Mass Of Unit Cell Research Articles

An improvement of generalized Bloch mode synthesis method-based model order reduction technique for band-structure computation of periodic structures

In this paper, an improvement of generalized Bloch mode synthesis (GBMS) method-based model order reduction technique is presented for accelerating the band-structure computation of periodic structures. The improvement is implemented through employing firstly the algebraic substructuring, which enables the mass and stiffness matrices of a single unit cell partitioned into a set of small substructures automatically in the algebraic perspective without considering the physical domain. Then these substructures are reduced via the enhanced Craig-Bampton (ECB) method. The boundary sets are divided into interior boundary and exterior boundary components. The reduced interior modes and interior boundary sets are regarded as the new interior sets relative to the exterior boundary component, and its size can be further reduced via the second level ECB method. Local-level characteristic constraint (L-CC) mode reduction technique is employed for exterior boundary reduction for further reducing the computational cost. The coupling residual modes of the two-level ECB processes are compensated after imposing the Bloch boundary conditions. Two numerical examples are presented to demonstrate the potential benefits of the proposed GBMS-based model-order reduction technique in accelerating band-structure computation of periodic structures. The results show that the proposed method can effectively improve the computational efficiency and have good computational convergence compared with the generalized Bloch mode synthesis with algebraic condensation method.

The effect of phonon anharmonicity on the lattice thermal conductivity of rare-earth pyrochlores: A first-principles study

In thermal barrier coating materials, there are several conventional routes to suppress the lattice thermal conductivity in order to improve the thermal insulation efficiency, such as doping, increasing the mass of unit cells, or using complex lattice structures. However, in spite of the fact that the mass of unit cell of La2Sn2O7 is much larger than that of La2Zr2O7 and these two pyrochlores have very similar crystal structure, the thermal conductivity of the former is much higher. By comparing the phonon spectra, phonon density of states and Grüneisen parameter of each phonon mode calculated using first-principles methods, it is found that the La2Zr2O7 has stronger anharmonicity than La2Sn2O7 because of the anharmonic potential of La atoms in the (111) plane. The difference between La potentials of the two pyrochlores is attributed to the fact that the electron cloud around Zr atoms is more easily polarized than that around Sn by the displacement of La atoms, which induces stronger long-range interaction and decreases the potential energy of La. These results provide intuitive insights towards the interplay between the electronic structure, anharmonicity and lattice thermal conductivity of rare-earth pyrochlores.

Optical bandgap tuning of ferroelectric semiconducting BiFeO3-based oxide perovskites via chemical substitution for photovoltaics

Ferroelectrics exhibit bulk photovoltaic effect, of which spontaneous electric polarization separates photo-excited carriers and results in above-bandgap open-circuit voltage. Compared with organometal halide perovskites, ferroelectric oxide perovskites are much more stable in a wide range of mechanical, chemical and thermal conditions and able to fabricate using low-cost facilities. The bottlenecks for ferroelectric photovoltaic applications are their poor optical absorption of visible light and high electric resistivity owing to wide bandgap. In this work, bandgap of BiFeO3-based oxide perovskites was judiciously tuned in a range of 1.10-2.06 eV through B-site chemistry, which makes ferroelectric semiconducting oxide perovskites feasible for photovoltaic solar cell applications with wide light absorption and high efficient energy conversion. In particular, La and Mn co-substituted BiFeO3 solid solution exhibits a narrow direct bandgap around 1.20 eV, which extends photon absorptions over ∼80% sunlight spectrum covering ultraviolet-visible-infrared region and increases electrical conductivity (σdc) up to about 10-5 S/m. The electronic subshell configuration, the reduced mass of unit cell, and the tolerant factor/octahedral factor related to ionic size were attempted as descriptors to classify and map out the relationship between compositions and bandgap properties of BiFeO3-based oxide perovskites. This essay paves the road for ferroelectric semiconducting oxide perovskites to make photovoltaic solar cells with power conversion efficiency promising beyond the Shockley-Queisser limit of conventional p-n junction solar cells.

Effects of substitution element on physical properties of BiFeO<sub>3</sub>-BaTiO<sub>3</sub> ferroelectric ceramics driven by data-mining oxide perovskites

Data-mining from reliable extant data on basis of material informatics can lead to unveil those hidden qualitative and quantitative rules and make predictions of new materials faster and cheaper, which would reduce human synthesis effort than traditional trial-and-error way. In contrast to traditional scenario of material research initiated by scientists intuition and in a chain of synthesis-observation-analyses of mechanism, modern material research will change into productions and applications of data provoked by data science paradigm, while the present bottleneck of material informatics is still limited by the number of data. In the field of functional materials of ferroelectric piezoceramics, there are two ongoing trends of developing lead-free piezoceramics with thermal aging stability and high temperature piezoceramics with higher response for applications like in Internet-of-Things (IoT) and smart systems. Recent years, data-mining on oxide perovskites and designing new perovskite-type high temperature piezoceramics have been carried out and progresses were firstly reviewed in a short. Based on those quantitative relationships and rules data-mined out previously, various kinds of substitution elements for BiFeO3-BaTiO3(BF- x BT) solid solution ferroelectrics were proposed on basis of the mass and cation size features of atom and their effects on ferroelectric phase transition temperature, dielectric and piezoelectric properties observed experimentally. The compositions were tentatively chosen near the rhombohedral-pseudocubic structural phase boundary in order to check predominant contribution from substitution element. Among those elements of Al, Sc, Zn1/2Ti1/2 substitution for Fe and Bi1/2Nb1/2, Al1/2Nb1/2, Zn1/3Nb2/3, Zr, Sn for Ti, it was demonstrated experimentally that Bi(Zn1/2Ti1/2)O3 (BZT) exhibits a very promising effect in the BF- x BT system to obtain piezoceramics prepared using conventional solid state electroceramic processing with robust insulating and low dielectric loss properties fulfilled for the requirements of engineering applications. For those low dielectric loss BF- x BT- y BZT ternary solid solution ceramics, their ferroelectric Curie temperature ( T C) follows a quadratic polynomial relation of T C ( μ ) = a + b μ + c μ 2 and reduced mass of unit cell ( μ ) is a good descriptor to represent BF- x BT and BF- x BT- y BZT solid solution perovskite numerically. Full poling for piezoelectricity was obtained in Mn-doped BF- x BT- y BZT ( x ≤0.32, y 0.05) ceramics. So far, two ferroelectric piezoceramic materials were selected out: one is Mn-doped 0.74BF-0.22BT-0.04BZT exhibiting piezoelectric constant d 33~75 pC/N, dielectric constant e 33T/ e 0~260, dielectric loss tan δ ~0.01, thickness electromechanical coupling factor kt ~0.42, radial electromechanical coupling factor kp ~0.29 and T C~630°C, four times piezoresponse of commercial K-15 Aurivillius-structured bismuth titanate ceramics; another is Mn-doped 0.69BF-0.27BT-0.04BZT exhibiting d 33~145 pC/N, e 33T/ e 0~570, tan δ ~0.03, kt ~0.41, kp ~0.33, T C~510°C, 50% beyond K-81 tungsten-bronze-structured lead metaniobate ceramics. Similar to PbZr1- x Ti x O3(PZT) system, BF-BT-BZT system exhibits a structural phase boundary and thus their piezoelectric performance and thermal stability could be modified in a wide range through adjusting composition and doping. At this point, it is time performing systematical characterizations on BF-BZT-BT perovskite-type piezoceramics for high temperature engineering application testing. Based on the correlation between d 33 and e 33 for piezoceramics data-mined out previously, how to reduce synthesis effort for designing new materials was also argued using bad dielectric data of those high dielectric loss samples. Our essay demonstrates data-mining driven designing based on material informatics sure able to reduce time-to-insight and human effort on synthesis, accelerating new materials discovery and deployment.

Ferroic phase transition of tetragonal Pb0.6−xCaxBi0.4(Ti0.75Zn0.15Fe0.1)O3 ceramics: Factors determining Curie temperature

In this paper, ferroelectric phase transitions of Pb0.6−xCaxBi0.4(Ti0.75Zn0.15Fe0.1)O3 with x ≤ 0.20 ceramics were experimentally measured and a change from first-order to relaxor was found at a critical composition x ∼ 0.19. With increasing Ca content of x ≤ 0.18, Curie temperature and tetragonality was found decrease but piezoelectric constant and dielectric constant increase in a quadratic polynomial relationship as a function of x, while the ferroic Curie temperature and ferroelastic ordering parameter of tetragonality are correlated in a quadratic polynomial relationship. Near the critical composition of ferroic phase transition from first-order to relaxor, the Pb0.42Ca0.18Bi0.4(Ti0.75Zn0.15Fe0.1)O3 and 1 mol % Nb + 0.5 mol % Mg co-doped Pb0.44Ca0.16Bi0.4(Ti0.75Zn0.15Fe0.1)O3 ceramics exhibit a better anisotropic piezoelectric properties than those commercial piezoceramics of modified-PbTiO3 and PbNb2O6. At last, those factors including reduced mass of unit cell, mismatch between cation size and anion cage size, which affect ferroic Curie temperature and ferroelastic ordering parameter (tetragonality) of tetragonal ABO3 perovskites, are analyzed on the basis of first principle effective Hamiltonian and the reduced mass of unit cell is argued a more universal variable than concentration to determine Curie temperature in a quadratic polynomial relationship over various perovskite-structured solid solutions.

Structural phase transitions of robust insulating Bi1-xLaxFe1-yTiyO3 multiferroics

In contrast to leaky BiFeO3, of which two structural phase transitions of cycloidal modulated antiferromagnetic-paramagnetic and ferroelectric-paraelectric are observed below 850 °C, chemical co-substitution to form Bi1−xLaxFe1−yTiyO3 ternary solid solution makes these multiferroic ceramics become robust insulating low dielectric loss and exhibit rich structural phase transitions. Differential thermal analysis and temperature-dependent X-ray diffraction measurements probe four first-order structural phase transitions, e.g., TH = 305 °C, TN = 365 °C, TC = 810 °C, and TS = 830 °C observed in the Bi0.98La0.02Fe0.99Ti0.01O3 system, which are reasonably attributed to Brazovskii-type cycloidal modulated antiferromagnetic-helimagnetic (at TH) and helimagnetic-paramagnetic (at TN) magnetic phase transitions, ferroelectric-paraelectric (at TC) and rhombohedral-cubic (at TS) structural phase transitions, respectively. Magnetic phase transition temperatures change a little but ferroelectric and lattice structural phase transition temperatures decrease gradually with increasing co-substitution up to composition-induced rhombohedral-(pseudo-)cubic structural phase boundary. The first-order nature of magnetic phase transition and emergence of helimagnetic phase were attributed to Ti3+ d1 magnetic disorder distribution in the Fe3+ d5-O-Fe3+ d5 chains, while the first-order nature becomes weak with increasing co-substitution, owing to decreased ferroelectric rhombohedral lattice distortion. An intermediate rhombohedral paraelectric phase is discovered intervening between ferroelectric rhombohedral and paraelectric cubic phase, of which the temperature range defined by difference between TC and TS increases with increasing co-substitution. It was found that TC and TS are able to be predicted quantitatively by reduced mass of unit cell. These findings enrich our understanding of ferroic phase transitions and advance designing novel high temperature multiferroic compounds.