zn concentration. Both undoped and Zn doped Y₂O₃ showed a broad visible emission from blue to green region due to various defects and impurities present in it. Broad PL excitation spectrum inferred the possibility to attain the visible emission under the excitation of light with wide range of wavelength from near UV to blue region. Excitation of pure Y₂O₃ under near UV (375 nm) LED chip lead to the emission of yellow light whereas Zn doped Y₂O₃ emitted warm white light with color coordinate of (0.42, 0.35), colour rendering index of 77.6 and correlated color temperature (CCT) of 2840 K. Hence, Zn doped Y₂O₃ discussed in the present work can be a better replacement for various rare earth doped phosphors in the application of phosphor converted WLED (pc-WLED).     

Correlation between the physical properties and the novel applications of Mg<Subscript>0.7</Subscript>Cu<Subscript>0.3</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nano-ferrites

Nano-ferrite of the general formula Mg_0.7Cu_0.3Fe₂O₄ was prepared by citrate-gel auto combustion method. The structure was studied by X-ray diffraction, Brunauer–Emmet–Teller, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy analyses. The crystallite size of the investigated nano ferrite was ?39 nm. The magnetic hysteresis measurements at different temperatures (100, 170, 240, and 300 K) were performed using a vibrating sample magnetometer. A correlation between magnetic behavior and lattice strain has been established. Arrott plot has been employed to understand the magnetic behavior of nano-crystalline Mg_0.7Cu_0.3Fe₂O₄. The magnetic susceptibility was carried out using Faraday’s method. Magnetic constants such as Curie temperature, effective magnetic moment, saturation magnetization, and coercivity were obtained and reported. Based on UV diffuse reflectance spectroscopy studies, the optical band gaps are in the range from (1.3–1.9 eV), hence the investigated samples could act as visible light driven photo catalysts.     

Light emitting mechanisms dependent on stoichiometry of Si-rich-SiN<Subscript>x</Subscript> films grown by PECVD

Light emission and morphology of silicon-rich silicon nitride films grown by plasma-enhanced chemical vapor deposition were investigated versus film’s stoichiometry. The excess silicon content in the films was controlled varying the NH₃/SiH₄ gas flow ratio from 0.45 up to 1.0. High-temperature annealing was employed to form the silicon quantum dots (QDs) and to enhance the photoluminescence (PL) in visible spectral range. The PL spectrum was found to be complex. The competition of five PL bands leads to the non-monotonous variation of total PL peak position in the range of 1.55–2.95 eV when the Si excess content increases. The shape of PL spectra depends also on an excitation light wavelength. It is shown that for the films fabricated with R ≤ 0.56 and R ≥ 0.67 the dominant contribution into PL spectra is given by native SiN_x defects, whereas in the films obtained with R = 0.59–0.67 the Si-QDs form the main radiative channel. The highest PL intensity is detected in Si-rich SiN_x films grown at R = 0.59–0.67 as well. PL mechanisms are discussed in terms of the contribution of different radiative channels in the light emission process that can show the ways for the optimization of SiN_x light-emitting properties.     

A Non-destructive FTIR Method for the Determination of Ammonium and Sulfate in Urban PM<Subscript>2.5</Subscript> Samples

Traditionally, the atmospheric particle composition is analyzed using destructive methods. In general, the destructive methods lead to the destruction of the samples, higher cost of the analysis and larger analysis time. In view of aforesaid, in current work, we present a method for the non-destructive analysis of atmospheric particles using open path-Fourier transform infrared spectroscopy (OP-FTIR). The developed method has been used for the measurement of ammonium and sulfate in atmospheric particles without destroying the samples. Here, we targeted the said species because of their relative importance for air pollution episode formation. Particulate sulfate plays a major role in formation of haze. However; particulate acidity is an important factor in this process, which is governed by particulate ammonium concentration. Therefore, both SO₄^2? and NH₄⁺ are important as far as atmospheric chemistry of haze formation is concerned. In the present study, the qualitative and quantitative estimation of ammonium and sulfate ions in PM_2.5 (particulate matter with aerodynamic diameter less than 2.5 µm) was carried out using OP-FTIR with the developed method. The seasonal average concentration of NH₄⁺ and SO₄^2? were measured to be 12.00 ± 5.80, 31.71 ± 12.71 µg/m³ respectively for winters, 3.00 ± 0.85 and 8.00 ± 2.28 µg/m³ respectively for summers and 2.60 ± 1.90 and 7.00 ± 5.21 µg/m³ respectively for monsoon season. The observed results are found to be in good agreement with that of other studies using destructive methods.     

Combustion synthesis and photoluminescence study of UV emitting LaBaB<Subscript>9</Subscript>O<Subscript>16</Subscript> phosphors

The borate phosphor LaBaB₉O₁₆ doped with Ce³⁺ ion intentionally and successfully synthesized using solution combustion rout using metal nitrates as precursors and urea as fuel. The phosphors were characterized by X-ray diffraction (XRD), Scanning electron microscopy and photoluminescence spectroscopies. The XRD patterns of the phosphor confirmed the successful crystallization of LaBaB₉O₁₆. The average crystallite size calculated using the Debye Scherer equation. The PL excitation spectra of LaBaB₉O₁₆ exhibited broad spectra peaking at 275 nm. Upon excitation with ultraviolet (UV) radiation at 274 nm the phosphor exhibited a broad band UV emission peaking at a wavelength of 335 nm corresponding to the 4f⁰5d¹??4f¹ transition of the Ce³⁺ ion. Moreover the influence of concentration of Ce³⁺ ion on luminescence properties has also been studied. Optimum concentration of Ce³⁺ ions in the prepared phosphor was found to be 0.05 mol. For this concentration the critical distance R₀ was calculated to be 22.04 Å. Finally, the Stokes shift for the synthesized phosphor was calculated to be 6512 cm^??1 using corresponding excitation and emission.     

Synthesis of CuO nanoflower and its application as a H<Subscript>2</Subscript>O<Subscript>2</Subscript> sensor

CuO three-dimensional (3D) flower-like nanostructures were successfully synthesized by a simple method at 100°C with Cu(NO₃)₂·3H₂O and NH₃·H₂O for 6 h in the absence of any additives. We found that NH₃·H₂O amount was critical for CuO morphology evolution. The phase analysis was carried out using X-ray diffraction (XRD) and the result confirmed that the CuO nanoflowers were single-phase. The morphological investigations by field emission scanning electron microscope (FESEM) revealed that the CuO nanoflowers were mono-dispersed in a large quantity and consisted of nanosheets. And then, CuO nanoflowers were successfully used to modify a gold electrode to detect H₂O₂ with cyclic voltammetry (CV) and amperometric (AC). It was found that CuO nanoflowers may be of great potential for H₂O₂ electrochemical sensing.     

Polyethyleneimine functionalised nanocarbons for the efficient adsorption ofcarbon dioxide with a low temperature of regeneration

Polyethyleneimine (PEI) conjugates with a range of nanocarbons (NCs) have been prepared, and their performances with regard to carbon dioxide absorption and liberation are compared. PEI-functionalised multi-walled carbon nanotubes (PEI-MWNTs) prepared by the reaction of branched PEI (25,000 Da) with F-MWNTs in the presence of pyridine, showed a lower CO₂ capacity at 25 °C (5 wt%, 1.1 mmol CO₂/g adsorbent) as compared to PEI-SWNTs (9.2 wt%, 2.1 mmol CO₂/g adsorbent), consistent with the interior layers of the MWNTs adding weight to the base NC without adding functionality. PEI-functionalised graphite/graphene was prepared by three routes: fluorinated graphite intercalation compounds, prepared from natural graphite powder, were reacted with PEI in EtOH with pyridine; exfoliated natural graphite powder was reacted with Boc–Phe(4-N₃)–OH, and subsequently PEI to give PEI-Phe(4-N-G); graphite oxide (GO) was reacted with PEI in the presence of NEt₃ to give PEI-GO. The CO₂ capacity of PEI-GO at 25 °C (8 wt%, 1.8 mmol CO₂/g adsorbent) was comparable to that of PEI-SWNTs making GO a valid and cheaper alternative to the SWNT scaffold. The temperature of CO₂ desorption of the PEI-NCs was 75 °C, providing a lower energy load for regeneration compared to current amine-based scrubbing units. The rate of CO₂ uptake is seen to depend on the curvature of the NC substrate.     

Microwave absorption characteristics of CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript> perovskite/carbon nanotube composites

The CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃PbI₃/carbon nanotube (MC) composite have been successfully synthesized by a facile in situ solution method, which are investigated as the microwave absorption materials. For the MAPbI₃ particles, the minimum reflection loss is only ?4.9 dB around 16.4 GHz due to the poor relative complex permittivity. Then, the relative complex permittivity of MC composites could be adjusted by changing the mass fraction of CNTs in composite, which is a vital role for the dielectric loss. The reflection loss of MC-5 composite (MAPbI₃/CNT, 5:1 wt%) can be improved to ?35.7 dB with thickness of 1.3 mm at 13.1 GHz. When the thickness is <3.0 mm, the microwave absorption bandwidth of MC-5 is 11.8 GHz (5.0–16.8 GHz) under the reflection loss lower than ?20 dB. The quarter-wavelength (λ/4) matching model is used to discuss the microwave absorption mechanism of MC composites. These results indicate that MC-5 composite could be used as the microwave absorption materials with strong reflection loss, lightweight and broad bandwidth.     

Photoluminescence properties of a novel red-emitting phosphor Ba<Subscript>2</Subscript>LaV<Subscript>3</Subscript>O<Subscript>11</Subscript>:Eu<Superscript>3+</Superscript>

Ba₂LaV₃O₁₁:Eu³⁺ phosphors were firstly synthesized by the traditional solid-state reaction method at 1100 °C. Their luminescence properties were investigated by photoluminescence excitation and emission spectra. The excitation spectrum shows a broad band centered at about 275 nm in the region from 200 to 370 nm, which is attributed to an overlap of the charge transfer transitions of O^2??→?V⁵⁺ and O^2??→?Eu³⁺. The phosphors exhibit the red emissions of Eu³⁺ and the emission intensity ratio of ⁵D₀?→?⁷F₂ to ⁵D₀?→?⁷F₁ is dependent on the Eu³⁺ concentration due to an environment change about Eu³⁺ ions. Concentration quenching occurs at 30 mol% in the phosphors and exchange interaction is its main mechanism. Ba₂LaV₃O₁₁:Eu³⁺ displays tunable CIE color coordinates from yellow orange to red depended on Eu³⁺ content, which may have a potential application for illuminating and display devices.     

EPR and optical investigation of ultraviolet-emitting Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet

Gd₃Ga₅O₁₂ garnet was prepared by a solution combustion method and characterized using powder X-ray diffraction, electron paramagnetic resonance (EPR) and optical spectroscopic techniques. EPR spectrum of the samples at ambient and low temperatures exhibited resonance signal at g?≈?2 attributed to Gd³⁺ ions disposed in an octahedral symmetry. The optical absorption spectrum showed a band centered at 274 nm attributed to ⁸S_7/2→⁶I_J transition of the Gd³⁺ ions. The excitation spectrum showed a maximum at 273 nm along with two relatively weaker peaks at 276 and 279 nm. These three excitation bands were assigned to the transitions ⁸S_7/2→⁶I_J. The emission spectrum showed two peaks centered at 311 and 306 nm that were assigned to the transitions ⁶P_7/2→⁸S_7/2 and ⁶P_5/2→⁸S_7/2 of Gd³⁺, respectively. To evaluate the phosphor performance, several magnetic and thermodynamic parameters for the system were calculated adopting standard procedure.     

Photoluminescence properties and charge compensation investigations of novel orange-emitting Eu<Superscript>3+</Superscript> doped Na<Subscript>4</Subscript>Ca<Subscript>4</Subscript>(Si<Subscript>6</Subscript>O<Subscript>18</Subscript>) phosphors

A series of polycrystalline Na₄Ca₄(Si₆O₁₈):Eu³⁺ orange emitting phosphors were synthesized by a conventional high-temperature solid-state reaction. The phase formation was confirmed by X-ray power diffraction analysis. The excitation spectra show a strong host absorption indicating an efficient energy transfer process from O^2? to Eu³⁺ ions. Upon NUV radiation, the phosphors showed strong red emission around 610 nm (⁵D₀ → ⁷F₂) and orange emission around 591 nm (⁵D₀ → ⁷F₁), but the ⁵D_1,2,3 emission nearly can not be seen. Compared with the luminescence properties of Li⁺, Na⁺, and K⁺ co-doped samples, we deduced that Na⁺ ions probably prefer to dope into the intrinsic Na vacancies rather than Ca²⁺ ions vacancies in Na₄Ca₄(Si₆O₁₈) crystal. Thermal stability properties, quantum efficiency and chromaticity coordinates of the phosphors have been investigated for the potential application in white LEDs.