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导读：D.manager’s134 Theofficerinchargeoftheengineering_____thewatchtotherelievingofficerifthereisreasontobelievethatthelatterisobviouslynotcapableofcarryingoutthewatch-keepingdutieseffe
D. manager’s 134 The officer in charge of the engineering _____ the watch to the relieving officer if there is reason to believe that the latter is obviously not capable of carrying out the watch-keeping duties effectively. A. shall take over
B. shall not take over C. shall hand over
D. shall not hand over 135 When deciding the composition of the engineering watch, which may include _____appropriately, many factors shall be taken into account. A. satisfied engineers
B. qualified ratings C. satisfied chief engineer
D. qualified chief engineer 136 When deciding the composition of the engineering watch, which may include appropriately qualified ratings, following criteria shall be taken into account. I. the type of ship.II. the type and condition of the machinery.III. the safety of life, ship, cargo and port.IV. the observance of international, national and local regulations. A. I +II
B. II+III C. II+III+IV
D. I+II+III+IV137 The officer in charge of the engineering watch shall continue to be responsible for machinery operations, _____ the presence of the chief engineer in the machinery space. A. as long as
B. unless C. despite
D. till 138 All members of the engineering watch shall be familiar with assigned watch-keeping duties. In addition, every member shall, with respect to the ship they are serving in have knowledge of_____. I. the use of appropriate internal communication systems.II. the use of GMDSS.III. the escape routes from machinery spaces.IV. the number location and types of fire-fighting equipment in the machinery spaces. A. I+III+IV
B. II+III+IV C. III+IV
D. I+II+III+IV139 When the machinery spaces are in the periodic unmanned condition, _____ shall be immediately available and on call to attend the machinery space. A. the chief engineer B. the designated engineer from shipyard C. the designated duty officer in charge of the engineering watch D. the designated surveyor from classification society 140 Changes in direction or speed of the main propulsion units shall be recorded, _____ an Administration has determined that the size or characteristics of a particular ship make such recording impracticable. A. in addition to
B. in addition that C. except for
D. except where141 The officer in charge of the engineering watch shall notify the chief engineer without delay_____. A. in any emergency or if in any doubt as to what decision or measures to take B. in the event of any impending action in machinery spaces that may cause reduction in ship’s speed C. when isolating and bypassing machinery to be worked on D. co-operating with any engineer in charge of maintenance work142 When the ship is in restricted visibility, the officer in charge of the engineering watch shall ensure that_____.I. bridge ord
rders relating to changes in speed or direction of operation are immediately implemented. II. auxiliary machinery used for maneuvering is readily available. III. permanent air or steam pressure is available for sound signals. A. I+II
D. I+II+III143 The officer in charge of the engineering watch shall ensure that all machinery involved with the maneuvering of the ship can be immediately be placed in the manual mode of operation when notified that the ship is _____. A. at anchor
B. at unsheltered anchorage C. in congested waters
D. in an open road-stead144 The International Sewage Pollution Prevention Certificate shall be issued for a period specified by the Administration, which shall not exceed _____ from the date of issue. A. six months
B. one year C. three years
D. five years 145 The _____ Convention is the first to establish basic requirements on training, certification and watch-keeping for seafarers on an international level. A. STCW78
B. STCW95 C. SOLAS74
D. SOLAS88146 One of the major features of the STCW95 is the adoption of a new STCW Code, to which many ______ have been transferred. A. safety regulations
B. technical regulations C. minimum requirements
D. standard requirements147 If a hydraulic pump is overheating, the cause may be _______. A. excessive internal leakage in the pump B. low discharge pressure and fluid flow C. excessive fluid level in the hydraulic reservoir D. operation of the pump at 100% efficiency148 Zinc rods are installed in the refrigeration system _______. A. liquid strainer
B. liquid receiver C. saltwater condenser
D. evaporator 149 Accidental flooding of the engine room bilges by the bilge main is prevented by _____. A. stop-check valves installed in the bilge suction manifolds B. using a positive displacement reciprocating bilge pump C. installing eductors in all bilge rose boxes D. installing a swing check before each bilge valve 150 Under the provisions of applicable international conventions, ships entitled to fly the flag of a Party are subject, while in the port of a Party, to control by officers duly authorized by the Party. Ships of non-parties or below convention size shall be given ______. A. more favorable treatment
B. less favorable treatment C. no more favorable treatment
D. the most favorable treatment 151 In a refrigeration system, if the cooling water to the condenser fails, the __________. A. box temperature solenoid valve will close
B. expansion valve will close C. compressor will shutdown
D. king valve will open 152 The Garbage Record Book, as a part of the ship’s official logbook, shall be kept on board and preserved for a period of ___
__. A. 3 years after making the first entry
B. 2 years after the last entry C. 3 months after last entry
D. 3 years after the last entry153 If liquid refrigerant accidentally comes in contact with the eyes, _____ immediately. A. rub the eyes
B. irritate the eyes C. wash the eyes with a heavy boric acid solution
D. obtain medical help 154 Any carbon built up on surfaces must be washed away by _____ additives and held in suspension by a _____ additive. A. dispersant /detergent
B. detergent/dispersant C. dispersant /dispersant
D. detergent/detergent 155 The _____ of an oil refers to its ability to mix with water and then release the water in a centrifuge. A. oxidation resistance
B. carbon-forming tendency C. demulsibility（抗乳化性）
D. acidity 156 Your vessel has run aground and upon taking fuel oil tank soundings, you find that a fuel tank level has increased. You therefore should suspect ____. A. condensation in the fuel tank B. a crack in the hull portion of the fuel tank C. a load of bad fuel D. contamination from the saltwater flushing system157 The term ‘discharge', as it applies to the pollution regulations, means _______. A. spilling
B. leaking C. dumping
D. All of the above. 158 When adding oil to a refrigeration system, precautions must be taken to ensure that _______. A. the compressor suction pressure is not too high B. all air is purged from the pump and charging fittings C. the high pressure cutout switch is held open to prevent accidental starting D. the condenser is completely shutdown first 159 To add refrigerant to the high side of an air conditioning system, you should introduce the refrigerant through the _____. A. discharge service valve as a vapor B. suction service valve as a liquid C. charging valve as a liquid D. condenser purge valve as a vapor160 If a refrigeration system were short of refrigerant, the condition would result in _____. A. continuous running of the compressor B. high suction pressure C. high discharge pressure D. short cycling of the compressor on the water failure switch161 Which of the statements listed concerning heat transfer is correct? A. Heat is always transferred at a constant rate. B. Heat transfer rate increases as temperature difference increases. C. The rate of heat transfer is not affected by temperature difference. D. The high temperature region is known as a heat sink.162 The flash point of a petroleum product is an indication of its ____. A. viscosity
B. pour point C. volatility
D. lower explosive limit 163 The flash point of a liquid refers to the temperature ________. A. at which a liquid will give off inflammable vapors B. at which a liquid will burn steadily C. at which a liquid will explode D. that a liquid
must reach before it will flow readily164 Now that the valve is not worth _____, I suggest it _____. A. to repair/renewing
B. repairing/renewing C. to repair/ be renewed
D. repairing/be renewed 165 _____, the following possible causes should be investigated. A. Should the engine stop without apparent reason B. The engine should stop without apparent reason C. The engine stops without apparent reason D. The engine is stopped without apparent reason166 _____, the second one will start automatically. A. If the first stand-by set failed B. If the first stand-by set fails C. If the first stand-by set will fail D. If the first stand-by set doesn’t fail 167 The difference between a supercharged engine and an un-supercharged engine is that _____ operates at an increased pressure. A. the later
B. the latter C. the former
D. the farmer 168 A supercharged engine differs from an un-supercharged engine _____ it operates at an increased pressure. A. in which
B. on which C. at which
D. in that 169 When a refrigeration compressor is in the 'off' cycle, the thermal expansion valve will_________. A. always be wide open when the compressor restarts B. continue to operate as if the system were in operation C. remain open until evaporator pressure equalizes, then close until the compressor restarts D. always be completely closed until the compressor restarts170 What is the harmful effect of sulfur in a fuel? A. It causes excessive smoking and soot at low firing rates. B. It doesn't readily burn when combined with oxygen. C. It clogs fuel oil strainers more often. D. It forms a corrosive acid when mixed with water or water vapor.171 A solenoid valve used in a refrigeration system should be installed _______. A. with the axis of the solenoid horizontal, sensing temperature of the box, and upstream of the thermal expansion valve B. upright, sensing temperature of the box, and upstream of the thermal expansion valve C. upright, sensing temperature of the box, and downstream of the thermal expansion valve D. upright, sensing superheat of the tail coil, and upstream of the thermal expansion valve 172 The maximum Blood Alcohol Concentration (BAC) rate on board shall not be more than ____ by weight any time when being tested. But watch-keepers are not allowed to drink any alcoholic beverage _____ before their watch. A. 0.08%/ 4 hours
B. 0.08%/ 2 hours C. 0.04%/ 4 hours
D. 0.04%/ 2 hours 173 The Port State Control officer may witness a fire drill carried out by the crew assigned to these duties on the _____. A. Navigation Log Book
B. Engine Room Log Book C. Oil Record Book
D. Muster List 174 The fire drill must, as far as practicable, be conducted as if _____. A. there is a minor emergency
B. there is an actual em
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Our range of services include: contract R & D; ca customer training & equipment hiredisplacement allows ferroelectric switching in a bismuth-based perovskiteReceived
23rd December 2011
, Accepted 20th February 2012First published on 20th February 2012Bi(Fe2/8Mg3/8Ti3/8)O3 (BFTM) is a member of a small class of pure Bi3+ A site perovskites which are stable without recourse to high pressure synthesis. BFTM is polar in the R3c space group, but ferroelectric switching of the polarisation is not attainable in bulk ceramics. Formation of solid solutions with BaTiO3 produces enhanced functional behaviour. The composition 0.75Bi(Fe2/8Mg3/8Ti3/8)O3–0.25BaTiO3 displays ferroelectric hysteresis loops and piezoelectric response (high field d33 of 85 pC N-1 at 0.1 Hz and low field d33 of 16 pC N-1). This change in functional behaviour is associated with significant changes in the average structure, where the rhombohedral distortion is reduced and transformed to a pseudo-cubic R3m space group, as substitution of the larger Ba2+
suppresses tilting of the BO6 octahedra. Polar Bi displacements are refined solely along the pseudocubic &111&p direction of the perovskite subcell, with the off-axis displacements characteristic of BFTM being suppressed. The local structure deviates from the average structure in a similar way to PZT as shown by diffuse scattering in , suggesting correlated &111&p displacements along directions other than the average rhombohedral unique axis. The switchable polarisation measured by PUND measurements is considerably smaller than the remanence measured in P(E) loops.
Lead based perovskite-structured piezoelectric ceramics, in particular Pb(ZrxTi1-x)O3 (PZT) have been studied extensively due to their high performance in a wide range of electronic devices such as actuators, sensors, and transducers. Piezoelectric materials generate an electric field in response to applied stress or change dimensions under an applied field. The required polar structures are generated by the 6s2 lone pair of the , with displacements along the [001]p and [111]p directions of the parent cubic perovskite unit cell producing tetragonal (T) and rhombohedral (R) ferroelectric structures respectively. Combinations of these displacements produce a monoclinic structure which has also been assigned to coexisting nanodomains of the R and T structures. There are two possible R structures in space groups R3m and R3c depending on whether antiphase octahedral tilting about [111]p is present, affording R3c. In PZT, the piezoelectric properties quantified by the piezoelectric coefficient, d33 (ranging from 200–600 pC N-1 in commercial PZT) and strain (0.3%) are optimal at the morphotropic phase boundary (MPB) where the ferroelectric rhombohedral and tetragonal phases of a solid solution meet. Piezoelectrics containing lead are highly effective primarily because the 6s2 lone pair can easily become stereoactive resulting in a large structural distortion and strong coupling between the electronic and structural degrees of freedom. The large piezoelectric response of PZT is assigned to polarization rotation at the MPB or in the monoclinic phase and to a high contribution (up to 70%) from the domain wall motion to the piezoelectric properties.
There is current interest in evaluating lead-free materials as alternatives to PZT for regulatory reasons, although the demands in coming up with a readily processable material to approach PZT performance metrics are considerable. A-site bismuth perovskites could be a viable alternative, as Bi3+ ions will also have lone-pair distortions similar in magnitude to their lead-based counterparts. Although replacing lead with bismuth may seem straightforward, it adds a layer of complexity as few A-site bismuth perovskites are stable under ambient pressure (due to the small size of the Bi3+ ion, which can cause instability in the AO12 polyhedra) with BiFeO3, Bi2(Mn4/3Ni2/3)O6, and Bi(Fe2/8Mg3/8Ti3/8)O3 as the only examples.
Bi(Fe2/8Ti3/8Mg3/8)O3 (BFTM) crystallizes in the rhombohedral R3c space group which arises from A-site and B-site displacements along [111]p in relation to the Pmm parent perovskite structure. In this space group, the BO6 polyhedra belong to the tilt system a-a-a-, which means that the tilt angles are of equal magnitude and each octahedron rotates with an anti-phase tilt in relation to its neighbouring octahedra. The hysteresis loop of BFTM is not saturated and the remanent polarization is only ~0.5 μC cm-2, because the polarisation is not switchable at fields below dielectric breakdown. However, literature suggests that it is possible to achieve improved hysteresis loops and piezoelectric response through either doping and/or forming solid solutions. The purpose of this study is to synthesize solid solutions between Bi(Fe2/8Mg3/8Ti3/8)O3 and BaTiO3. BaTiO3 is a classical ferroelectric material, which crystallizes in the P4mm tetragonal space group, due to the polar Ti displacement from the centroid of the
octahedron.
Starting materials of BaCO3 (Alfa Aesar, 99.997%), Bi2O3 (Alfa Aesar, 99.9995%), Fe2O3 (Alfa Aesar, 99.998), MgO (Alfa Aesar 99.998%), and TiO2 (Alfa Aesar, 99.995%) were mixed in stoichiometric amounts under
with an agate mortar and pestle then calcined at 600 °C for 12 h. The resulting powders were pressed into pellets and placed in an alumina crucible lined with Pt foil on the bottom to prevent reaction with the alumina while heating. The pellets were then fired at different temperatures with
and repelletizing in the intermediate stages. The synthetic procedure is based on the synthesis of BFTM: 750 °C 12 h, 800 °C 12 h/800 °C 3 h, 850 °C 12 h/850 °C 3 h, 900 °C 12 h.
the starting materials produced an Aurivillius type phase as an impurity. Phase pure powder used for physical measurements was ball milled overnight in an
slurry with a binder of 2 wt% polyvinyl butyral resin. The
was done in a planetary mill with eight 10 mm yttria-stabilized
balls, for 45 cycles with each cycle consisting of 15 min forward, 15 min rest, then 15 min reverse milling direction at a rate of 350 rpm. Pellets were pressed in a 10 mm die then vacuum sealed in a latex bag and pressed at 2000 bar using an isostatic cold press and sintered at 950 °C 1 875 °C for 15 h to achieve a density greater than 95% as measured by an Archimedes balance. The pellets were then polished down to &0.5mm thickness (for ferroelectric and piezoelectric measurements) and polished to a mirror finish on a polishing wheel with a series of P800, P1200, then P2400 grit
paper (Buehler, Germany).
were collected using a Panalytical X'Pert Pro
with Co Kα1 radiation (λ = 1.78901 ?). Data were collected over a 2θ range from 20°–130°.
were collected at ESRF on the ID31 beamline with λ = 0.39986 ? at room temperature and 10 K. Data were collected over a 2θ range from 2–44° at room temperature in a
capillary.
data were collected at the ISIS spallation
source at the Rutherford Appleton Laboratory on the HRPD beam line. Data collected on the backscattering banks were used in the refinement. Low temperature
data was also collected at the ISIS spallation
source on the GEM beam line at a temperature of 10 K. Refinements were performed using the software GSAS/EXPGUI and Topas Academic.
were collected with a JEOL 2000FX
at room temperature. Samples were prepared by crushing the powder in
and the crystallites in suspension were deposited onto a holey carbon film.
To prepare the pellets for impedance measurements, Pt paste electrodes were applied and fired at 800 °C for 0.5 h. Pt wires were used to make electrical contact. Impedance measurements were performed using a Solartron 1255B Frequency Response Analyzer and a Solatron 1296 dielectric interface. Measurements were taken over the frequency range of 1 Hz to 1 MHz over a temperature range from 25 °C to 900 °C on heating and cooling.
Ferroelectric characterization was performed with a Radiant Ferroelectric Tester Precision
with a Radiant Precision high voltage interface and a Trek 609B high voltage . Measurements were performed at room temperature on thin pellets with gold sputtered electrodes at a frequency of 1 Hz and P(E) loops were collected at successively higher voltages until eventually dielectric breakdown occurred. Positive-up negative-down (PUND) measurements were also performed with the same instrument. A series of 5 voltage pulses were applied with a delay time of 1000 ms and a pulse width of 1000 ms. The net switched polarization is calculated by subtracting the non-switched polarization (P^) from the total polarization of the switching pulse (P*). High-field converse piezoelectric measurements were performed using a Fotonic sensor MTI-2100 combined with a commercial Radiant Ferroelectric Tester Precision , in line with a Trek 5/80 high voltage , and Radiant high voltage interface. In order to measure the piezoelectric coefficient d33, poling was performed on pellets sputtered with Au electrodes on the parallel plane faces. The pellet was poled in silicon oil at 200 °C. The voltage was increased slowly to up to 110 kV cm-1 and left for 20 min. Then the sample was slowly cooled to room temperature with the applied electric field before removing from the silicon oil, taking approximately three hours. d33 was determined with a conventional Berlincourt meter (Piezotest PM300, London, UK). The measurements were carried out by applying an alternating force of 0.25 N and a frequency of 110 Hz.
BFTM-BT materials were synthesised at 10, 25, 50 and 75% BT content. The substitution of 10% BT into BFTM to afford Bi0.9Ba0.1(Fe0.22Ti0.44Mg0.34)O3 (0.9BFTM–0.1BT) changes the diffraction pattern significantly, with weakening of the R3c tilting peak (absent in the XRD data). At this composition, two rhombohedral phases are apparent in the neutron diffraction pattern together with a weakened
1/2 (111)p reflection characteristic of the R3c phase. Le Bail analysis shows that this material consists of two phases: R3c and R3m. The weakening of the tilt signature compared with the BFTM parent is shown in .
from a) BFTM, b) 0.9BFTM-0.1BT and c) 0.75BFTM-0.25BT and d)
of 0.75BFTM–0.25BT showing the evolution from the R3c to R3m space group. The arrows in the
patterns indicate the presence/absence of a tilting peak. The
of 0.75BFTM–0.25BT can be indexed solely as a cubic perovskite. The white circle in the SAED pattern shows where the superstructure spot would be expected if tilting were present in the structure.
Bi0.75Ba0.25(Fe0.188Mg0.281Ti0.531)O3 (0.75BFTM-0.25BT) does not have the R3c space group of the parent BFTM as shown by the absence of the R3c
1/2 (111)p tilting peak in the
data and the lack of the tilt reflection in the [110] zone axis in the selected area electron diffraction () pattern, . The rhombohedral distortion is also strongly suppressed and the diffraction pattern appears cubic. Room temperature X-ray and
were fitted to several different models by Rietveld refinement. The models used were Pmm, the “distorted” Pmm model applied to PMN, and R3m. Upon inspection of the data, the space group Pmm was chosen for the initial refinement as there was no characteristic peak splitting to indicate a lowering of symmetry. The fits of the combined X-ray and
data gave an Rwp value of 7.01 (χ2 = 5.56) but showed a large mismatch of the model intensities of several of the peaks, which indicated that the space group was incorrect as shown in Figure S1 (Supporting Information). The distorted Pmm model, which gave an Rwp of 6.61 (χ2 = 4.19) was based on an ideal cubic perovskite where the atoms are displaced from their special positions along the common disordered directions of [100], [110], and [111] as shown in Figure S2. The R3m refinements were run with the standard atomic positions with A and B-site displacements along the &111&p direction, but different models were also examined where 1) the A-site was changed from the (x,x,x) special position to the (x,y,z) general position corresponding to displacement away from the [111]p direction, 2) the three B-site
were refined with each on a separate position or 3) the magnesium and iron were refined to the same position with the titanium being allowed to refine separately. All of the models except the standard R3m space group resulted in very unreasonable thermal parameters or diverged upon refining. The best refinement came from the R3m space group with an Rwp = 6.19 (χ2 = 3.67). Great care was taken in the refinements to keep all instrument parameters (background and profile) the same so that any difference in the goodness of fit was based on a difference in the model only. Therefore, R3m was chosen as the space group for the average structure and the combined X-ray/ Rietveld refinement is shown in . The lattice parameters show the unit cell is nearly cubic with a = 4.00361(1) ?, α = 90.0025(3)°. The refined parameters and bond lengths are given in
Rietveld refinement of combined a)
patterns for 0.75BFTM–0.25BT. The circles represent the observed data while the solid line represents the model and the difference curve is below.
Table 1 Atomic parameters for 0.75BFTM–0.25BT from Rietveld refinement at 300 K. Space group: R3m, lattice parameters: a = 4.00361(1) ?, α = 90.0025(3)°
-0.0476(3)
-0.0476(3)
-0.0476(3)
0.19/0.28/0.53
Table 2 Bond lengths and angles of 0.75BFTM–0.25BT at 300 K
Bond length/?
3 × 26480(4)
6 × 2.84050(5)
3 × 3.02056(6)
Fe/Mg/Ti–O
O–Fe/Mg/Ti–O
89.858(2) × 2, 93.249(2) × 2, 175.4738(8) × 2
O–Bi/Ba–O
55.779(2) × 3, 57.6677(8) × 6, 59.65929(2) × 3, 59.9043(2) × 3, 61.986(1) × 6, 64.766(2) × 3, 86.1161(3) × 6, 93.6193(3) × 6, 113.429(1) × 6, 119.3482(7) × 6, 119.626(2) × 6, 126.7215(9) × 6, 170.6873(5) × 3, 174.4914(9) × 3
Diffraction (10 K) and
(RT to 100 K—Fig. S4) was performed at low temperatures, but no structural phase transition occurred. Combined / refinements were performed and the R3m model gave the best fit with an Rwp of 3.213% (χ2 = 2.12) (Fig. S3 in the Supporting Information). The refined displacement parameters (Beq(A-site) = 6.64(4) ?2, Beq(B-site) = 0.84(2) ?2, and Beq(O) = 2.36(4) ?2) are similar to those at room temperature suggesting static displacements at low temperature.
of 0.75BFTM–0.25BT shows that the material has a perovskite unit cell with nearly cubic lattice parameters which is consistent with the overall structure proposed by the Rietveld refinements. Along with the Bragg diffraction, diffuse streaks in SAED patterns along minor axes are observed, as shown in
which consists of four patterns along the [321], [139], [431] and [71] zone axes as examples. Diffuse scattering is typically very weak, and these patterns can only be observed in the higher order zone axes or when tilted slightly off-axis away from the lower zone axes. This is due to a number of factors which are described in detail elsewhere but one of the main reasons is that when looking down a low order axis with strong Bragg reflections, any disorder is averaged out over the depth of the atomic columns, resulting in zero net modulation.
Diffuse streaks present in SAED patterns of the a) [321], b) [139], c) [431] and d) [371] zone axes of 0.75BFTM–0.25BT. The intense diffuse streaks are marked by the arrows with indexes showing their directions in reciprocal space. The indexing is according to the cubic ideal perovskite.
In the case of the [321] pattern (), diffuse streaks are observed along [25]* and [15]* which are perpendicular to [11] and [11], respectively. And for the [139], [431] and[71] patterns (), the diffuse streaks are along [1]* and [5]*, [37]* and [57]*, [415]*, and [5]* which are perpendicular to [11] and [11], [11] and [11], [11] and [111] respectively. These transverse polarized diffuse streaks running through the Bragg reflections build up continuous {111}* sheets in reciprocal space. The transverse polarized nature of the diffuse streaks occurs when the displacement eigenvector of the most heavily displaced atoms is perpendicular to the modulation wave vector itself and requires that the atomic displacements are perpendicular to the sheets, which means the displacements producing the diffuse scattering occur along the &111&p directions.
Room temperature impedance measurement of both relative permittivity and loss show frequency dispersion (). The permittivity increases with increasing BT content from 144 for BFTM to 265 for 0.9BFTM–0.1BT, and 345 for 0.75BFTM–0.25BT at 1 kHz at room temperature. The loss however, does not follow this trend, with values at 1 kHz of 0.040 for BFTM, 0.076 for 0.9BFTM–0.1BTO and 0.049 for 0.75BFTM–0.25BT.
a) Dielectric constant and b) loss tangent as a function of frequency at room temperature for BFTM and 10% and 25% BT substituted materials.
The temperature dependence of the permittivity from 0.75BFTM–0.25BT shows a broad peak at around 400 °C, and a second sharper peak at about 600 °C (). The broad lower temperature peak shifts to higher temperatures as the frequency is increased while the second sharper peak does not shift significantly with increasing frequency if the data taken on heating is considered. However, the second higher temperature peak is suppressed (i.e. the magnitude of the permittivity maximum is decreased) as the frequency is increased. The higher temperature peak shows hysteresis on heating/cooling which does not occur at lower temperature.
Relative permittivity of 0.75BFTM–0.25BT as a function of temperature.
Ferroelectric P–E hysteresis loop measurements were performed for both 0.9BFTM–0.1BT and 0.75BFTM–0.25BT as shown in . P(E) loops for both compositions show a change in slope as the applied voltage is increased, suggesting non-linearity and therefore genuine domain wall motion. However, the shape of the loop for 0.9BFTM–0.1BT is not near saturation, possibly due to the inability to apply sufficient electric fields and therefore the loop shows sub-switching field behaviour. In contrast to the loop of 0.9BFTM–0.1BT, it is evident from the shape that the loop for 0.75BFTM–0.25BT is nearly saturated at a field of 90 kV cm-1. The real switched polarization (ΔP) values are obtained using PUND measurements by subtracting the non-switched polarization (P^) from the total polarization of the switching pulse (P*) and dividing by 2 and is shown in . The pulse width used was 1000 ms and the delay time between the pulses was 1000 ms. The switched ΔP obtained from PUND is small (2.4 μC cm-2 compared to the total maximum polarization of 20 μC cm-2).
P(E) hysteresis for a) 0.9BFTM–0.1BT and b) 0.75BFTM–0.25BT. c) PUND data for 0.75BFTM–0.25BT showing the true switched polarization.
Strain–electric field measurements for 0.75BFTM–0.25BT show a characteristic butterfly strain loop () which is evidence for ferroelectric switching and bulk piezoelectricity.
shows the strain response under unipolar drive at both 1 Hz and 0.1 Hz. The measured displacement is larger at 0.1 Hz, which suggests that application of electric field of lower frequency enables more efficient switching of domains in the material. This type of frequency dependence is observed for BiFeO3, PZT, PMN-PT, and PZN-PT and explained by switching of predominantly non-180° domains. The high field piezoelectric coefficient d33 calculated from the unipolar strain data is approximately 53 pC N-1 at 1 Hz and 85 pC N-1 at 0.1 Hz. The piezoelectric coefficient, d33 measured was 16 pC N-1 with the sample poled at 110 kV cm-1 for 20 min at 200 °C. The d33 of BFTM was also measured and was found to be 0.6 pC N-1.
a) Butterfly loop from strain-electric field measurements of 0.75BFTM–0.25BT and b) strain response of 0.75BFTM–0.25BT under unipolar drive.
The introduction of the Ba2+
into BFTM produces significant structural and functional changes. The octahedral tilting is suppressed, producing a change in space group from R3c to a pseudocubic structure best described on average as R3m (), but with strong indications from diffuse scattering of a more intricate local structure. In a perovskite, BO6 octahedra are prone to tilting when there is a need to optimise the anion coordination because the A-cation is too small for the 12 coordinate site of the AO12 polyhedra as is the case in BFTM (tolerance factor = 0.948). In the solid solution of 0.75BFTM-0.25BT (tolerance factor = 0.98), a percentage of the bismuth ions which have a 12 CN ionic radius of 1.34 ? are replaced by
which have a larger ionic radius of 1.61 ? increasing the VA/VB ratio of the polyhedra which decreases the average tilt angle to zero, resulting in a transition of the space group from R3c to R3m. The A-site is occupied by bismuth and barium with average A–O bond lengths of 3 × 2.64799(4) ?, 6 × 2.84050(5) ?, 3 × 3.02056(6) ? which give a highly under-bonded bismuth atom (bond valence = 2.2) and a highly over-bonded barium atom (bond valence sum 2.7). The octahedral B site is occupied by iron, titanium, and magnesium in a disordered manner with bond lengths of 3 × 1.94386(4) ? and 3 × 2.06299(4) ?. The bond lengths result in bond valence sums of 3.1 for iron, 2.6 for Mg which is over-bonded and 3.7 for Ti which is under-bonded. Both the A and B
are displaced along the &111&p direction, with average structure displacements from the centroids of their anion polyhedra of 0.23 ? (A ) and 0.10 ? (B ). The B-site displacement is smaller than in BFTM (0.24 ?) but similar to that in the R3m phase of PZT (0.11 ?). The A-cation displacement is much smaller than that found in BFTM (0.66 ?) and just over half that of R3m PZT (0.40 ?). This difference in the A-site displacement between BFTM-BT and both BFTM and PZT is assigned to occupation of 25% of the sites in BFTM-BT by barium, which is not prone to the lone pair distortion seen in bismuth and lead. This is also apparent when comparing mean A–O bond lengths of 2.9(4) ? in BFTM and 2.84(2) ? in 0.75BFTM-0.25BT, where the spread in distances is greatly reduced in the Ba-doped system. The distribution of B–O bond lengths (BFTM 2.08(8) ? vs. 2.00(6) ? in 0.75BFTM-0.25BT) is almost unchanged, reflecting the decreased A-site distortion in the average structure. The nature of the A site displacements from the anion centroids differs from the parent BFTM, where there are disordered static local Bi displacements away from the threefold axis along the &110&p direction, which are not observed in the average structure of 0.75BFTM–0.25BT. The absence of octahedral tilting and the introduction of the Ba2+
which prefers higher symmetry environments may be linked to the suppression of these off-axis displacements. The pseudocubic nature of the diffraction pattern and the ambiguity of the refined displacement directions (i.e. the similar figures of merit established for refined A site displacements along [001]p and [111]p) suggest similarities from the average structure analysis to PMN and other pseudo-cubic relaxor materials where well defined long-range order of polar displacements does not occur.
a) BFTM in the R3c space group with octahedral tilting, b) 0.75BFTM-0.25BT in R3m, which is untilted c) BFTM viewed down the [001] axis to show the tilting scheme. d) 0.75BFTM-0.25BT viewed down the [111] direction, showing the loss of the octahedral tilt. e) A-site polyhedra of BFTM, multiple bismuth atoms are visible because of the multiplicity of the off-axis displacement f) A-site polyhedra in 0.75BFTM–0.25BT, g) B site octahedra of BFTM and h) B site octahedra of 0.75BFTM–0.25BT. The centroid of each polyhedra is represented as the white sphere. In e and f, bonds are colored according to short (green), medium (black) and long (blue) bond lengths.
The presence of static or dynamic disorder is normally signalled in average structure analyses by the atomic displacement parameters. The displacement parameters of the B-site
can be considered normal with a Beq value of 1.22(5) ?2 (U = 0.015 ?2). However, the oxygen and A-site thermal parameters are abnormally large and temperature-independent, suggesting a significant static disorder component which is superimposed on the average structure. The large displacement parameters of the A-site and the oxygen combined with the severe over/under bonding of the A-site
are indicative of considerable local strain, which is consistent with the diffuse scattering in the electron diffraction patterns.
Despite the reduced displacements in 0.75BFTM-0.25BT compared to BFTM, the solid solution has enhanced ferroelectric properties. This can be associated with the change in structure due to the suppression of the octahedral tilt which is caused by the addition of barium atoms on the A-site. In the lead-based PZT system, diffraction contrast imaging by Randall and Eitel shows that the antiphase boundaries (APB) associated with the octahedral tilt coincide with non-180° ferroelectric domain walls, consistent with the conclusion from the frequency dependence of the strain–field measurements that this is the switching path. These APB regions have a strain mismatch between each tilt variant which pin the domain wall motion and suppress the extrinsic response in the R3c phase, but are not present in the R3m phase due to the absence of tilting. The introduction of barium on the A site has the same effect on the average structure of BFTM, providing an explanation for the enhanced ease of polarisation switching here.
On a local scale, the pattern of diffuse scattering in 0.75BFTM–0.25BT is similar to that of the well-known piezoelectric materials Pb(Zr,Ti)O3 (PZT) and Pb,La(Zr,Ti)O3 (PLZT). The transverse polarized diffuse streaks indicate the
displacements run perpendicular to the diffuse sheets along the [111]p axes. This means the A-site and B-site displacements are strongly correlated along a one dimensional chain in different &111&p directions with little to no correlation between each chain. The correlation length is estimated approximately as 5–10 unit cells calculated from the width, in reciprocal space, of the diffuse streaking—instrumental factors which are not easily measurable contribute to the width of the streaking. While this result seems to be in conflict with the average structure, which shows a displacement in only one &111&p direction, a study of the diffuse scattering of PZT by Welberry et al. used Monte Carlo techniques to show how this disordered model can include the local diffuse scattering while at the same time satisfying the Bragg conditions of the average structure. They developed models with more than 50% of the
being displaced along a single &111&p direction with the remaining
displacements distributed between each of the other seven &111&p directions. It was shown that these models will produce a diffraction pattern where the Bragg peaks fit an average structure that assumes no disorder while at the same time giving rise to the diffuse scattering patterns that were observed with . It is important to note that diffuse scattering can still be observed despite the fact that there will be a random distribution of 25% barium atoms on the A-site and 28% magnesium on the B-site, neither of which are prone to distortion. This does not affect the presence of diffuse scattering, but could potentially affect the correlation length.
The combined average structure and diffuse scattering information show that the present material is distinct from both ferroelectric PZT and relaxor PMN. The diffuse scattering in PMN condenses into rods, corresponding to correlated sheets of displacement, around the &110& type Bragg peaks rather than sheets in reciprocal space, whereas the average structure in PZT permits decisive identification of the displacement directions. In BFTM–BT the mean A
displacement along the rhombohedral [111]p direction of 0.24 ? is less than the static incoherent displacement of 0.30 ? deduced from the temperature invariance of the displacement parameters. The component of this three-dimensionally incoherent displacement along the non-rhombohedral [111]p directions (i.e. those body diagonal directions of the original cubic cell not selected for the three dimensionally correlated distortion) is correlated along each direction but not between them as shown by the pattern of diffuse scattering. The Pmm refinements show that other displacement directions are likely to be present but the diffuse scattering patterns indicate that these displacements are not correlated. These structural differences from the canonical relaxor and ferroelectric materials should impact the functional behaviour.
The ferroelectric properties of 0.75BFTM–0.25BT change quite profoundly from the parent material BFTM. The impedance data from BFTM shows a sharp single peak at 730 °C indicating a ferroelectric (FE) to paraelectric (PE) transition from R3c to the ideal cubic perovskite with the space group Pmm. The addition of BT (Tc = 120 °C) lowers the transition temperature as expected, but in the case of BFTM–BT, there are two distinct peaks that can be seen in the variable temperature impedance data. The frequency dispersive nature of the low temperature anomaly suggests that it can be attributed to the freezing of polar nanoregions. On the other hand, the position of the high temperature anomaly is invariant and more experiments will be needed to clarify its origin, but it is indicative of a polar transition to a polar nanoregion () state. This is a common effect and can be seen in NBT–BT, NBT–KBT, and BFO–BT. The high temperature peak is attributed to the phase transition while the low temperature peak is described as a relaxation associated with a reorientation of the polarization of the polar nanoregions due to the thermal fluctuations and both peaks are part of the same phenomenon.
The room temperature permittivity as a function of frequency shows that 0.75BFTM–0.25BT has a much higher permittivity than both BFTM and 0.9BFTM–0.1BT across the whole frequency range. As the BT content increases, the coercive field and Curie temperature decreases and ferroelectric hysteresis loops evolve as is discussed in the following paragraph. Therefore, it is expected that the relative permittivity i.e. electrical polarizability increases with increasing BT content. The relative permittivity shows a strong frequency dependence increasing with a decrease in frequency. Dielectric loss of the samples decreases with increasing frequency in the low frequency regime (with the exception of the 0.9BFTM–0.1BT composition, where a broad relaxation peak dominates the low frequency behaviour), while an increase is obvious at high frequencies. The increase at low frequencies can be assigned to hopping conduction, while that at the highest frequencies may be due to inductive contribution from the wires. For BFTM, a broad loss peak at high frequencies is evident, suggesting the presence of a relaxation phenomenon. A similar peak in loss is also observed for 0.9BFTM–0.1BT at low frequencies, while in 0.75BFTM–0.25BT this feature cannot be seen. Corresponding to the peaks in the dielectric loss, the slope of the dielectric constant curves changes with frequency. Cole–Cole plots (Figure S5) reveal clearly one semicircle for BFTM, showing that dielectr the tails at high and low frequency ends correspond to the increase in dielectric loss at the lowest and highest frequencies. For both 0.9BFTM–0.1BT and 0.75BFTM–0.25BT compositions, a semicircle can also be assigned, while the shape is not well-defined due to the contribution from high and low frequency tails. The magnitude of dielectric constant rules out the possibility of contribution from extrinsic charge carriers of interfacial charges at grain boundaries or electrodes. Thus, we conclude that the dipoles that take part in the dielectric relaxation process, which are responsible for frequency dispersion of dielectric constant and loss, are intrinsic. The loss values at 1 kHz do not seem to follow a trend with values of 0.040, 0.076, and 0.049 for BFTM, 0.9BFTM–0.1BT, and 0.75BFTM–0.25BT respectively. The loss of other well known ferroelectrics is: BaTiO3 (0.014), PZT (0.015), and 0.67BFO–0.33PZT (0.18). The loss of BiFeO3 has been reported at 0.03 in non-quenched samples which do not have saturated P(E) loops and 0.1 in quenched samples with saturated P(E) loops.
The ferroelectric properties evolve significantly upon addition of BT to BFTM. The P(E) loop for BFTM clearly does not reach saturation even at applied electric fields of approximately 140 kV cm-1. This is likely due to the high Tc of BFTM and a large coercive field. Although the Pr value for 0.9BFTM–0.1BT (2.3 μC cm-2) is higher than BFTM (0.5 μC cm-2), the P(E) curve of 0.9BFTM–0.1BT is comparable to the parent compound as both show non-saturated loops at sub-switching fields. 0.75BFTM–0.25BT shows the best ferroelectric properties of the three materials. 0.75BFTM–0.25BT, which has a smaller Tc than BTFM has a P(E) loop which indicates switching behaviour and becomes nearly saturated at an applied field of 90 kV cm-1. The loops of this material show a large remanent polarization despite a coercive field of 44 kV cm-1 at the frequency of the measurement. PUND measurements were essential to characterise the switchable component of this polarisation, which is of the order of 10% of the total. The difference between the remanent and saturation polarisations in P(E) (which contain field-driven contributions such as conductivity, capacitance and non-permanent polarisation) and the true permanent switchable polarisation in PUND indicates that although the switchability is improved over BFTM there are mechanisms which dissipate the polarisation in 0.75BFTM-0.25BT. One of the main contributions to the discrepancy between the observed remanence and switchable polarization could be from PNRs which reorient during the P–E cycle, adding to the hysteresis and observed remanent polarization, but thermally relax between the pulses of the PUND measurement. The regions producing the polarisation response also generate the observed piezoelectric behaviour.
In regards to the piezoelectric properties, the maximum strain value obtained is 0.068%, which is lower than that of PZT (0.3%) and BiFeO3 (0.17%) but is of a similar magnitude to NBT-BT solid solution (0.06%). The measured d33 value of 16 pC N-1 for 0.75BFTM–0.25BT is a significant improvement from the d33 of BFTM which was found to be 0.6pC N-1. The d33 of 0.75BFTM-0.25BT is one-third of the high field value obtained from the unipolar strain field measured at 1Hz. This is not uncommon as piezoelectric coefficients obtained from induced strain measurements can be significantly higher than those obtained from direct measurements due to the possibility of better poling under larger fields that cannot be reached before breakdown or the domain walls may be more prone to displacement under large unipolar electric fields. The dielectric and ferroelectric results show that the sample has a high Tc and large Ec, which also causes the difficulties in poling the sample completely. A similar phenomenon was also found in other lead-free piezoelectrics, such as (Bi0.5Na0.5)TiO3 (Ec = 73kV cm-1), and Bi0.5K0.5TiO3 (Ec = 52.5 kV cm-1). Two materials that can easily be poled are BaTiO3 which has an Ec of 26 kV cm-1 and PZT which has an Ec of 7kV cm-1. The d33 of common piezoelectric materials are BiFeO3: 16 pC N-1, BaTiO3: 190 pC N-1, and PZT 53/47: 220 pC N-1. The functional properties of the Pb-based systems vary sharply with composition e.g. the PbTiO3 end member of PZT is not switchable, suggesting that further property improvement will require more detailed compositional variation.
The introduction of BaTiO3 into BFTM increases the tolerance factor, suppresses the octahedral tilting and induces a phase transition from the polar space group R3c to a pseudocubic structure (at 25% BT doping) best described by polar space group R3m. Although the displacements in 0.75BFTM–0.25BT are smaller, the absence of the octahedral tilt allows for easier ferroelectric switching, which enhances the ferroelectric and piezoelectric properties over those of BFTM. The switchable polarisation is only 10% of the total observed in P(E) measurements. The combination of average and local structure shown by the Bragg diffraction and diffuse scattering shows that the nature of the A-site displacements in BFTM-BT differs from the classical ferroelectric and relaxor materials. Displacements are correlated three-dimensionally along a privileged [111]p axis which confers a small metric rhombohedral distortion but the displacements along the other 7 initial cubic [111]p directions are one-dimensionally correlated, producing a pseudo-cubic average structure with different extents of [111]p displacement correlation combined with displacements along other directions. This combination of long-range and local distortions affects the average structure by making the component of the total Bi/Ba displacement which is not three-dimensionally correlated along the unique rhombohedral axis larger than the coherent displacement along the rhombohedral direction. This structural change is coupled with the observed enhancement of the functional behaviour.
This work is funded by the European Research Council (ERC Grant agreement 227987 RLUCIM), the Knowledge Centre for Materials Chemistry and the EPSRC (/H000925).
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Footnotes+ Electronic supplementary information (ESI) available. See DOI: ? This work is funded by the European Research Council (ERC Grant agreement 227987 RLUCIM) and the EPSRC.This journal is (C) The Royal Society of Chemistry 2012