Ca-doped Samples Research Articles

The spontaneous exchange bias effect in La2−xCaxCoMnO6 series

Structural, electronic and magnetic properties of polycrystalline La2-xCaxCoMnO6 (0 $\leq$ x $\leq$ 0.75) compounds are investigated by x-ray diffraction and magnetometry. All the samples have an orthorhombic structure and show a slight decrease in the unit cell with Ca-doping. Temperature-dependent magnetization measurements reveal a complex magnetic behavior with two ferromagnetic transitions. These transitions are ascribed to Co2+--Mn4+ and Co3+--Mn3+ couplings and suggest the presence of additional antiferromagnetic couplings in these disordered compounds. Field-dependent magnetization curves, measured after cooling the samples in a zero external magnetic field, reveal the spontaneous exchange bias (SEB) effect for the Ca-doped samples. The strengthening of the uncompensated magnetic coupling at the interfaces, caused by the increased antiferromagnetic phase, explains the increase of SEB with increasing the Ca-content.

Bandgap engineering of NiO, and enhanced photocatalytic performance by Ca doping

Wide bandgap NiO is vital for the photodegradation of organic molecules like dyes, bacteria, viruses, and for water decontamination. Calcium oxide is another efficient catalyst due to the presence of greater active sites. In this study, CaO molecules were incorporated into the NiO matrix by calcium doping. It was assumed that the incorporation of CaO would enhance the photocatalytic performance of NiO. Pure and calcium substituted NiO samples were prepared by simple co-precipitation method. The samples were characterized by X-ray diffractometer, SEM, UV–visible, and FTIR spectrometer. XRD profiles show the successful formation of NiO and Ca-doped NiO cubic nano crystallites. UV–visible spectroscopy revealed a shift in absorbance peaks toward longer wavelengths and decreased bandgap in Ca-doped samples. Photodegradation study revealed superior photocatalytic performance of doped samples under similar conditions. The improved photocatalytic performance of Ca-doped NiO is attributed to greater active sites and decreased bandgap due to the formation of new energy levels by the impurity addition. This study demonstrates that calcium doping in nickel oxide has enhanced the photodegradation efficiency from 45 % to 98.15 %.

New insights into the formation mechanism of zircon in a ZrO2–SiO2 nanocrystalline glass-ceramic: A TEM study

We have previously observed that doping of Ca ions was beneficial to the formation of zircon (ZrSiO4). It is well known that synthetic ZrSiO4 is typically formed via a solid-state reaction between ZrO2 and SiO2, in which the interfaces between the reactant and resultant play an important role. However, the interfaces are lacking detailed microstructural observation. This follow-up study aims at exploring the formation mechanism of ZrSiO4 by inspecting the interfaces at the nano and atomic scales with TEM techniques. Results demonstrated that ZrSiO4 was formed in the Ca-doped sample after sintering at 1200 °C, whereas, no ZrSiO4 was formed in the undoped sample even after sintering at 1230 °C. The Ca-doped sample consisted of a continuous ZrSiO4 matrix with dispersed ZrO2 nanocrystallites. Doping of Ca ions promoted the formation of ZrSiO4 by causing lattice distortion and oxygen vacancies in ZrO2 lattices. Thin amorphous grain boundary complexions were found between ZrO2 nanocrystallites and between ZrO2 and ZrSiO4 crystallites. These amorphous complexions acted as reaction sites and an intermediate metastable state for the solid-state reaction. A detailed formation mechanism of ZrSiO4 at the nanometer scale and atomic scale has been proposed.

Growth and characterization of large (Y,La)TiO3 and (Y,Ca)TiO3 single crystals

The Mott-insulating rare-earth titanates ($R{\mathrm{TiO}}_{3}$, with $R$ being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of ${\mathrm{Ti}}^{3+}$ to oxidize to ${\mathrm{Ti}}^{4+}$. We describe our efforts to grow sizable single crystals of ${\mathrm{YTiO}}_{3}, {\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ ($x\ensuremath{\le}0.25$), and ${\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y}{\mathrm{TiO}}_{3}$ ($y\ensuremath{\le}0.35$) with the optical traveling-solvent floating-zone technique. We present sample characterization via chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO.

Elucidating the Effect of the Dopant Ionic Radius on the Structure and Electrochemical Performance of Ni-Rich Layered Oxides for Lithium-Ion Batteries.

The merits of Ni-rich layered oxide cathodes in specific capacity and material cost accelerate their practical applications in electric vehicles and grid energy storage. However, detrimental structural deterioration occurs inevitably during long-term cycling, leading to potential instability and capacity decay of the cathodes. In this work, we investigate the effect of the doped cation radius on the electrochemical performance and structural stability of Ni-rich cathode materials by doping with Mg and Ca ions in LiNi0.8Co0.1Mn0.1O2. The results reveal that an increase in the doping ion radius can enlarge the interlayer spacing but lead to the collapse of the layered structure if the ion radius is too large, which undermines the cycling stability of the cathode material. Compared with the Ca-doped sample and the pristine material, Mg-doped LiNi0.8Co0.1Mn0.1O2 presents improved structural stability and superior thermal stability due to the pillar and glue roles of medium-sized Mg ions in the lithium layer. The results of this study suggest that a suitable ionic radius of the dopant is critical for stabilizing the structure and improving the electrochemical properties of Ni-rich layered oxide cathode materials.

Poisoning Effects of Alkali and Alkaline Earth Metal Doping on Selective Catalytic Reduction of NO with NH3 over the Nb-Ce/Zr-PILC Catalysts

The poisoning effects of alkali metals (K and Na) and alkaline earth metals (Ca and Mg) on catalytic performance of the 2Nb4Ce/Zr-PILC catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated, and physicochemical properties of the catalysts were characterized by means of the X-ray diffraction XRD (XRD), Brunner−Emmet−Teller (BET), hydrogen temperature-programmed reduction (H2-TPR), X-ray Photoelectron Spectroscopy (XPS), ammonia temperature-programmed desorption (NH3-TPD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) techniques. Doping of M (M = K, Na, Ca, and Mg) deactivated the 2Nb4Ce/Zr-PILC catalyst according to the sequence of 0.8 K > 0.8 Na > 0.8 Ca > 0.8 Mg (M/Ce molar ratio = 0.8). The characterization results showed that the decreases in redox ability, NH3 adsorption, Ce3+/Ce4+ atomic ratio, and amount of the chemisorbed oxygen (Oβ) were the important factors influencing catalytic activities of the alkali metal-and alkaline earth metal-doped samples. Consequently, compared with the Mg- and Ca-doped samples, doping of K caused the 2Nb4Ce/Zr-PILC sample to possess the lowest redox ability, NH3 adsorption, and amount of the Oβ species, which resulted in an obvious deactivation effect.

Effect of calcium or yttrium doping on cation ordering and electrochemical performance of Li(Ni0.80−xCo0.15Al0.05Mx)O2 (M = Ca, Y) as a Li-ion battery cathode

The electrochemical performance of Li(Ni0.80Co0.15Al0.05)O2 (NCA) cathode was demonstrated to be improved by calcium and yttrium doping. Samples were synthesized by ultrasonic sound-assisted co-precipitation method which satisfied precipitation of nano-sized particles around 20 nm. Doping elements were introduced to the structure by solid-state reaction route and single phase, layered Li0.993(Ni0.804Co0.154Al0.035Y0.014)O2±σ and Li0.991(Ni0.801Co0.152Al0.037Ca0.019)O2±σ samples were successfully synthesized. Lattice parameters and site occupancy of the atoms were calculated by Rietveld refinement. Occupancy refinement of the atomic sites confirmed that doping elements occupied the transition metal layer as it was aimed. The presence of doping elements in the transition metal layer stabilized nickel and decreased the amount of cation mixing. Y-doped NCA sample exhibited the highest initial specific discharge capacity (157 mAh g−1) and Ca-doped sample showed the highest rate capability. Pristine NCA showed the lowest capacity, in any case, indicating the necessity of further treatment to achieve improvement in NCA cathode.

In-depth distribution of elements and chemical bonds in the surface region of calcium-doped diamond-like carbon films

In the present work we determined surface composition and chemical bonds at the top surface and in a shallow sub-surface region of the calcium-doped diamond-like carbon (DLC) layers using angular-resolved photoelectron spectroscopy (ARXPS) aided by the Maximum Entropy Method (MEM). The resulting non-destructive depth profiles showed that the composition and the chemical bonding vary significantly on the way from the top surface to the shallow sub-surface region. We demonstrate, for the first time, that Ca doping induced (i) the accumulation of the calcium carbonate, CaCO3, at the surface, while the calcium oxide, CaO, is locatedin a deeper sub-surface region, (ii) structural conversion from the C sp2-rich top surface observed in the undoped DLC layer to the C sp3-rich top surface of the Ca-doped DLC samples, and (iii) conversion in the opposite direction which occurred in a sub-surface region. Our conclusions are consistent with Raman and Ion Surface Scattering spectra. Obtained results can be useful for deeper understanding the interaction between doped DLC layer surfaces with surroundings and particularly with living tissue.

Chemical pressure induced near-complete suppression of spin-wave excitations in Bi0.9A0.1FeO2.95 (A = Ba, Ca)

We have probed the changes in the phonon and magnon modes of BiFeO3 on substitution of Ba and Ca at the A-site using infrared reflectance spectroscopy. The systematic hardening of the phonon modes in the doped samples is seen to correlate well with the volume reduction of the unit cell observed from the Rietveld measurements. This is attributed to the generation of chemical pressure in these samples on account of substitution of the dopants. In addition, a near-complete suppression accompanied by a complete lifting of the degeneracy of the magnon modes as compared to the pristine BiFeO3 is detected for the Ca-doped samples. These results suggest that aliovalent doping at A-site affects the crystal field of FeO6 octahedra of BiFeO3, which eventually modifies its magnetic nature. These doping-induced modifications are mainly responsible for the improved magnetic and magnetoelectric properties of the Ca-doped BiFeO3 samples.

Transparent and conductive CdS:Ca thin films for optoelectronic applications

This paper presents the structural, morphological, optical and electrical evolution of Ca-doped CdS thin films. Non-doped and Ca-doped CdS samples with various amounts of Ca atoms (from 0 to 10 at.% with an increasing step of 2 at.%) were grown by spray pyrolysis route on glass slides. The structural investigation by X-ray diffraction showed that Ca-doping distorted CdS structure until 8 at.% Ca-doping and then a slight improvement in the intensity of (101) peak was obtained for 10 at.% Ca-doping compared to the other Ca-doping samples. Morphological analysis displayed a grain growth for a low amount of Ca-doping whereas higher concentration of Ca-doping led to a reduction in the grain size of CdS thin films. More stoichiometric CdS specimens were obtained after various amounts of Ca-doping according to energy dispersive X-ray spectroscopy data. Transparency of the CdS samples enhanced remarkably with the incorporation of Ca atoms in CdS with a particular concentration of 10 at.%. Tauc’s plot investigation illustrated that the bandgap score of samples changed from 2.54 eV for non-doped CdS to 2.48 eV for 4 at.% Ca-doped CdS thin films. Further increase of Ca-doping doesn’t vary the bandwidth of CdS samples. Photoluminescence data indicated that Ca-doped CdS thin films had lower intrinsic defects compared with non-doped CdS one. The electrical examination demonstrated that the carrier density of CdS thin films increased till 6 at.% Ca-doping and then decreased further increase of Ca-doping. However, resistivity values exhibited the opposite behavior accordingly. In conclusion, it can be pronounced that 6 at.% Ca-doped CdS thin films are the optimum specimen to be used as an effective transparent and conductive material in the optoelectronic devices.

Improved ferromagnetism and magnetodielectric properties of relaxor ferroelectric Ba4-Ca Nd2Fe1.5Co0.5Nb8O30 ceramics with tungsten bronze structure

Abstract The structural, magnetic, dielectric and magneto-dielectric properties of tungsten bronze Ba4-xCaxNd2Fe1.5Co0.5Nb8O30 (0 ≤ x ≤ 0.9) were investigated. All the samples can be indexed with the tetragonal structure and the space group of P4bm. The incorporation of Ca ions improves the structural distortion and promotes the grain’s growth. The magnetic curves at 300 K indicate the superposition of ferromagnetic (FM) and antiferomagnetic (AFM) state in all the samples, the observed FM phase can be attributed to the spin canting of AFM coupling of Fe- and Co-based sublattices. The Ca-doped samples exhibit an improved FM state due to large lattice distortion. The dielectric constant in anomaly region obeys well with Curie-Weiss law with γ = ∼1.6, implying that all the samples are relaxor ferroelectrics. The dielectric loss relaxations obey with Vogel-Fulcher relation and can be ascribed to the local polarization fluctuation induced by chemical heterogeneities. In addition, the relative change in magnetic field dependent dielectric constant follows with the change in the square of magnetization M2, suggesting that the observed magneto-dielectric is caused by the intrinsic magnetoelectric (ME) coupling effect. These results manifest that tungsten bronze compounds may be the potential candidates for the application in ME devices.

Pb-doped p -type Bi2Se3 thin films via interfacial engineering

Due to high density of native defects, the prototypical topological insulator (TI), Bi$_2$Se$_3$, is naturally n-type. Although Bi$_2$Se$_3$ can be converted into p-type by substituting 2+ ions for Bi, only light elements such as Ca have been so far effective as the compensation dopant. Considering that strong spin-orbit coupling (SOC) is essential for the topological surface states, a light element is undesirable as a dopant, because it weakens the strength of SOC. In this sense, Pb, which is the heaviest 2+ ion, located right next to Bi in the periodic table, is the most ideal p-type dopant for Bi$_2$Se$_3$. However, Pb-doping has so far failed to achieve p-type Bi$_2$Se$_3$ not only in thin films but also in bulk crystals. Here, by utilizing an interface engineering scheme, we have achieved the first Pb-doped p-type Bi$_2$Se$_3$ thin films. Furthermore, at heavy Pb-doping, the mobility turns out to be substantially higher than that of Ca-doped samples, indicating that Pb is a less disruptive dopant than Ca. With this SOC-preserving counter-doping scheme, it is now possible to fabricate Bi$_2$Se$_3$ samples with tunable Fermi levels without compromising their topological properties.

Synthesis and Characterization of Ca-doped Li4Ti5O12 Using CaCO3 from Chicken Eggshell as a Dopant for Lithium-Ion Battery Anode Material

In order to improve the performance of Li4Ti5O12 (LTO) anode, this research was focused on Ca2+ion doping as a substitute to Li+ ion to form Li4-xCaxTi5O12with values of x=0, 0.05, 0.075, and 0.125 using solid-state reaction. The Ca2+ion source was CaCO3which synthesized from the chicken eggshell. The LTO was prepared by a solid-state method using TiO2 Degussa. The pristine LTO and Ca-doped LTO sample powder was characterized by XRD, SEM, and were also tested its electrochemical performance by EIS, CV and CD. The CaCO3 dopant characterization results showed CaCO3 in calcite polymorph as the main phase, with agglomerated fine particulate morphology and high purity. Characterization of LTO sample powder with XRD revealed that dopant Ca successfully enter the structure of LTO spinel, with maximum addition level x=0.05, which excessive addition led to CaTiO3 impurity forming. SEM result showed all Ca-doped LTO have almost similar morphology, which was agglomerated particulate. Ca-doped LTO samples have smaller particle size compared to pristine LTO. Electronic conductivity improvement was spotted at all of Ca-doped LTO sample, with Li3.75Ca0.125Ti5O12 (LCaTO-3) showed the lowest charge transfer resistance of 29.5 Ω. Li3.75Ca0.125Ti5O12(LCaTO-3) also had the highest initial discharge capacity of 168.2 mAh/g. Nevertheless, in high rate application, the best performance was showed by Li3.85Ca0.075Ti5O12(LCaTO-2) with the discharge capacity of 30.2 mAh/g at 12 C, which capacity retention percentage of 21.43% compared to its discharge capacity at 0.2 C.

Магнитные свойства нанокристаллов Bi-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-FeO-=SUB=-3-delta-=/SUB=-

Nanocrystalline magnetically ordered materials based on Ca-doped bismuth orthoferrite were synthesized by glycine-nitrate combustion method. The samples were characterized by X-ray diffractometry, helium pycnometry, Brunauer–Emmett–Teller method, scanning electron microscopy and elemental energy dispersive microanalysis. It was shown that all samples were isostructural to bismuth orthoferrite. The particles of nanopowders (with average particle size of about 0.3 m) were composed of nanocrystals (with average crystallite size of about 40 nm). The difference between magnetic properties of Ca-doped samples and pure bismuth orthoferrite were demonstrated by Mössbauer spectroscopy and magnetometry. It was found that the samples contain nanocrystals with modified magnetic parameters and a small amount of fine fraction of other phases with a magnetic order.

Oxygen ion conducting pyrochlore oxides prepared by an ultrasound-assisted wet chemistry route: Ca-doped Gd2Ti2O7 nanocrystals

Abstract Different compositions of pyrochlore-type oxides of general formula Gd2-xCaxTi2O7 (x = 0, 0.05 and 0.1), interesting solid oxide ion conducting materials, were prepared by a novel wet-chemistry method assisted by ultrasound. Titanium isopropoxide (TTIP), and Gd3+ and Ca2+ hydrated nitrates were used as metal sources; whereas, ethanol and ethylene glycol were used as solvents. The results corroborate the feasibility of this method to obtain nanometric particles of lanthanide titanates. Electrical properties of uniaxially pressed and sintered pellets (1200 and 1500 °C) were measured as a function of temperature (280–700 °C) and frequency (100 Hz-1MHz) by using impedance spectroscopy. The Analysis of electrical properties revealed an increasing of dc conductivity (σ) for higher contents of Ca2+ in the system, reaching a maximum of 3.55 × 10−3 S cm−1 at 700 °C for x = 0.1. Calculated activation energies for oxygen diffusion in the Ca-doped samples, are in the 0.68–0.89 eV range, well below that of pristine Gd2Ti2O7 (2.23 eV), which explains their enhanced conductivity. Increasing of dc conductivity is promoted by substitution of Gd3+ by Ca2+ in these titanates, generating oxygen vacancies in the structure, which facilitates the ion migration process.

Enhanced electrochemical performance of Ca-doped NdBa1-xCaxCoCuO5+δ as cathode material for intermediate-temperature solid oxide fuel cells

Exploring advanced cathode materials that have high electrochemical activity and superior thermal compatibility with other components is of great importance for the application of intermediate-temperature solid oxide fuel cells. In the present study, influences of Ca doping on electrochemical and physicochemical properties of NdBa1-xCaxCoCuO5+δ were studied systematically. A relatively low thermal expansion coefficient (16.8 × 10−6 °C−1) was obtained for the Ca-doped sample over the temperature range of 25–800 °C. Moreover, Ca doping significantly enhanced the electrochemical performance of NdBa1-xCaxCoCuO5+δ. The highest densities of power for NdBa1-xCaxCoCuO5+δ-based single cells at the temperatures of 800 °C increased from 1.42 W cm−2 for x = 0.0 to 1.84 W cm−2 for x = 0.3. This increase in electrochemical performance of NdBa1-xCaxCoCuO5+δ cathode with Ca doping can be ascribed to enhanced concentrations of oxygen vacancies and higher electrical conductivity.

Fabrication of Calcium Doped PlSZT Ceramics using High Planetary Mill Method

Ca-doped modified Lead Lanthanum Strontium Zirconium Titanate (PLSZT) ceramic, was synthesized with different concentrations of dopant in order to enhance the properties of PZT. Calcium carbonate, CaCO3 have been used as a source of Ca dopant in the preparation of PCLSZT using high planetary ball mill, palletized and sintered at 1200 °C for 3 hours. The uses of high planetary milling benefit in shorten the time for powder preparation where the calcination was not required. The concentration of CaCO3 was varied from 0.01, 0.03, 0.05 and 0.10 mol% and their effects on phase formation, surface morphology, density and porosity have been studied using X-ray diffraction (XRD) and Scanning electron microscopy (SEM). XRD results showed that all Ca-doped PLSZT samples present a tetragonal phase. Whereas SEM results showed that the grain size of the samples was increased with increasing calcium concentration. In addition, the density was observed to be decreased with increasing amount of Ca concentrations.

Effect of Calcium Concentration on the Microstructure and Electrical Properties of Ca-Doped PLSZT (Pb-La-Sr-Zr-Ti) System

Lanthanum and strontium modified Lead Zirconate Titanate (PLSZT) was doped with Ca2+ to improve the dielectric properties. Ca-doped PLSZT was prepared using high planetary ball mill, palletized and sintered at 1200 °C for 3 hours with heating rate 5 °C per minute. XRD results showed that all Ca-doped PLSZT samples consist of tetragonal and rhombohedral phase whereas SEM results showed that the grain size of the samples was decreased with an increase in calcium concentration but began to decrease at 8 mol% of calcium. The density was observed to decrease and the dielectric constant increased with the increase in concentration of calcium.

Unique magnetic properties of perovskite manganites La0.95T0.05CrxMn1−xO3 (T = Ca, Sr)

Powder samples of La0.95T0.05CrxMn1−xO3 (T = Ca or Sr, 0.00 ≤ x ≤ 0.30), two series of ABO3 perovskite manganites, were synthesized using the sol–gel method. X-ray diffraction analysis showed that all the samples had a single ABO3 perovskite phase with space group R3‾c. Unique magnetic properties were obtained for the oxides: (i) the Curie temperature of each series of samples first increased and then decreased with increasing Cr content (x); (ii) the average molecular magnetic moment reached the maximum value when x = 0.08 for the Ca-doped samples, but it monotonically decreased with increasing x for the Sr-doped samples. This behavior was explained using an O 2p itinerant-electron model for magnetic oxides.

Scintillation and dosimeter properties of Ca-doped MgO transparent ceramics

We have synthesized 0.001, 0.01 and 0.1% Ca-doped MgO transparent ceramics by the Spark Plasma Sintering (SPS) method, and then we investigated the optical, scintillation and dosimeter properties. In scintillation spectra, some emission bands appeared around 340, 400, 440, 600 and 750 nm. After X-ray irradiation, all the Ca-doped samples showed TSL with a glow peak ∼140 °C, and the 0.01% Ca-doped sample showed an additional broad peak ∼250 °C. The TSL spectrum consists of a broad peak over 300–500 nm and some peaks around 600 and 750 nm. The TSL response was confirmed to linearly increase with irradiation dose over a dose range from 0.1 to 100 mGy.