CaAl2O4 Phase Research Articles

Performance enhancement of free CaO containing magnesia by in situ intergranular CaAl2O4 and MgAl2O4

Magnesia refractory raw material with free CaO has inherent weaknesses of low resistance to hydration, thermal shock and slag penetration. In this work, micro-sized Al2O3 powder was added into such magnesia with the purpose of improving its comprehensive performances. The results showed that with increasing Al2O3 content, CaAl2O4 and MgAl2O4 phases were sequentially formed at the MgO grain boundaries, which enlarged the MgO grains and reduced the apparent porosity. Owing to the transformation of free CaO to CaAl2O4, reduction of apparent porosity and covering of intergranular MgAl2O4 on MgO grains, hydration of the specimens was inhibited effectively by introducing the Al2O3 addition. The formed CaAl2O4 and MgAl2O4 phases lowered the thermal expansion coefficient and induced crack deflection, and therefore residual flexural strength ratio after thermal shock tests increased remarkably from 15.1% to 33.2% with increasing Al2O3 addition from 0 wt% to 7 wt%. Moreover, slag penetration resistance of the specimens was improved effectively with raising the Al2O3 content to 3 wt%, attributing to the increase in MgO grain size, decrease in apparent porosity and formation of high stable intergranular CaAl2O4 phase. However, further increasing Al2O3 content would dramatically accelerate the slag penetration owing to the decrease in grain size and formation of more unstable intergranular MgAl2O4 phase.

Phase Relations in the CaO-Nd2O3-Al2O3 System in Application for Rare Earth Recycling

Phase relations in the CaO-Al2O3-Nd2O3 system were investigated in the temperature range between 1330°C and 1600°C using equilibration technique followed by characterization using x-ray diffraction and scanning electron microscopy combined with energy-dispersive x-ray spectroscopy (SEM/EDX). Melting relations were investigated using differential thermal analysis followed by microstructure study using SEM/EDX. It was found that CaNdAlO4 (τ1) compound melted congruently at 1808°C, while CaNdAl3O7 (τ2) compound melted incongruently at 1766°C. The sample in the stability fields adjacent to lowest eutectic in the CaO-Al2O3 system (CaAl2O4 + Ca3Al2O6) and τ2 as well as samples in the stability field τ2 + CaAl2O4 + CaAl4O7 and τ1 + τ2 + Ca3Al2O6 were completely melted at 1600°C. The Ca12Al14O33 phase considered as not stable in anhydrous atmosphere was found instead of Ca3Al2O6 phase after heat treatment at 1330°C. After melting Ca12Al14O33 formed glass. The obtained results are important for selection of conditions for process of rare earth recycling from waste permanent NdFeB magnets.

New linear high-temperature thermistors based on the xAl2O3-(1-x)CaMnO3 ceramics

It is very important for a thermistor to have linear electrical properties, which can effectively reduce the complexity of circuit design. Phase structure modulation through introducing a second phase is crucial to achieve high performance electronic ceramics. Herein, new linear high-temperature thermistors based on the xAl2O3-(1-x)CaMnO3 ceramics were synthesized by the solid phase reaction method. With increasing Al2O3 content, CaMn2O4 and CaAl2O4 phase were observed, which formed internal conducting network and induced linear electrical properties. The ceramics exhibited negative temperature coefficient (NTC) thermistor characteristics in the temperature range of 523 K–1173 K. Their resistance-temperature (R-T) relationship displayed a good linear characteristic, especially for x = 0.3, which showed the highest linearity (R2: 0.9984) in the temperature range of 523 K–1173 K, covering medium to high temperature region. The mechanism of linear electrical properties was explained by a parallel conductive model in combination with complex impedance analysis. This approach could be generalized for designing linear high-temperature thermistors with a wide temperature range.

Photoluminescence properties and energy transfer of blue-green CaAl2O4: Tb3+, Bi3+ phosphors

A series of Tb3+ (or Bi3+) doped and Tb3+, Bi3+ co-doped CaAl2O4 phosphors were successfully prepared by the conventional solid state reaction method. The properties of the as-prepared samples, including the crystal structure, morphology, composition and photoluminescence properties are characterized by the X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), energy dispersive spectrometer (EDS), high-angle annular dark field (HAADF) image and spectroscopy technology, etc. The diffraction patterns of the doped samples display the peaks corresponding to the CaAl2O4 phase. The EDS and HAADF indicate that the Ca, Al, O, Tb and Bi elements are distributed uniformly in the Tb single-doped and the Tb, Bi co-doped CaAl2O4 samples. In order to investigate the energy transfer from Bi3+ to Tb3+, the critical (or optimal) doped concentrations of Tb and Bi in CaAl2O4 are obtained experimentally in sequential order. The critical doped concentrations (single-doped: 3 ​mol%Tb3+, and co-doped: 3 ​mol%Tb3+, 0.3 ​mol%Bi3+) for CaAl2O4 hosts have been determined. A strong blue-green luminescence corresponding to 5D4→7F5 transition of Tb3+ by near ultra-violet (367 ​nm) excitation and the obvious energy transfer from Bi3+ to Tb3+ in the co-doped CaAl2O4 can be observed. The energy transfer mechanism has been discussed. The CIE coordinates and decay lifetimes for the typical samples have also been investigated. It is found that CaAl2O4: Tb3+, Bi3+ is a blue-green emitting phosphor and has higher efficiency for the operation with near ultra-violet excitation.

Phase transformation involved in the reduction process of magnesium oxide in calcined dolomite by ferrosilicon with additive of aluminum

AbstractMetal magnesium is mainly produced from the calcined dolomite by the silicothermic production. However, in this process, the reduction temperature is higher while the reaction speed is slow, which results in higher energy consumption and serious environmental problems. In this paper, adding aluminum into the ferrosilicon reducing agent is expected to lower the reaction temperature so as to solve the problems above. The phase transition involved in the whole reduction process including with and without aluminum addition were investigated in details by theoretical calculation and experimental research. The influence of aluminum on the magnesium oxide reduction path was analysis to clarify the internal mechanism. The results show that aluminum added into the ferrosilicon would first react with magnesium oxide to form magnesium vapor and alumina under vacuum pressure of 10 Pa when the temperature rises to 720°C. Then, calcium aluminate would be formed by the reaction of aluminum oxide and calcium oxide. Once the temperature reaches 1150°C, silicon begins to reduce the magnesium oxide to create the silicon oxide that will finally react with calcium oxide to form calcium silicate. When the temperature rises above 1150°C, both the aluminum and silicon will participate in the reduction of magnesium oxide. In the process of heating up, the mixture of aluminum, ferrosilicon and calcined dolomite forms Mg2Al4Si5O18 and Ca3Al2(OH)12 phase with the components in calcined dolomite. Mg2Al4Si5O18 and Ca3Al2(OH)12 phase finally form Ca12Al14O33 phase. The interaction between aluminum and ferrosilicon in the mixture is less; the mixture of aluminum and ferrosilicon first forms Al3FeSi2 phase, and finally has the trend of forming Al4.5FeSi phase. There is a great difference between the phase transformation of aluminum in the mixture of aluminum, ferrosilicon and calcined dolomite and that of aluminum in the mixture of aluminum and ferrosilicon.

Towards Blue Long-Lasting Luminescence of Eu/Nd-Doped Calcium-Aluminate Nanostructured Platelets via the Molten Salt Route.

Calcia-alumina binary compounds doped with rare earths and some transition metals cations show persistent luminescence from the visible to the infrared range. Specifically, the blue light can be obtained through the Eu2+ activator center in a potential host, such as dodecacalcium hepta-aluminate (Ca12Al14O33) and monocalcium aluminate (CaAl2O4). By doping with Nd3+, the persistent luminescence can be substantially prolonged; for this reason, the Eu/Nd pair is a potential choice for developing long-lasting blue luminescence. Herein, the phase evolution of the calcia-alumina system via molten salt synthesis is reported as a function of the synthesis temperature and the atmospheric environment. The fraction of CaAl2O4 phase increases when the temperature is higher. Synthesized microparticles of platelet-type morphology represent isolated nanostructured ceramic pieces. Under visible light, the particles are white. This indicates that the followed process solves the dark-gray coloring of phosphor when is synthesized in a reduced atmosphere at high temperature. As regards the synthesis mechanism, which is assisted by the molten flux, the dissolution−diffusion transport process is promoted at the surface of the alumina microparticles. In fact, the emission intensity can be modulated through the phase of the Eu-doped calcium-aluminate discrete platelets synthesized. Consequently, the photoluminescence intensity depends also on the oxidation state of the Eu ion. X-ray absorption near-edge structure and photoluminescence measurements corroborate the Eu reduction and the grain coarsening with the enhancement of the blue emission. The doped phosphors with Eu/Nd show a broad and strong absorption in the region of 320–400 nm and a broad emission band at around 440 nm when they are excited in this absorption range. From a broader perspective, our findings prove that the Ca12Al14O33 and CaAl2O4 phases open new opportunities for research into the design of blue long-lasting emitters for a wide range of fields from ceramic to optoelectronic materials.

Leaching characteristics and mechanism of the synthetic calcium-aluminate slags for alumina recovery

The leaching characteristics and mechanism of synthetic CaO-Al2O3 slags in alkaline solution at atmospheric pressure have been studied. The purpose of the study is to have a better understanding of the leaching part of the Pedersen process, as an alternative to the Bayer process for alumina production. The crystalline slags containing CaAl2O4, Ca3Al2O6, CaAl4O7, and Ca12Al14O33 phases, and leaching residues (predominantly CaCO3) are characterized by X-ray Diffraction and semi-quantitative analysis. Of the leaching characteristics in a solution containing 120 g/L Na2CO3, the slag with the highest amount of Ca12Al14O33 phase is the most leachable one in the CaO-Al2O3 system with about 95% of alumina extraction. The leaching extent is confirmed employing Inductively Coupled Plasma-High Resolution-Mass Spectrometer (ICP-HR-MS) analysis, and it decreases by 0.4% for every percent of the bayerite (Al(OH)3) formation during the leaching. The less stable form of CaCO3, i.e., vaterite, is formed over the leached slag particles that consist 33–49 wt% CaO, while Ca3Al2(OH)12 (tricalcium alumina hydrate) precipitated at relatively low concentrations (< 6 wt%) in all residue. The non-bridging oxygen (NBO) over tetrahedral structure (T) index shows that the atomic structure may affect the leaching extent of the slags, the lower NBO/T index of the phase is the more difficult for the phase to leach or depolymerize. However, the Ca12Al14O33 phase is an exception case where it has “free” O-ions at the center of the cage structure, which makes it easily depolymerize, therefore, the NBO/T index for the Ca12Al14O33 phase becomes irrelevant. Furthermore, the morphology and size evolution of the obtained residue measured with laser particle analyzer indicates the agglomeration behavior of the residue particles during the leaching process.

Highly-porous mayenite-based ceramics by combined suspension emulsification and reactive sintering

This work focuses on demonstration of reliable and versatile method for processing cellular mayenite (Ca12Al14O33) -based ceramics with designed porosity, with emphasis on potential catalytic applications. Highly porous (up to 82% of open porosity) monoliths were obtained by emulsification of liquid paraffin in aqueous suspensions of mixed CaCO3 and Al2O3 powders followed by burnout of the organic phase and reactive firing at various temperatures. The results have shown that this method is particularly suitable for processing of mayenite and allows simultaneous retaining of highly open cellular structure with reproducible pore size and a major content of Ca12Al14O33 phase in a wide range of processing conditions. The correlations between firing conditions, minor nominal aluminium excess, porous structure and phase composition were analysed based on Taguchi experimental planning.

Sorption-enhanced reforming of tar: Influence of the preparation method of CO2 absorbent

To remove tar and produce environment-friendly H2, one of the promising routes is the sorption-enhanced steam reforming (SESR) process, in which the CO2 sorbent is a key element. We prepared the CO2 sorbents with Ca12Al14O33 as carrier with various methods. Their characterizations were examined, and the sample prepared by solgel (SG) method showed the strongest CaO and Ca12Al14O33 phases and the most excellent pore structure among all the samples. Then, a thermogravimetric experiment was conducted, and the results showed that the sample prepared by sol-gel (SG) method had the best CO2 adsorption capacity and excellent long-term cyclic stability. Finally, the sorbent was used into the steam reforming experiments of tar. Under the action of the sorbent, the reforming reaction was enhanced in-situ, with the H2 yield and concentration improved obviously, and especially, H2 concentration can reach over 98.85%.

Erosion behavior and luminescence properties of Y3Al5O12:Ce3+‐embedded calcium bismuth borate glass‐ceramics for WLEDs

AbstractCurrent white light emitting diodes (WLEDs) have poor thermal stability and lack red‐light, which restrict their applications in high‐power and high‐color‐rendering‐index solid‐state lighting. YAG glass‐ceramics provide an efficient way to resolve these problems. Herein, novel YAG‐embedded calcium bismuth borate glass‐ceramics (YAG‐GCs) with Eu3+ doping were prepared using a rapid melt quenching technique. The precursor glass exhibits superior YAG refractive index matching and high transmittance. Differential scanning calorimeter simulations verify YAG particles react with the precursor glass. The degree of YAG erosion is slight but monotonously increases with the co‐sintering temperature from 640 to 700°C. The erosion products probably contain YAB (Al3Y(BO3)4), Al3Eu(BO3)4, Bi24B2O39, and Ca12Al14O33 phases, and the Bi ion valence state is maintained during the reaction process. The energy transfer from Ce3+ to Eu3+ is suppressed. The YAG‐GC PL intensities monotonously increase as the co‐sintering temperature decrease from 640 to 700°C and the YAG content increase from 2.5 to 7 wt.%. The optical parameters of a WLEDs packed by YAG‐GCs and blue chips are a luminous efficiency of 105.3 lm/W, correlated color temperature of 3940 K and color rendering index of 70.1. The as‐prepared YAG‐GCs are promising candidates for high‐power, warm WLEDs due to their superior thermal stability, high quantum efficiency, and low cost.

CaO-based sorbent derived from lime mud and bauxite tailings for cyclic CO2 capture.

Using aluminum nitrate (AlN) and bauxite tailings (BTs) as different dopants, and lime mud (LM) as calcium source, a series of CaO-based sorbents were prepared for CO2 capture by dry mixing method; then, the carbonation conversions of multiple carbonation/calcination cycles were detected in a thermogravimetric analyzer (TGA). Effects of different dopants, dopant contents, precalcination conditions, and a long series of cycles on CO2 absorption properties were scrutinized, and the phase composition and morphologies were tested by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Durability studies show that the sample doped with AlN remains a higher absorption conversion (30.88%) after 30 carbonation/calcination cycles. In the meantime, the sorbent doped with BTs showed a lower conversion, which is probably resulted from the impurities from waste BTs. However, the sample BT has a better cyclic absorption stability. In addition, the incorporation of BTs, as a kind of solid waste, not only decreases the preparation cost but also is good for environment. The occurrence of Ca12Al14O33 phase is considered to provide a stable framework inhibiting inactivation of CaO, and improve the CO2 adsorption stability. Graphical abstract ᅟ.

Effect of carbon coating on spontaneous C12A7 whisker formation

A carbon nanoreactor concept was applied to study the stabilization effect of carbon shell on phase composition and morphology of dodecacalcium hepta-aluminate Ca12Al14O33. The starting C12A7 powder was obtained using aluminum and calcium hydroxides as precursors. Carbon shell was formed by a chemical vapor deposition of divinyl at 550 °C. After the calcination at 1400 °C, the product was characterized by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM). It was observed for a first time that spontaneous formation of calcium aluminate whiskers take place under the conditions described. Each whisker consists of a ‘head’ (globular particle of 0.5 microns in diameter) and a ‘tail’ (prolonged whisker of few microns in length and 0.1–0.2 microns in diameter). According to HRTEM, the ‘head’ is characterized with microcrystal lattice of Ca12Al14O33 compound. XRD data show the presence of CaAl2O4 phase traces. The ‘head’ and ‘tail’ of the whisker are covered with structured graphene layers of 10 nm and 3 nm, correspondingly.

Correlation of Structure, Tunable Colors, and Lifetimes of (Sr, Ca, Ba)Al₂O₄:Eu2+, Dy3+ Phosphors.

(Sr, Ca, Ba)Al2O4:Eu2+, Dy3+ phosphors were prepared via a high temperature solid-state reaction method. The correlation of phase structure, optical properties and lifetimes of the phosphors are investigated in this work. For the (Sr, Ca)Al2O4:Eu2+, Dy3+ phosphors, the different phase formation from monoclinic SrAl2O4 phase to hexagonal SrAl2O4 phase to monoclinic CaAl2O4 phase was observed when the Ca content increased. The emission color of SrAl2O4:Eu2+, Dy3+ phosphors varied from green to blue. For the (Sr, Ba)Al2O4:Eu2+, Dy3+ phosphors, different phase formation from the monoclinic SrAl2O4 phase to the hexagonal BaAl2O4 phase was observed, along with a shift of emission wavelength from 520 nm to 500 nm. More interestingly, the decay time of SrAl2O4:Eu2+, Dy3+ changed due to the different phase formations. Lifetime can be dramatically shortened by the substitution of Sr2+ with Ba2+ cations, resulting in improving the performance of the alternating current light emitting diode (AC-LED). Finally, intense LEDs are successfully obtained by combining these phosphors with Ga(In)N near UV chips.

Effects of calcining temperatures of Eu2+ and Dy3+ ion-codoped calcia-alumina binary compounds on their phase transition and luminescence properties

In this study, phase evolution as a function of calcining temperature in calcia-alumina binary compound phosphors was examined to interpret their luminescence properties. The binary compounds were prepared through a coprecipitation method employing potassium carbonate as the precipitant to obtain precursors with a high-precision stoichiometric composition for the calcination. The results indicate that the morphology, surface properties, and infrared transmittance of the prepared phosphors were affected by the calcining temperature. X-ray diffraction analysis results enabled identification of Ca12Al14O33, CaAl2O4, and CaAl4O7 phase transitions at various calcining temperatures. The amount of the CaAl2O4 phase increased with the calcining temperature within the range of 700–1060 °C. The Ca12Al14O33 and CaAl4O7 phases exhibited trends opposite to that of the CaAl2O4 phase. When the calcining temperature reached 980 °C, the CaAl2O4 phase (60.5%) was determined to be the main phase in the structure, and excellent emission intensity at an emission band of 449 nm was observed as a result of the complete substitution of Eu2+ for Ca2+. The emission intensity corresponding to Eu2+ 4f65d1 → 4f7 decreased slightly when the temperature reached 1060 °C because of more monoclinic reciprocal CaAl4O7 phase (81.5%) formation, causing the transfer of some Eu2+ to Eu3+, during which strong photoluminescence spectra of Eu3+5D0 → 7Fj (j = 0, 1, 2, 3, 4) within the wavelength range of 570–720 nm were observed. Because the strong photoluminescence spectra of the Eu2+ and Eu3+ emissions were together within the wavelength range of 449–720 nm in this phosphor, the photoluminescence was white light. One moderately intense emission band in the infrared region was observed and attributed to the Dy3+4F9/2 → 6Hj (j = 11/2, 13/2, 15/2) transition when the temperature was higher than 900 °C. Finally, the energy transitions of Eu2+, Eu3+, and Dy3+ and the nephelauxetic effect and crystal field are discussed.

Investigation of CaO-based sorbents derived from eggshells and red mud for CO2 capture

A series of CaO-based sorbents, using low-cost eggshells and red mud (Rm) pretreated by the acid leaching process as raw materials, were synthesized by the dry mixing method. The effects of different sodium leaching ratios, Rm doping contents, and eggshells as calcium precursors on CO2 capture properties of the CaO-based sorbents are discussed in this paper. The cyclic carbonation/calcination performance of the as-prepared sorbents was evaluated by a thermogravimetric analyzer. The phase compositions and morphologies of the sorbents were characterized by XRD and SEM, respectively. The as-prepared sorbents doped with 10 wt% Rm3 (with a sodium leaching ratio of 99.91%) showed a superior cyclic conversion of 46.67% after 100 carbonation/calcination cycles. The existence of the Ca12Al14O33 phase was probably responsible for the excellent CO2 cyclic performance.

Laser sintering of persistent luminescent CaAl2O4:Eu2+Dy3+ ceramics

Described herein is the use of a laser sintering technique to produce persistent luminescence CaAl2O4:Eu2+Dy3+ ceramics exhibiting enhanced translucency in the visible/near infrared spectral range. In this technique, a CO2 laser was used as the main heating source for sintering with no atmospheric control. The ceramics sintered at a power density of 1.2 W/mm2 yielded single monoclinic CaAl2O4 phase, homogeneous grain size distributions and transmittance up to 45% in the range of 400 nm–900 nm. Upon conventional sintering in air, the ceramics exhibited the characteristic blue emission from the Eu2+ ions corresponding to the 5d → 4f transition at 440 nm, and weak emission from Eu3+ ions between 550 nm and 700 nm, corresponding to the 5D0 → 7FJ transitions.

Development of sintering-resistant CaO-based sorbent derived from eggshells and bauxite tailings for cyclic CO2 capture

Carbon dioxide, one of the major greenhouse gases, are believed to be a major contributor to global warming. As a consequence, it is imperative for us to control and remove CO2 emissions. The CaO, a kind of effective CO2 sorbent at high temperature, has attracted increasing attention due to some potential advantages. The main drawback in practical application is the deterioration of CO2 capture capacity following multiples cycles. In the present study, novel low-cost porous CaO-based sorbents with excellent CO2 absorption–desorption performance were synthesized using bauxite tailings (BTs) and eggshells as raw materials via solid-phase method. Effect of different BTs content on CO2 absorption–desorption properties was investigated. Phase composition and morphologies were analyzed by XRD and SEM, and CO2 absorption properties were investigated by the simultaneous thermogravimetric analyzer. The as-prepared CaO-based sorbent doped with 10 wt% BTs showed superior CO2 absorption stability during multiple absorption–desorption cycles, with being >55% conversion after 40 cycles. This improved CO2 absorption performance was attributed to the particular morphologies of the CaO-based sorbents. Additionally, during absorption–desorption cycles the occurrence of Ca12Al14O33 phase is considered to be responsible for the excellent CO2 absorption performance of CaO-based sorbents. In the meanwhile, the use of solid waste eggshell and BTs not only decreases the release of solid waste, but also moderates the greenhouse effect resulted from CO2.

Transesterification of glycerol with dimethyl carbonate over calcined Ca-Al hydrocalumite

A series of Ca-Al hydrocalumite with different Ca/Al ratios (1–6) were synthesized and used in the transesterification of glycerol with dimethyl carbonate (DMC) to glycerol carbonate (GC) under mild conditions. The calcined Ca-Al hydrocalumites were active with a selectivity toward GC that reached 97% at 93% conversion of glycerol over the sample with Ca/Al = 2 at 70 °C, 3 h, and DMC/glycerol = 3. The glycerol conversion depended mainly on the proportion of strong basic sites in the calcined Ca-Al catalysts. The Ca12Al14O33 phase in the calcined catalysts was stable, but CaO was lost in recycle experiments and thus brought deactivation.

Study of CO2 cyclic absorption stability of CaO-based sorbents derived from lime mud purified by sucrose method.

Using lime mud (LM) purified by sucrose method, derived from paper-making industry, as calcium precursor, and using mineral rejects-bauxite-tailings (BTs) from aluminum production as dopant, the CaO-based sorbents for high-temperature CO2 capture were prepared. Effects of BTs content, precalcining time, and temperature on CO2 cyclic absorption stability were illustrated. The cyclic carbonation behavior was investigated in a thermogravimetric analyzer (TGA). Phase composition and morphologies were analyzed by XRD and SEM. The results reflected that the as-synthesized CaO-based sorbent doped with 10 wt% BTs showed a superior CO2 cyclic absorption-desorption conversion during multiple cycles, with conversion being >38 % after 50 cycles. Occurrence of Ca12Al14O33 phase during precalcination was probably responsible for the excellent CO2 cyclic stability.

Structural and luminescent properties of Eu2+ and Nd3+-doped mixed alkaline earth aluminates prepared by the sol-gel method.

Alkaline earth aluminates with the overall nominal compositions Mg0.5Sr0.5Al2O4 (MSA), Ca0.5Mg0.5Al2O4 (CMA) and Ca0.5Sr0.5Al2O4 (CSA) doped with 0.5 mol% of Eu2+ and 0.25 mol% of Nd3+ ions were obtained by a modified aqueous sol-gel method and annealed in a reducing atmosphere at 900, 1000, 1100 and 1300 °C. The sample structures were investigated by XRD. Solid solubility was only confirmed for the CSA samples. UV-excited luminescence was observed in the blue region (λ = 440 nm) in the samples of CMA containing the monoclinic CaAl2O4 phase and in the green region (λ = 512 nm) in the samples of MSA containing hexagonal or monoclinic phases of SrAl2O4. The CSA samples, besides the blue region, exhibited an extended shoulder in the green region, which proved the existence of some pure strontium phases. Co-doped Nd3+ ions did not affect the wavelength of the emitted light, but the persistent luminescence at room temperature was greatly extended with respect to the aluminates doped with Eu2+ ions only.