Nanoscale Characterization of Lead-Free Piezoceramics Using Atomic Force Microscopy

The last 35 years have seen a tremendous advancement in atomic force microscopy (AFM) in terms of its versatility and resolving power in exploring the functional properties of materials. Among them, the introduction of the piezoresponse force microscopy (PFM) technique, pioneered by Güthner and Dransfeld in 1992, has turned into a mainstream method for probing and controlling the static and dynamic properties of nanoscale ferroic structures and devices. PFM enables non-destructive visualization and manipulation of ferroelectric nanodomains and direct measurements of the local physical characteristics of ferroelectrics. Using the PFM technique, the work in this thesis is dedicated to studying ferroelectric domain structure in lead-free piezoceramics from two perspectives. On the one hand, the underlying mechanisms of measured functional properties in piezoceramics have been probed by direct observation with PFM. On the other hand, the domain structure evolution of piezoceramics under external stimuli has been visualized, thereby revealing their potential applications. Several different AFM techniques, including standard PFM, Kelvin probe force microscope (KPFM), and switching spectroscopy PFM, have been utilized for making the comprehensive study. Firstly, the evolution of the domain structure of a lead-free Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) piezoceramic under temperature and electric field stimulation on micrometer and nanometer scales was studied. The PFM results highlight the critical role of wedge-shaped domains in domain evolution. Transitional domain structures with an increased density of nanodomains appear in both the thermal and poling cycles. Interestingly, the electric-field-dependent domain structure evolution at different temperatures shows better domain structure reversibility at high temperatures than at temperatures close to the phase boundary, implying a slow rate of fatigue for the functional properties in this temperature range. Next, the unipolar fatigue behavior of three BZT-xBCT compositions with different crystallographic structures, i.e., 40BCT(R), 50BCT(O) and 60BCT(T), were evaluated. PFM studies were performed to relate the fatigue behavior to the different strain mechanisms of each of the three studied compositions. PFM domain maps indicate that the high amount of extrinsic contributions to strain made orthorhombic 50BCT(O) and rhombohedral 40BCT(R) compositions most susceptible to fatigue during unipolar cycling. Unlike them, the tetragonal composition 60BCT(T) has a high amount of intrinsic contributions to strain, making it more resistant to electric fatigue, resulting in relatively stable electromechanical properties. Na1/2Bi1/2TiO3-based compositions have been another promising candidate for lead-free piezoceramics. With the inclusion of ZnO, the depolarization temperature of 0.94Na1/2Bi1/2TiO3-0.06BaTiO3:0.1ZnO relaxor ferroelectric/semiconductor composites is enhanced. Room temperature PFM data directly demonstrate a long-range ferroelectric order induced by the ZnO inclusion in NBT-6BT:0.1ZnO composites. Also, PFM results show a slow rate of depolarization after poling in NBT-6BT:0.1ZnO composites. Compared to pure NBT-6BT ceramics, site-specific PFM hysteresis loops were acquired to reveal the modification of local ferroelectricities with the ZnO inclusion. In addition, I tried to map the domain structure of BT under different creep mechanisms with the PFM technique. Taking advantage of the unique advantages of AFM in terms of good spatial resolution and versatility, this thesis presents four studies on lead-free piezoceramics in terms of structural morphology, domain structure, domain wall dynamic, local hysteresis properties, and local potential. It provides an in-depth understanding of functional behaviors in piezoceramics from the structure point of view, primarily the ferroelectric domain (wall) structure

High temperature creep‐mediated functionality in polycrystalline barium titanate

Dislocations in oxides can be described as charged line defects and means for one-dimensional doping, which can tune electrical and thermal properties. Furthermore, theoretically it was shown that dislocations can pin ferroelectric domain walls. Broader application of this concept hinges on the development of a methodology to avail this approach to polycrystalline ceramics. To this end, we use different creep mechanisms as a method to introduce multidimensional defects and quantify structural changes. A deformation map for fine-grained barium titanate is provided and the influences of the defects and creep regimes are correlated in this first study to modifications of electrical conductivity, dielectric, ferroelectric, and piezoelectric properties. A plastic deformation of 1.29% resulted in an increase in the Curie temperature by 5°C and a decrease in electromechanical strain by 30%, pointing toward electromechanical hardening by dislocations

Nanoscale Characterization of Lead-Free Piezoceramics Using Atomic Force Microscopy

The last 35 years have seen a tremendous advancement in atomic force microscopy (AFM) in terms of its versatility and resolving power in exploring the functional properties of materials. Among them, the introduction of the piezoresponse force microscopy (PFM) technique, pioneered by Güthner and Dransfeld in 1992, has turned into a mainstream method for probing and controlling the static and dynamic properties of nanoscale ferroic structures and devices. PFM enables non-destructive visualization and manipulation of ferroelectric nanodomains and direct measurements of the local physical characteristics of ferroelectrics. Using the PFM technique, the work in this thesis is dedicated to studying ferroelectric domain structure in lead-free piezoceramics from two perspectives. On the one hand, the underlying mechanisms of measured functional properties in piezoceramics have been probed by direct observation with PFM. On the other hand, the domain structure evolution of piezoceramics under external stimuli has been visualized, thereby revealing their potential applications. Several different AFM techniques, including standard PFM, Kelvin probe force microscope (KPFM), and switching spectroscopy PFM, have been utilized for making the comprehensive study. Firstly, the evolution of the domain structure of a lead-free Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) piezoceramic under temperature and electric field stimulation on micrometer and nanometer scales was studied. The PFM results highlight the critical role of wedge-shaped domains in domain evolution. Transitional domain structures with an increased density of nanodomains appear in both the thermal and poling cycles. Interestingly, the electric-field-dependent domain structure evolution at different temperatures shows better domain structure reversibility at high temperatures than at temperatures close to the phase boundary, implying a slow rate of fatigue for the functional properties in this temperature range. Next, the unipolar fatigue behavior of three BZT-xBCT compositions with different crystallographic structures, i.e., 40BCT(R), 50BCT(O) and 60BCT(T), were evaluated. PFM studies were performed to relate the fatigue behavior to the different strain mechanisms of each of the three studied compositions. PFM domain maps indicate that the high amount of extrinsic contributions to strain made orthorhombic 50BCT(O) and rhombohedral 40BCT(R) compositions most susceptible to fatigue during unipolar cycling. Unlike them, the tetragonal composition 60BCT(T) has a high amount of intrinsic contributions to strain, making it more resistant to electric fatigue, resulting in relatively stable electromechanical properties. Na1/2Bi1/2TiO3-based compositions have been another promising candidate for lead-free piezoceramics. With the inclusion of ZnO, the depolarization temperature of 0.94Na1/2Bi1/2TiO3-0.06BaTiO3:0.1ZnO relaxor ferroelectric/semiconductor composites is enhanced. Room temperature PFM data directly demonstrate a long-range ferroelectric order induced by the ZnO inclusion in NBT-6BT:0.1ZnO composites. Also, PFM results show a slow rate of depolarization after poling in NBT-6BT:0.1ZnO composites. Compared to pure NBT-6BT ceramics, site-specific PFM hysteresis loops were acquired to reveal the modification of local ferroelectricities with the ZnO inclusion. In addition, I tried to map the domain structure of BT under different creep mechanisms with the PFM technique. Taking advantage of the unique advantages of AFM in terms of good spatial resolution and versatility, this thesis presents four studies on lead-free piezoceramics in terms of structural morphology, domain structure, domain wall dynamic, local hysteresis properties, and local potential. It provides an in-depth understanding of functional behaviors in piezoceramics from the structure point of view, primarily the ferroelectric domain (wall) structure

Investigation on Start-Up Characteristics of Large Axial Flow Pump Systems Considering the Influence of Auxiliary Safety Facilities

A large number of operation practices show that the reliability and stability of large axial flow pump systems will face significant challenges during the start-up process. If the start-up control mode or safety auxiliary facilities of large axial flow pump stations are unreasonable, start-up failure will easily follow. In order to find a scientific control strategy for the start-up of large axial flow pump stations, the start-up characteristics of large axial flow pump stations must be fully understood first. In this paper, based on the secondary development of Flowmaster software, a simulation study of the start-up process of a large axial flow pump system equipped with different safety aids is carried out. It is found that it is a very dangerous start-up control mode to delay the opening of the rapid-drop gate to reduce the maximum reflux value and reflux duration when the pump system is initially started. When the rapid-drop gate opens with a delay of 4 s, the power overload coefficient reaches 23.49, indicating that the possibility of start-up failure of the large axial flow pump system increases sharply the longer the gate delay is opened. The method of adding a flap valve to the rapid-drop gate can significantly weaken the instantaneous impact power of the unit and prevent the unit from overload. When safety auxiliary facilities with an additional disc valve on the fast descending gate are adopted, the backflow coefficient is within 0.2, the impact head coefficient is within 2, and the power overload coefficient is less than 0. The research results will provide an important reference value for comprehensively understanding the start-up characteristics of large axial flow pump stations and finding scientific and safe start-up control strategies

Investigation on Start-Up Characteristics of Large Axial Flow Pump Systems Considering the Influence of Auxiliary Safety Facilities

A large number of operation practices show that the reliability and stability of large axial flow pump systems will face significant challenges during the start-up process. If the start-up control mode or safety auxiliary facilities of large axial flow pump stations are unreasonable, start-up failure will easily follow. In order to find a scientific control strategy for the start-up of large axial flow pump stations, the start-up characteristics of large axial flow pump stations must be fully understood first. In this paper, based on the secondary development of Flowmaster software, a simulation study of the start-up process of a large axial flow pump system equipped with different safety aids is carried out. It is found that it is a very dangerous start-up control mode to delay the opening of the rapid-drop gate to reduce the maximum reflux value and reflux duration when the pump system is initially started. When the rapid-drop gate opens with a delay of 4 s, the power overload coefficient reaches 23.49, indicating that the possibility of start-up failure of the large axial flow pump system increases sharply the longer the gate delay is opened. The method of adding a flap valve to the rapid-drop gate can significantly weaken the instantaneous impact power of the unit and prevent the unit from overload. When safety auxiliary facilities with an additional disc valve on the fast descending gate are adopted, the backflow coefficient is within 0.2, the impact head coefficient is within 2, and the power overload coefficient is less than 0. The research results will provide an important reference value for comprehensively understanding the start-up characteristics of large axial flow pump stations and finding scientific and safe start-up control strategies

Water-blocking Asphyxia of N95 Medical Respirator During Hot Environment Work Tasks With Whole-body Enclosed Anti-bioaerosol Suit

Background: During hot environment work tasks with whole-body enclosed anti-bioaerosol suit, the combined effect of heavy sweating and exhaled hot humid air may cause the N95 medical respirator to saturate with water/sweat (i.e., water-blocking). Methods: 32 young male subjects with different body mass indexes (BMI) in whole-body protection (N95 medical respirator + one-piece protective suit + head covering + protective face screen + gloves + shoe covers) were asked to simulate waste collecting from each isolated room in a seven-story building at 27-28°C, and the weight, inhalation resistance (Rf), and aerosol penetration of the respirator before worn and after water-blocking were analyzed. Results: All subjects reported water-blocking asphyxia of the N95 respirators within 36-67 min of the task. When water-blocking occurred, the Rf and 10-200 nm total aerosol penetration (Pt) of the respirators reached up to 1270-1810 Pa and 17.3-23.3%, respectively, which were 10 and 8 times of that before wearing. The most penetration particle size of the respirators increased from 49-65 nm before worn to 115-154 nm under water-blocking condition, and the corresponding maximum size-dependent aerosol penetration increased from 2.5-3.5% to 20-27%. With the increase of BMI, the water-blocking occurrence time firstly increased then reduced, while the Rf, Pt, and absorbed water all increased significantly. Conclusions: This study reveals respirator water-blocking and its serious negative impacts on respiratory protection. When performing moderate-to-high-load tasks with whole-body protection in a hot environment, it is recommended that respirator be replaced with a new one at least every hour to avoid water-blocking asphyxia

Genome-Wide Analysis of Differentially Expressed microRNA in <i>Bombyx mori</i> Infected with Nucleopolyhedrosis Virus

<div><p><i>Bombyx mori</i> nucleopolyhedrosis virus (BmNPV) is a major pathogen that threatens the growth and sustainability of the sericulture industry. Since microRNAs (miRNAs) have been shown to play important roles in host-pathogen interactions, in this study we investigated the effects of BmNPV infection on silkworm microRNAs expression profile. To achieve this, we constructed and deep-sequenced two small RNA libraries generated from BmNPV infected and un-infected larvae. The results revealed that 38 silkworm miRNAs were differentially expressed after BmNPV infection. Based on the GO analysis, their predicted target genes were found to be involved in diverse functions such as binding, catalytic, virion and immune response to stimulus suggesting their potential roles in host-virus interactions. Using the dual-luciferase reporter assay, we confirmed that Bmo-miR-277-5p, up-regulated in BmNPV-infected larvae, targeted the <i>B</i>. <i>mori</i> DNA cytosine-5 methyltransferase (Dnmt2) gene which may play potential role in silkworm-BmNPV interaction. These results provide new insights into exploring the interaction mechanism between silkworm and BmNPV.</p></div

Differentially expressed miRNAs in BmNPV infected silkworm

<p>Differentially expressed miRNAs in BmNPV infected silkworm</p