Seed Recalcitrance:a Current Assessment

Seeds have been categorized as orthodox, recalcitrant and intermediate seeds according to their dehydration behaviors. Identification of desiccation-tolerance and -sensitivity of seeds is the basis making storage strategy of seeds and long-term conservation of species gene resources. In addition to the inherent characteristics of the species, developmental status of the seeds, dehydration rate, and the conditions under which they are dried and subsequently re-imbibed are very important factors influencing desiccation tolerance of seeds. Survival, electrolyte leakage rate, and germination/growth rate produced by survived seeds are a excellent synthetic parameter when discussing desiccation tolerance of seeds. Desiccation tolerance of seeds is a quantitative feature. The term 'critical water content' is incorrect and has caused some confusion in assessment of seed recalcitrance. A new working approach to quantify the degree of seed recalcitrance has been proposed in this paper.

The Effect of Replacement of Zn 2+ Cation with Ni 2+ Cation on the Structural Properties of Ba sub>2Zn sub>1–x Ni sub>xWO sub>6 Double Perovskite Oxides (X = 0, 0.25, 0.50, 0.75, 1)

The Ba2Zn1-xNixWO6 double perovskite oxides were synthesized using solid state reaction method. The effect of replacement of Zn2+ with Ni2+ cation on the structural properties was investigated by X-ray diffraction (XRD) at room temperature. From the X-ray diffraction and by means of standard Rietiveld method, the samples showed the same cubic crystal structure with (Fm-3m) space group and the crystallite size ranging from 71.91 nm to 148.71 nm. The unit cell volume was found to decrease as a result of the replacement, while there was no significant difference in the value of tolerance factor of the samples. This is may be due to the convergence of ionic radii of Ni2+ and Zn2+ cations. The Fourier Transform Infrared Spectroscopy (FTIR) was performed for the samples and the resultant characteristic absorption bands confirmed the double perovskite structure.

Enhanced energy storage properties and antiferroelectric stability of Mn-doped NaNbO_(3)-CaHfO_(3) lead-free ceramics:Regulating phase structure and tolerance factor

NaNbO_(3)-based ceramics usually show ferroelectric-like P-E loops at room temperature due to the irreversible transformation of the antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase,which is not conducive to energy storage applications.Our previous work found that incorporating CaHfO_(3) into NaNbO_(3) can stabilize its antiferroelectric phase by reducing the tolerance factor(t),as indicated by the appearance of characteristic double P-E loops.Furthermore,a small amount of MnO_(2) addition effectively regulate the phase structure and tolerance factor of 0.94NaNbO_(3)-0.06CaHfO_(3)(0.94NN-0.06CH),which can further improve the stability of antiferroelectricity.The XRD and XPS results reveal that the Mn ions preferentially replace A-sites and then B-sites as increasing MnO_(2).The antiferroelectric orthorhombic phase first increases and then decreases,while the t shows the reversed trend,thus an enhanced antiferroelectricity and the energy storage density Wrec of 1.69 J/cm^(3) at 240 kV/cm are obtained for 0.94NN-0.06CH-0.5%MnO_(2)(in mass fraction).With the increase of Mn content to 1.0%from 0.5%,the efficiency increases to 81% from 45%,although the energy storage density decreases to 1.31 J/cm^(3) due to both increased tolerance factor and non-polar phase.

Relationship between tolerance factor and temperature coefficient of permittivity of temperature-stable high permittivity BaTiO_(3)–Bi(Me)O_(3) compounds

The temperature coefficient of permittivity(TCε)of BaTiO_(3)–Bi(Me)O_(3) solid solutions were investigated.It was determined that as the tolerance factor was decreased with the addition of Bi(Me)O_(3),the TCε shifted from large negative values to TCε values approaching zero.It is proposed that the different bonding nature of the dopant cation affects the magnitude and temperature stability of the permittivity.This study suggests that the relationship between tolerance factor and TCε can be used as a guide to design new dielectric compounds exhibiting temperature-stable high permittivity characteristics,which is similar to past research on perovskite and pyrochlore-based microwave dielectrics.

Realizing simultaneously excellent energy storage and discharge properties in AgNbO_(3)based antiferroelectric ceramics via La^(3t)and Ta^(5+)co-substitution strategy

AgNbO_(3)based antiferroelectric(AFE)ceramics have large maximum polarization and low remanent polarization,and thus are important candidates for fabricating dielectric capacitors.However,their energy storage performances have been still large difference with those of lead-based AFEs because of their room-temperature ferrielectric(FIE)behavior.In this study,novel La^(3+)and Ta^(5+)co-substituted AgNbO_(3)ceramics are designed and developed.The introduction of La^(3+)and Ta^(5+)decreases the tolerance factor,reduces the polarizability of B-site cations and increases local structure heterogeneity of AgNbO_(3),which enhance AFE phase stability and refine polarization-electric field(PeE)loops.Besides,adding La^(3+)and Ta^(5+)into AgNbO_(3)ceramics causes the decrease of the grain sizes and the increase of the band gap,which contribute to increased Eb.As a consequence,a high recoverable energy density of 6.79 J/cm3 and large efficiency of 82.1%,which exceed those of many recently reported AgNbO_(3)based ceramics in terms of overall energy storage properties,are obtained in(Ag0.88La0.04)(Nb0.96Ta0.04)O_(3)ceramics.Furthermore,the discharge properties of the ceramic with discharge time of 16 ns and power density of 145.03 MW/cm3 outperform those of many lead-free dielectric ceramics.

Crystal structure and dielectric properties of layered perovskite-like solid solutions Bi_(3-x)Gd_(x)TiNbO_(9)(x=0,0.1,0.2,0.3)with high Curie temperature

The Aurivillius phases(APs)[Bi_(2)O_(2)]2t[A_(m-1)B_(m)O_(3m+1)]2-are well-known ferroelectrics with high Curie temperatures.Hightemperature piezoceramics Bi_(3-x)Gd_(x)TiNbO_(9)(BiGdTiNb,x=0,0.1,0.2,0.3)were prepared by a solid-state reaction method.The structural and electrophysical characteristics of BiGdTiNb ceramics have been studied.According to the data of powder X-ray diffraction,all the compounds are single-phase with the structures of two-layer APs(m=2)with the orthorhombic crystal lattice(space group A21am).The temperature dependence of the relative permittivityε/ε_(0)(T)of the compounds was measured and showed that the Curie temperature of perovskite-like oxides Bi_(3-x)Gd_(x)TiNbO_(9) increases linearly with an increase in the substitution parameter x to T_(C)=950℃.The activation energies of charge carriers have been found in different temperature ranges.

Crystal structure and dielectric properties of layered perovskite-like solid solutions Bi_(3-x)Gd_(x)TiTaO_(9)(x=0.0,0.1,0.2,0.3)with high Curie temperature

The Aurivillius phases[Bi_(2)O_(2)][A_(n−1)B_(n)O3_(n+1)]are well-known ferroelectrics with high Curie temperatures T_(C).High-temperature piezoceramics Bi_(3-x)Gd_(x)TiTaO_(9)(BGdTTa,x=0.0,0.1,0.2,0.3)were prepared by a solid-state reaction method.The structural and electrophysical characteristics of BGdTTa ceramics have been studied.According to the data of powder X-ray diffraction,all the compounds are single-phase with the structures of two-layer Aurivillius phases(m=2)with the orthorhombic crystal lattice(space group A2_(1)am).The temperature dependence of the relative permittivity ε/ε_(o)(T)of the compounds was measured and showed that the Curie temperature T_(C) of perovskite-like oxides Bi_(3-x)Gd_(x)TiTaO_(9)increases linearly with an increase in the substitution parameter x to TC=925℃.The activation energies of charge carriers have been found in different temperature ranges.

Structure and dielectric properties of solid solutions Bi_(7−2x)Nd_(2x)Ti_(4)NbO_(21)(x=0.0,0.2,0.4,0.6,0.8,1.0)

The electrophysical and structural characteristics of bismuth titanate oxides of a number of phases of solid solutions of the Aurivillius phases Bi_(7−2x)Nd_(2x)Ti_(4)NbO_(21)(x=0.0,0.2,0.4,0.6,0.8,1.0)having a layered structure of the perovskite type have been investigated.According to the XRD data,all studied compounds are single-phase and have a mixed-layer structure of Aurivillius phases(m=2.5)with a rhombic crystal lattice(space group I2cm,Z=2).A relationship has been established between changes in the chemical composition of solid solutions and orthorhombic and tetragonal distortions of perovskite-like layers.The temperature dependences of the relative permittivityε/ε_(o)(T)are measured.It was found that the change in the phase transition temperature-Curie temperature T_(C)synthesized Aurivillius phases Bi_(7−x)Nd_(2x)Ti_(4)NbO_(21)(x=0.0,0.2,0.4,0.6,0.8,1.0)has a close to linear dependence on the change in the parameter x.The activation energies of charge carriers in different temperature ranges were calculated.It was found that three clearly defined temperature ranges with different activation energies can be distinguished,which is associated with the different nature of charge carriers in the studied solid solutions of the perovskite type.The effect of substitution of Nd^(3+)ions for Bi3+ions is investigated.

Structure and dielectric properties of solid solutions Bi_(7)Ti_(4+x)WxNb_(1-2x-0.1)V_(0.1)O_(21)(x=0.1-0.4)

The structural and electrophysical characteristics of a number of solid solutions of layered oxides of the perovskite-type Bi_(7)Ti_(4+x)WxNb_(1-2x-0.1)V_(0.1)O_(21)(x=0.1-0.4)are studied.According to X-ray powder diffraction data,all the compounds studied are single-phase and have the structure of Aurivillius phases(m=2.5)with a rhombic crystal lattice(space group I2 cm,Z=2).Changes in tetragonal and rhombic distortions of perovskite-like layers in compounds were considered depending on their chemical composition.The temperature dependences of the relative permittivityε(T)were measured.It was shown that the Curie temperature T_(C) of the perovskite-type oxides Bi_(7)Ti_(4+x)WxNb_(1-2x-0.1)V_(0.1)O_(21)(x=0.1-0.4)linearly decreases with increasing parameter x.The activation energies of charge carriers were obtained in different temperature ranges.It was found that there are three temperature regions with very different activation energies due to the different nature of the charge carriers in the studied compounds.The effect of substitution of Nb^(5+) ions by V^(5+) ions is investigated.It was found that for a number of compounds,the substitution of niobium ions by vanadium ions led to an increase in the dielectric constant and a decrease in the dielectric loss tangent.