Synthesis and Characterization of Tb[N(CN)2]3·2H2O and Eu[N(CN)2]3·2H2O:

Two new rare-earth dicyanamides, namely, Tb[N(CN)2]3·2H2O and Eu[N(CN)2]3·2H2O, have been prepared by ion exchange in aqueous solution, followed by evaporation of the solvent at room temperature. The structures of both compounds have been solved and refined from single-crystal and powder X-ray diffraction data, respectively. The two compounds are isostructural and are built up from irregular quadratic antiprismatic LnN6O2 polyhedra connected to each other by three crystallographically independent dicyanamide ([N(CN)2]3-) ions (Tb[N(CN)2]3·2H2O, P21/n, Z = 4, a = 7.4632(15) Å, b = 11.523(2) Å, c = 13.944(3) Å, β = 94.06(3)°, V = 1196.2(4) Å3; Eu[N(CN)2]3·2H2O, P21/n, Z = 4, a = 7.4780(3) Å, b = 11.5429(5) Å, c = 13.9756(7) Å, β = 93.998(4)°, V = 1203.41(10) Å3). Annealing of the hydrated phases of Ln[N(CN)2]3·2H2O (Ln = Eu, Tb) at 150 °C under an argon atmosphere leads to the formation of nonhydrated Ln[N(CN)2]3 (Ln = Eu, Tb). Both the hydrated (Eu[N(CN)2]3·2H2O) and nonhydrated (Eu[N(CN)2]3) europium(III) dicyanamides show red luminescence due to the dominant intensity of 5D0−7FJ (J = 1, 2, 4) emission lines by excitation at 365 nm. The broad excitation band of europium(III) dicyanamide (fwhm = 8000 cm-1) ranging between 260 and 420 nm with λmax ≈ 30000 cm-1 is ascribed to a Eu−N charge-transfer transition, which is significantly shifted to lower energy compared to that of oxo compounds due to the nephalauxetic effect. Similarly, both the hydrated (Tb[N(CN)2]3·2H2O) and nonhydrated (Tb[N(CN)2]3) terbium(III) dicyanamides show green emission at λexc = 365 nm, arising mainly from the dominant 5D0−7F4 transition. However, unlike europium(III) dicyanamide, the broad excitation band of terbium(III) dicyanamide ranging between 250 and 400 nm with a maximum at 33000 cm-1 can be assigned to the 4f8−4f75d1 transition of Tb3+

Enhancement of Microwave Absorption Properties of Hexaferrite/Epoxy Composites on the Addition of Non-magnetic Oxides

The effect of non-magnetic oxides such as Al2O3, TiO2 and ZnO on the microwave absorption properties of magnetoplumbite barium hexaferrite (BaFe11.8Co0.2O19) is analyzed. Barium hexaferrite nanoparticles are synthesized through the sol-gel auto-combustion method. BaFe11.8Co0.2O19-Al2O3, BaFe11.8Co0.2O19-TiO2 and BaFe11.8Co0.2O19-ZnO composites are synthesized in a 1:1 ratio through mechanical mixing and heat treatment. The epoxy composites are fabricated with 50% loading of BaFe11.8Co0.2O19-Al2O3, BaFe11.8Co0.2O19-TiO2 and BaFe11.8Co0.2O19-ZnO in epoxy matrix followed by room temperature curing. The powder XRD analyses showed homogeneous distribution of BaFe11.8Co0.2O19 and Al2O3 in BaFe11.8Co0.2O19-Al2O3 composite while TiO2 and ZnO phases dominate in BaFe11.8Co0.2O19-TiO2 and BaFe11.8Co0.2O19-ZnO composites, respectively. Scanning electron microscopy shows the evenly distributed BaFe11.8Co0.2O19 and Al2O3 in BaFe11.8Co0.2O19-Al2O3 composites. The electromagnetic characterization calculated from experimental permittivity and permeability shows reflection loss RL ≤ -10 dB (≥ 90% absorption) for a very small thickness of 0.5 mm over the entire X-band (8-12 GHz) for BaFe11.8Co0.2O19-Al2O3 composites. BaFe11.8Co0.2O19-TiO2 and BaFe11.8Co0.2O19-ZnO show RL < - 8 dB with a thickness of 2.5 mm over the frequency range 8–9.7 GHz and RL < - 8 dB with a thickness of 3.6 mm over 8.7-11.1 GHz, respectively. Further, when compared with BaFe11.8Co0.2O19 alone (RL < -7 dB at 3.2 mm in 8-11 GHz), the BaFe11.8Co0.2O19-Al2O3 composite is superior both in terms of the thickness of the coating as well as the percentage absorption in the X-band

Oxide thermoelectric materials: A structure-property relationship

Recent demand for thermoelectric materials for power harvesting from automobile and industrial waste heat requires oxide materials because of their potential advantages over intermetallic alloys in terms of chemical and thermal stability at high temperatures. Achievement of thermoelectric figure of merit equivalent to unity (ZT �1) for transition-metal oxides necessitates a second look at the fundamental theory on the basis of the structure–property relationship giving rise to electron correlation accompanied by spin fluctuation. Promising transition - metal oxides based on wide-bandgap semiconductors, perovskite and layered oxides have been studied as potential candidate n- and p-type materials. This paper reviews the correlation between the crystal structure and thermoelectric properties of transition-metal oxides. The crystal-site-dependent electronic configuration and spin degeneracy to control the thermopower and electron–phonon interaction leading to polaron hopping to control electrical conductivity is discussed. Crystal structure tailoring leading to phonon scattering at interfaces and nanograin domains to achieve low thermal conductivity is also highlighted

Synthesis and Characterization of Tb[N(CN)2]3·2H2O and Eu[N(CN)2]3·2H2O:

Two new rare-earth dicyanamides, namely, Tb[N(CN)2]3·2H2O and Eu[N(CN)2]3·2H2O, have been prepared by ion exchange in aqueous solution, followed by evaporation of the solvent at room temperature. The structures of both compounds have been solved and refined from single-crystal and powder X-ray diffraction data, respectively. The two compounds are isostructural and are built up from irregular quadratic antiprismatic LnN6O2 polyhedra connected to each other by three crystallographically independent dicyanamide ([N(CN)2]3-) ions (Tb[N(CN)2]3·2H2O, P21/n, Z = 4, a = 7.4632(15) Å, b = 11.523(2) Å, c = 13.944(3) Å, β = 94.06(3)°, V = 1196.2(4) Å3; Eu[N(CN)2]3·2H2O, P21/n, Z = 4, a = 7.4780(3) Å, b = 11.5429(5) Å, c = 13.9756(7) Å, β = 93.998(4)°, V = 1203.41(10) Å3). Annealing of the hydrated phases of Ln[N(CN)2]3·2H2O (Ln = Eu, Tb) at 150 °C under an argon atmosphere leads to the formation of nonhydrated Ln[N(CN)2]3 (Ln = Eu, Tb). Both the hydrated (Eu[N(CN)2]3·2H2O) and nonhydrated (Eu[N(CN)2]3) europium(III) dicyanamides show red luminescence due to the dominant intensity of 5D0−7FJ (J = 1, 2, 4) emission lines by excitation at 365 nm. The broad excitation band of europium(III) dicyanamide (fwhm = 8000 cm-1) ranging between 260 and 420 nm with λmax ≈ 30000 cm-1 is ascribed to a Eu−N charge-transfer transition, which is significantly shifted to lower energy compared to that of oxo compounds due to the nephalauxetic effect. Similarly, both the hydrated (Tb[N(CN)2]3·2H2O) and nonhydrated (Tb[N(CN)2]3) terbium(III) dicyanamides show green emission at λexc = 365 nm, arising mainly from the dominant 5D0−7F4 transition. However, unlike europium(III) dicyanamide, the broad excitation band of terbium(III) dicyanamide ranging between 250 and 400 nm with a maximum at 33000 cm-1 can be assigned to the 4f8−4f75d1 transition of Tb3+

Effect of homologue impurity phases on thermoelectric transport properties of heavily doped ZnO

A comprehensive study of polycrystalline samples of dual-doped Zn1-xAlx/2Inx/2O (x = 0.02, 0.04 and 0.06), Zn1-xGax/2Inx/2O (x = 0.02, 0.04 and 0.06) and triple-doped Zn1-xAlx/3Gax/3Inx/3O (x = 0.03, 0.06 and 0.09) systems synthesized through the solid-state reaction is presented in the light of structure-property correlations. Rietveld refinement of powder XRD data confirmed the presence of impurity phases on highly doped compositions (x ≥ 0.4) for Zn1-xAlx/2Inx/2O and Zn1-xAlx/3Gax/3Inx/3O systems and scanning electron microscopy microstructural analyses showed the presence of elongated morphological feature in all the compositions associated with ZnO homologue systems. Raman studies confirmed presence of both impurity phase and ZnO homologue phase. No visible traces of the presence of impurity phase in Zn1−xGax/2Inx/2O causes relatively low electrical resistivity (ρ ~ 4–5 mΩ cm) in this composition. On the other hand, Zn1−xAlx/2Inx/2O and Zn1-xAlx/3Gax/3Inx/3O systems had electrical resistivity in the range of 10–20 mΩ cm that is one order of magnitude higher than Zn1−xGax/2Inx/2O system. This is arising from the presence of the insulating secondary phases in Zn1−xAlx/2Inx/2O and Zn1−xAlx/3Gax/3Inx/3O systems. Contrary to electrical resistivity, thermal conductivity of Zn1−xAlx/2Inx/2O and Zn1-xAlx/3Gax/3Inx/3O (6–8 Wm−1K−1) systems is one order of magnitude lesser than Zn1-xGax/2Inx/2O (12 Wm−1K−1) systems. The impurity phase present causes phonon–phonon and phonon-interface scattering in Zn1−xAlx/2Inx/2O and Zn1-xAlx/3Gax/3Inx/3O systems which in turn stands beneficial in reducing the total thermal conductivities of the system. Therefore, chemical doping acts as an important parameter for controlling the interdependent electrical and thermal transport properties in ZnO system resulting in relatively superior thermoelectric (TE) performance in Zn0.94Ga0.03In0.03O system. Further, lowering of electrical resistivity and thermal conductivity through doping in Zn0.94Ga0.03In0.03O system causes four times improvement of TE performance in comparison with un-doped ZnO

Role of interface states associated with transitional nanophase precipitates in the photoluminescence enhancement of SrTiO3 : Pr3+,Al3+

SrTiO3:Pr3+,Al3+ phosphor samples with varying ratios of Sr/Ti/Al were prepared by the gel-carbonate method and the mechanism of enhancement of the red photoluminescence intensity therein was investigated. The photoluminescence (PL) spectra of SrTiO3:Pr3+ show both D-1(2) --> H-3(4) and P-3(0) --> H-3(4) emission in the red and blue spectral regions, respectively, with comparable intensity. The emission intensity of D-1(2) --> H-3(4) is drastically enhanced by the incorporation of Al3+ and excess Ti4+ in the compositional range Sr(Ti,Al-y)(O3+3y/2):Pr3+ (0.2 less than or equal to y less than or equal to 0.4) and SrTi1+xAlyO3+z:Pr3+ (0.2 less than or equal to x less than or equal to 0.5; 0.05 less than or equal to y less than or equal to 0.1; z = 2x + 3y/2) with the complete disappearance of the blue band. This cannot be explained by the simple point defect model as the EPR studies do not show any evidence for the presence of electron or hole centers. TEM investigations show the presence of exsolved nanophases of SrAl12O19 and/or TiO2 in the grain boundary region as well as grain interiors as lamellae which, in turn, form the solid-state defects, namely, dislocation networks, stacking faults and crystallographic shear planes whereby the framework of corner shared TiO6 octehedra changes over to edge-sharing TiO5-AlO5 strands as indicated from the Al-27 MAS NMR studies. The presence of transitional nanophases and the associated defects modify the excitation-emission processes by way of formation of electronic sub-levels at 3.40 and 4.43 eV, leading to magnetic-dipole related red emission with enhanced intensity. This is evidenced by the fact that SrAl12O19:Pr3+,Ti4+ shows bright red emission whereas SrAl12O19:Pr3+ does not show red photoluminescence

Role of B2O3 on the phase stability and long phosphorescence of SrAl2O4 : Eu, Dy

The role of B2O3 addition on the long phosphorescence of SrAl2O4:Eu2+, Dy3+ has been investigated. B2O3 is just not an inert high temperature solvent (flux) to accelerate grain growth, according to SEM results. B2O3 has a substitutional effect, even at low concentrations. by way of incorporation of BO4 in the corner-shared AlO4 framework of the distorted 'stuffed' tridymite structure of SrAl2O4. which is discernible from the IR and solid-state MAS NMR spectral data. With increasing concentrations, B2O3 reacts with SrAl2O4 to form Sr4Al4O25 together with Sr-borate (SrB2O4) as the glassy phase, as evidenced by XRD and SEM studies. At high B2O3 contents, Sr4Al14O25 converts to SrAl2B2O7 (cubic and hexagonal), SrAl12O19 and Sr-borate (SrB4O7) glass. Sr4Al14O25:Eu2+, Dy3+ has also been independently synthesized to realize the blue emitting (lambda(em)approximate to490 nm) phosphor. The afterglow decay as well as thermoluminescence studies reveal that Sr4Al14O25:Eu, Dy exhibits equally long phosphorescence as that of SrAl2O4:Eu2+, Dy3+. In both cases, long phosphorescence is noticed only when BO4 is present along with Dy3+ and Eu2+. Here Dy3+ because of its higher charge density than Eu2+ prefers to occupy the Sr sites in the neighbourhood of BO4, as the effective charge on borate is more negative than that of AlO4. Thus. Dy3+ forms a substitutional defect complex with borate and acts as an acceptor-type defect center. These defects Eu2+ ions and the subsequent thermal release of hole at room temperature followed by the trap the hole generated by the excitation of recombination with electron resulting in the long persistent phosphorescence. (C) 2003 Elsevier Science B.V. All rights reserved

Transport properties of p-type Ca3-xLnxCo4O9-Ag (Ln = Lu, Yb; 0.1≤x≤0.2) oxides

Thermoelectric transport properties of p-type Ca3-xLnxCo4O9/yAg oxides (Ln = Lu & Yb; 0.1 ≤ x ≤ 0.2; 0.05 ≤ y ≤ 0.1) synthesized by sol-gel methodology were investigated in this paper. The structural analyses (SEM, XRD and TEM) confirmed the presence of two phases, viz, Ca3xLnxCo4O9 and Ag-metallic phases. The contribution of rare earth doping in one hand and presence of Ag as secondary phase on the other hand were studied. The resistivity measurements indicated the reduction of electrical resistance at the grain boundary leading to an overall decrease in electrical resistivity with increasing Ag-concentration. The enhancement of Seebeck coefficient is attributed to the substitution of Ln3+ at Ca2+ sites that in turn reduces hole concentration through formation Co3+ for charge concentration counter balance in Ca3-xLnxCo4O9/yAg matrix. The tuning of electrical transport properties through Ca3-xLnxCo4O9 and Ag-metallic bi-phasic formation resulted high power factor of 582 μW m−1 K−2 for Ca2.8Ln0.2Co4O9/0.05Ag and 548 μW m−1 K−2 for Ca2.8Yb0.2Co4O9/0.05Ag at 950 K highlighting its potential application on small scale energy harvesting to power sensor and wireless sensor network where requirement of power is in the milliwatt range

High temperature transport properties of co-substituted Ca1−xLnxMn1−xNbxO3 (Ln = Yb, Lu; 0.02 ≤ x ≤ 0.08)

The co-substituted Ca1�xLnxMn1�xNbxO3 (Ln = Yb, Lu; 0.02 � x � 0.08) are synthesized by solid-state reaction and the electronic transport properties are investigated. Rietveld refinement confirms the formation of single phase orthorhombic structure with gradual increase of cell parameters with doping level. The electronic transport properties such as Seebeck coefficient and electrical resistivity decrease with increasing the dopant concentration for both the co-substituted compositions. All the compositions of Ca1�xLnxMn1�xNbxO3 show nonmetal-like temperature dependence of resistivity; whereas metal-like temperature dependence of thermopower. This inconsistency is explained by the formation of oxygen vacancy associated defect centres that originates from partial reduction of Mn4+ to Mn3+ due to cosubstitution. The defect centres act as extrinsic carriers and cause additional contribution to the entropy of the system, leading to increase of Seebeck coefficient as a function of temperature. The transport mechanism of charge carriers is explained in the framework of Mott’s small polaron hopping mechanism

Effect of interface states associated with transitional nanophase precipitates in the enhancement of red emission from SrAl12O19:Pr3+SrAl_{12}O_{19}:Pr^{3+} by Ti4+Ti^{4+} incorporation

$SrAl_{12}O_{19}:Pr^{3+}, Ti^{4+}$ phosphor suitable for field emission displays is prepared by the wet chemical gel-carbonate method and the mechanism of enhancement in red photoluminescence (PL) intensity with $Ti^{4+}$ therein has been investigated. The PL spectra of $Pr^{3+}$ show both $^1D_2-^3H_4$ and $^3P_0-^3H_6$ emission in the red region with very weak intensity when excited at 355 nm. The emission intensity has increased by about 100 times at room temperature in the compositional range $SrAl_{12-x}Ti_xO_{19+x/2}:Pr^{3+}$, with $0.1 \leq x \leq 0.3$ in comparison to Ti-free $SrAl_{12}O_{19}:Pr^{3+}$. TEM investigations show the presence of exsolved nanophase of $SrAl_8Ti_3O_{19}$, the precipitation of which is preceded by the presence of defect centers at the interfacial regions between the semicoherent transient phase and the parent $SrAl_{12}O_{19}$ matrix. The presence of transitional nanophase and the associated defects modify the excitation–emission process by way of formation of electronic sub-levels at lower energy (3.5 eV) than the band gap of $SrAl_{12}O_{19} (\sim 7 eV)$ followed by non-resonance energy transfer to $Pr^{3+}$ level, leading to magnetic-dipole related red emission with enhanced intensity. The PL intensity of $Pr^{3+}$ decreases at high $Ti^{4+}$ concentrations $(x>0.3)$ due to higher extent of segregation of non- emissive $SrAl_8Ti_3O_{19}:Pr^{3+}$ phase