CaO Doping Research Articles

Mitigating low-temperature hydrothermal degradation of 2 mol% yttria stabilised zirconia and of 3 mol% yttria stabilised zirconia/nickel oxide by calcium oxide co-doping and two-step sintering

Hydrothermal degradation deteriorates the fracture toughness and strength of tetragonal stabilised zirconia. The phase transformation to the monoclinic phase is particularly critical for materials with lower stabiliser content such as 2 mol% and 3 mol% yttria stabilised zirconia (2YSZ and 3YSZ). In this work, two routes in two-step sintering and co-doping with calcium oxide are analysed to mitigate the degradation. Both strategies show a reduction in average grain size of the 2YSZ while maintaining a comparable densification. The achieved smaller grains suppress the hydrothermal degradation rates. In addition, a mitigating effect beyond the reduction in grain size of YSZ is found for CaO doping. 1.6 mol% CaO co-doped 2YSZ shows less than 4% of monoclinic phase after 50 h in an autoclave with H2O at 134 °C. Pure 2YSZ contrarily reaches 100% monoclinic phase after 20 h at the same conditions. A suppression of degradation by CaO doping was also observed for the composite of nickel oxide and 3 mol% yttria stabilised zirconia. Hence, CaO co-doping can be an interesting strategy to increase resistivity against hydrothermal degradation for both biomedical and renewable energy applications. The findings further outline a route to achieve tetragonal YSZ with lower yttria contents than 2 mol%.

Fabrication of CeO2/CaO nanocomposite as ultraviolet screening agent and its application in sunscreen

In order to improve CeO2 ultraviolet absorption performance and transparency, this paper puts forward that different proportions of calcium oxide are doped during the formation of CeO2. Doping calcium ions with larger ion size and lower valence leads to the formation of oxygen defects by inhibiting the precipitation of oxygen from cerium oxide and keeps the fluorite structure of cerium oxide stable. The effects of CaO doping on the crystal structure, morphology, and optical characteristics of CeO2 were analyzed by XRD, SEM, TEM, TGA, XPS, UV–vis spectrophotometry. The pH of sunscreen, spread ability and some physical as well as chemical evaluations were carried out, while to study its UV protection performance the sunscreen SPF test was performed. The UV–vis results revealed that CeO2/20%CaO showed highly efficient UV absorption capacity. After adding calcium oxide, the catalytic activity of the material is reduced, due to which the generation of free radicals is demoted, and the harm to human body is decreased. After the addition of 3 wt% nanomaterials into sunscreen, the sun protection factor（SPF ）values of pure cerium oxide and CeO2/20%CaO are 1.95 and 2.81, and their sun protection coefficient is increased by 69.39%. Therefore, it is an ideal candidate as a component of ultraviolet screening agent in sunscreens and provides a new idea for the research direction of cerium oxide in anti-ultraviolet applications.

Electrical properties, microstructure, and thermal conductivity of hot-pressed CaO-doped AlN ceramics

AlN ceramics exhibit good physical and chemical properties and are ideal materials for the dielectric layer of electrostatic chucks. However, they cannot generate a strong J−R-type electrostatic adsorption force because of their high resistivity. Therefore, adding other substances is necessary to regulate the electrical properties and enable their application. In this study, AlN ceramics were prepared using hot-pressed sintering with 0.2 wt% CaO as an additive at a sintering temperature range of 1700–1900 °C. The effects of CaO doping on the phase composition, microstructure, electrical properties, and thermal conductivity of the AlN ceramics were systematically investigated. The addition of CaO not only enhanced the sintering process of the AlN ceramics, but also significantly reduced the electrical resistivity and increased the thermal conductivity. The relative density of CaO-doped AlN ceramics reached 98.88 % at 1700 °C, with a thermal conductivity of 81.51 W m−1•K−1. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and AC impedance spectroscopy were used to analyse the impurities and defects in the AlN ceramics. The lattice parameters and fitting grain resistance indicated that the CaO-doped AlN presented a higher lattice oxygen concentration, increasing the concentration of Al vacancies and electrons. Moreover, the level of dissolved oxygen could be controlled by altering the sintering temperature, resulting in AlN ceramics with electrical resistivity ranging from 8.1 × 106 to 1.7 × 1012 Ω cm. Further research was conducted to investigate the effect of the electrical resistivity of AlN ceramics on the J−R-type electrostatic adsorption force, and it was found that AlN ceramics with a specific resistivity of 109 Ω cm exhibited the highest electrostatic adsorption of 351.7 g/cm2.

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y2O3–Al2O3–SiO2 (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y2Si2O7, mullite, and SiO2 (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.

Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

AbstractCalcium oxide (CaO) as sintering additive was first used to fabricate La0.4Gd1.6Zr2O7 transparent ceramics by a simple solid‐state reaction and one‐step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as‐fabricated La0.4Gd1.6Zr2O7 ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca2+ entered the lattice of La0.4Gd1.6Zr2O7 ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La0.4Gd1.6Zr2O7 ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).

Immobilization of Candida Rugosa lipase on Ca/Kit-6 used as bifunctional biocatalysts for the transesterification of coconut oil to biodiesel

Immobilization of Candida Rugosa lipase (CRL) on Ca doped Kit-6 solid was investigated via the adsorption method. Effects of the experimental parameters, such as pH value, temperature, time, and Ca content on the adsorption of CRL on Ca/Kit-6 were studied. In the Ca doped Ca/Kit-6 materials, Ca2+ coexisted in two environments: Ca2+ in CaO nanoparticles and in Ca-O-Si in the framework of Kit-6. There existed three kind of oxygen species: (1) lattice oxygen in CaO and SiO2; (2) Oxygen in structural defects; and (3) non-stoichiometric surface oxygen in OH form. The 0.5Ca/Kit-6 showed the most CRL immobilization capacity and stability under the optimal adsorption condition, which was correlated with its greater amount of basicity resulting from its more non-stoichiometric surface oxygen and oxygen in structural defects as well as more Ca2+ incorporation in the Kit-6 framework. In the production of biodiesel via transesterification of coconut oil, the catalysts showed better catalytic performance under weak basic than acidic condition. Under the optimal reaction conditions (40 °C, pH 7.5, 200 rpm, oil to methanol ratio 1:3 with stepwise addition of methanol, and 0.1 g of CRL/0.5Ca/Kit-6), the coconut oil conversion and biodiesel yield catalyzed with CRL/0.5Ca/Kit-6 was 95 and 98%, respectively. 10wt% water in the reaction mixture promoted the coconut oil transesterification to biodiesel. The CRL/0.5Ca/Kit-6 catalyst possessed bifunctional roles of the enzymatic catalysis role from immobilized CRL and the basic catalysis role from CaO doping. The CRL adsorption modes, water influence and the reaction mechanism of coconut oil transesterification were discussed.

Phase Formation and Stabilization Behavior of Ca-PSZ by Post-Heat Treatment

The phase formation and stabilization behaviors of calcia partially stabilized zirconia (Ca-PSZ) were investigated with regard to the CaO content and post-heat treatment. Sintered specimens were prepared by adding 2, 3, 4, and 5 mol% to CaO to ZrO2, and post-heat treatment were conducted. In the X-ray diffraction pattern, the monoclinic peak decreased, the tetragonal peak increased upon CaO doping, and no CaZrO3 peak was observed. Transmission electron microscopy images of the Ca-PSZ showed that the d-spacing of 4CSZ (200)m extended from 0.260 nm to 0.266 nm subsequent to post-heat treatment. The coefficient of thermal expansion gradually increased in accordance with the dopant concentration, in addition, it increased even after the post-heat treatment. These results are related to the increase in tetragonal phase, which has a relatively higher coefficient of thermal expansion than that of the monoclinc phase. According to the Vickers hardness measurement, the hardness of all specimens increased gradually as the concentration of CaO increased, and the hardness of the 5CSZ was improved from 676 to 774 Hv by the post-heat treatment.

Comparative X-ray diffraction study of the Yb2O3 stabilized zirconia ceramics doped with SrO and CaO

Ceramic samples with the compositions 97 mol% ZrO2, 3 mol% Yb2O3; 96 mol% ZrO2, 3 mol% Yb2O3,1 mol%SrO;96 mol% ZrO2, 3 mol% Yb2O3,1 mol% СаОwere obtained by the sol-gel method. The final sintering was carried out at 1450 or 1550 °C. A comparative study of the samples was carried out by the Rietveld method using X-ray diffraction data. The Stephens's approach was used to model the diffraction peaks profiles. The addition of SrO and CaO promotes the formation of thet'form during sintering of the samples at 1450 °C. These samples contain thetform (70 vol%) andt'form (30 vol%). The samples sintered at 1550 °C without doping and doped with SrO have approximately the same phase composition. In the samples sintered at 1550 °C with the addition of CaO, the content of thet'form is 45 vol%, and the content of thetform is 55 vol%. The addition of CaO or SrO leads to an increase in the unit cell volumes of thetforms during sintering at 1450 °C.For the samples sintered at 1550 °C, the unit cell volumes of thetforms are less than for the samples without doping. Based on the Williamson-Hall plots for thetforms, it was found that the CaO and SrO doping reduces the microstrain values for the [001] crystallographic direction.

Ca Doping Effect on the Competition of NH3–SCR and NH3 Oxidation Reactions over Vanadium-Based Catalysts

The Ca poisoning effect constrains vanadium-based catalysts from further application in high-calcium content flue gas, and the deactivation effect of CaO on V2O5–WO3/TiO2 (VWT) and V2O5/TiO2 (VT) catalysts has been investigated from a new perspective. As the NH3 selective catalytic reduction (NH3–SCR) results showed, the NO conversion at 400 °C declined by 71.7% for VT and by 34.8% for VWT after CaO doping, and the decrease in NO conversion for NH3–SCR at temperatures greater than 300 °C was mainly caused by the competition of the NH3 oxidation reaction. The characterization of catalysts by NH3–SCR, XRD, Raman spectroscopy, H2-TPR, NH3-TPD, and TEM showed that the number of surface acid sites decreased and that the oxidation and reduction properties of the catalysts all deteriorated after CaO doping, which is the primary reason for the decrease in NO conversion. CaO also affected NH3 oxidation, and the L-NH3 adsorbed on CaO favored the NH3 oxidation reaction, as DRIFTS showed. NH3 reaction pathways changed by CaO doping because NH3 oxidation becomes more competitive at temperatures greater than 300 °C. Density functional theory (DFT) calculations confirmed the NO formation pathway on CaO, and the competition mechanism of NH3–SCR and NH3 oxidation reactions has been described, which has hardly been reported.

The effect of CaO doping in activated carbon composite as a heavy metal adsorbent in water

Lead (Pb) is a dangerous heavy metal that can settle in marine animals, and it will have a bad impact on our health. One of the efforts to reduce the stream’s levels of hazardous metals is by using the adsorption method. This study aimed to determine the effect of CaO doping on activated carbon composites on the physical properties of water and its ability to absorb Pb. Atomic Absorption Spectroscopy was conducted to determine the remaining Pb content after the adsorption process. Based on the results, it was found that the doping of CaO in activated carbon composites affected the physical properties of water-based on parameters of pH, EC, TDS, temperature, and turbidity. The water quality test results have met the PerMenKes standard No: 416/MENKES/PER/IX/1990, with the water quality obtained having a pH value of 7, EC 313.00 µs/cm, TDS 156.50 ppm, temperature 29 , and turbidity 3.95 NTU. Besides that, the addition of CaO mass to activated carbon composites affects Pb metal’s adsorption ability, where CaO doping can increase the absorption of Pb.

Structure‐property relations for a phase‐pure, nanograined tetragonal zirconia ceramic stabilized with minimum CaO doping

AbstractDense (~97%) CaO‐stabilized ZrO2 ceramic was stabilized with minimum (3 mol%) doping (reported to date) and processed via conventional sintering at a low temperature (~1200°C); compositional analysis via X‐ray florescence confirmed the CaO doping accuracy. Phase‐pure tetragonal structure (characterized via both X‐ray diffraction and Raman spectroscopy) along with uniform nanograins (90 nm) of the ceramic ensured the evolution of no monoclinic phase even after vigorous low‐temperature degradation experiments (both thermal and hydrothermal aging for 80‐100 h). The sintered ceramic recorded a high hardness (~15 GPa); the indentation toughness value was also comparable to a 3 mol% yttria‐stabilized zirconia system. The remarkable structure–property correlations in the 3 mol% CaO‐stabilized ZrO2 ceramic suggests that the same may be worth examining for suitable future applications (e.g., in dental ceramics).

CaO doped ZnO–Bi2O3 varistors: Grain growth mechanism, structure and electrical properties

In this study, the impact of CaO doping on the microstructure, phase and electrical properties of the ZnO–Bi2O3 system was studied. The powder was synthesized by the solution combustion route. Reduction in the crystallite size of ZnO from 23 nm to 17 nm of the powder sample with CaO doping was noted. Conventional sintering was used to consolidate the powders. Solid state reacted compositions revealed, from X-ray diffraction, formation of two phases (a) Ca4Bi6O13 at 750 °C and (b) Ca0.89Bi3.11O5.56 at 900 °C. An increase in density from 4.21 to 5.45 gcc−1 and a decrease in grain size from 4.50 to 1.54 μm with increasing CaO doping were also observed. A grain growth mechanism in the ZnO–Bi2O3–CaO system was proposed. The breakdown field increased up to 1 wt % CaO (27 kV cm−1) and after that decreased. Impedance spectroscopic investigation, in the temperature range of 100300 °C and frequency range of 0.1 Hz -1M Hz, exhibited two relaxations. These were attributed to defects formed. The two-stage activation energy of conductivity was noticed, which was similar to that observed in complex ZnO varistor systems.

An economical dopant for improving the comprehensive electrical properties of ZnO varistor ceramics

The effects of CaO (Ca) doping on the microstructure and comprehensive electrical properties of ZnO varistor ceramics were studied. A small amount of Ca doping could increase the voltage gradient and nonlinearity of ZnO varistor ceramics and reduce the leakage current. The low-cost Ca could be an excellent substitute for some expensive doping elements of ZnO varistor ceramics. The electrical parameters of the best sample were as follows: breakdown voltage E1mA = 532 V/mm, leakage current JL = 0.11μA/cm2 and non-linear coefficient α = 96.

Investigation of the Effects of CaO Doping on the Microstructure and Electrical Properties of ZnO-Based Linear Resistance Ceramics

ZnO-Al2O3-MgO-SiO2-TiO2-CaO (ZnO-based) linear resistance ceramics were prepared by the conventional ceramics method with different amounts of CaO. The impact of CaO doping on the phase composition, microstructures and electrical properties of ZnO-based linear resistance ceramics was investigated in detail. The research showed that CaO doping not only limits the growth of ZnAl2O4 and ZnO grains—in particular, the grain size of ZnAl2O4 decreased from 4.40 μm to 1.38 μm—but also improves the electrical properties. The grain boundary barrier height and nonlinear coefficient decreased with the increase in CaO content, and the resistance temperature coefficient increased from −5.65 × 10−3/°C to 1.04 × 10−3/°C. The 4.00 mol.% CaO-doped ZnO-based linear resistance ceramics were found to possess excellent electrical properties: the resistivity is 44.87 Ω cm, the grain boundary barrier height is 0.011 eV, the nonlinear coefficient is 1.05 and the resistance temperature coefficient is −0.822 × 10−3/°C, a stable resistivity against frequency.

Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors

In present study, impact of CaO doping on microstructure and phase electrical properties of a ZnO varistor is being reported. The doped ZnO nanopowders were prepared by solution combustion followed by conventional sintering. The ZnO particle size of powders decreased from 22 to 17 nm with CaO doping was noted. Decrease in grain size of the sintered samples from 4.54 to 1.19 μm with CaO doping was observed. This is attributed to Ca4Bi6O13 and Ca0.89Bi3.11O5.58 secondary phase formations apart from pyrochlore and spinel phases. A breakdown field as high as 21 kVcm−1 in X = 1.00 sample was obtained. The Coefficient of nonlinearity (α) decreased from 95 to 42 is due to decrease in carrier concentrations. Increase in resistivity with CaO doping and decrease in resistivity beyond X = 2.5 were noted. The electric modulus plots yielded two relaxations at a temperature in the range 100–300 °C and frequency in the range 0.1 Hz-1MHz. Activation energy (Ea) decreased from 0.65 to 0.50 eV for a region I and from 0.89 to 0.75 eV for region II with CaO doping was observed.

Hydrogen Production by Ethanol Steam Reforming on Ni-Based Catalysts: Effect of the Support and of CaO and Au Doping

Nickel-based catalysts were investigated for hydrogen production by ethanol steam reforming. A suitable support was assessed among the metal oxides TiO2, ZrO2 and CeO2. Alongside catalytic testing with ethanol-water mixture, a series of characterization measurements were carried out: N2-physisorption, XRD, TPR, TPO, CO2-TPD, SEM, AAS and IC. Zirconium and cerium oxides proved to be suitable supports, even if both still suffer from coking phenomena. The effects of doping both by calcium oxide and gold nanoparticles were checked. Doping by calcium oxide prevented deactivation and highly increased the hydrogen yield. Moreover, the addition of gold nanoparticles greatly improved the hydrogen yield. The combination of the two dopants resulted in the best performing catalyst: the basic doping by CaO and the promotion of the water gas shift reaction by gold nanoparticles contributed to the highest hydrogen production.

Effects of CaO and La2O3 doping of (Zr0.8Sn0.2)TiO4 ceramics on the densifying behavior and microwave dielectric properties

The effects of CaO–La2O3 additives on sintering temperature and microwave dielectric properties of (Zr0.8Sn0.2)TiO4 (ZST) ceramics prepared by solid-state reaction method were investigated. Structural characterization and microstructure of materials were studied via X-ray diffraction and scanning electron microscopy. crystalline and single phase exhibiting orthorhombic symmetry were identified. Small quantities of CaO and La2O3 promoted the densification of the ceramics improving dielectric properties, otherwise, an excessive CaO and La2O3 amount gave an abnormal grain growth which deteriorated dielectrical parameters. Dense ZST ceramics doped with 0.5% CaO and 1 wt% La2O3 were attained after sintering at 1335 °C during 4h, in air atmosphere. Microwave dielectric parameters ofer = 39.56, Q × f = 44100 GHz (at 5.6 GHz), τf = −1.66 ppm/°C were derived.

Lowering the thermal conductivity of Sr(Ti0.8Nb0.2)O3 by SrO and CaO doping: microstructure and thermoelectric properties

Excess SrO and CaO were added to the Sr(Ti0.8Nb0.2)O3 thermoelectric material, which was structurally compensated by the formation of Ruddlesden–Popper-type planar faults with the compositions SrO and/or (Sr, Ca)O. Both types of doping significantly changed the original isotropic Sr(Ti0.8Nb0.2)O3 microstructure and resulted in the formation of lamellar Ruddlesden–Popper-type phases within the Sr(Ti0.8Nb0.2)O3 grains. Three-dimensional networks of single Ruddlesden–Popper-type faults were also observed in the Sr(Ti0.8Nb0.2)O3 for both types of doping. The combination of both structural features significantly lowered the thermal conductivity in comparison with Sr(Ti0.8Nb0.2)O3 due to the enhanced phonon scattering observed at the planar faults, which proves that introducing such defects is a promising method for lowering the thermal conductivity of the Sr(Ti0.8Nb0.2)O3 thermoelectric material. The highest figure of merit (ZT = 0.08) was achieved with CaO doping, since the significantly reduced thermal conductivity was accompanied by an increased power factor.

Sintering densification of CaO–UO2–Gd2O3 nuclear fuel pellets

CaO-doped UO2-10 wt% Gd2O3 burnable poison fuel was prepared by co-precipitation reaction method. It was found that 0.3 wt% CaO-doping significantly improved the sintered density, grain sizes and crushing strength of UO2–Gd2O3 fuel pellets at the sintering temperature of 1650 °C in the sintering atmosphere of hydrogen for 3.5 h. In addition, homogeneous solid solution without precipitation of free phases of CaO and Gd2O3 was successfully achieved. CaO doping in UO2–Gd2O3 fuel pellet system accelerated the thermally activated material transport, so the onset temperature of densification as well as the temperature of the maximum densification rate shifted to a lower temperature region.

Ab-initio study of metal-zirconia interfaces

A comparative theoretical study of metal-zirconia interfaces with BCC and FCC metals was performed using pseudopotential approach with LDA and GGA approximation for exchange-correlation functional. It was shown that the high adhesion can be achieved at the O-terminated Me/ZrO2(001) interface with BCC metals that is related to large charge transfer from metal film to substrate and increase of an ionic contribution in the chemical bonding. The structural and electronic factors which are responsible for decrease of adhesion at differently oriented metal-zirconia interfaces are discussed. The influence of CaO, MgO and Y2O3 doping on the work of separation (Wsep) at Me(001)/c-ZrO2(001) is analyzed.