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Unwanted changes in powder flow behavior can unexpectedly occur when a product is exposed to certain conditions of temperature and humidity. This can happen during production, but also during transport or storage. The work reported here demonstrates the novel approach of using an amended powder rheology set-up for measuring and predicting such changes in powder flow behavior. The developed methodology makes it possible to vary in-situ the temperature and the relative humidity of the air to which the product is exposed, thereby mimicking realistic conditions of production or related unit operations. An air flow capable of fluidizing the powder particles is controlled at a specific constant temperature and its relative humidity can be altered while measuring the torque in the fluidized powder bed in real time. The fluidization is necessary for generating a homogeneous introduction of temperature and relative humidity. Results obtained using citric acid and commercial coffee whitener products have proven this methodology to provide both similar and in certain instances dissimilar results compared to the more established methodology such as measuring the vapour adsorption isotherms. These observations are explained. In this way, it can be predicted under which combinations of temperature and humidity a product does or does not become sticky. The main advantages of our approach are that the flow properties are directly assessed, the interpretation of the obtained data is more straightforward and that the measurement times are shortened substantially.

Industrial powder coating processes offer a unique object of application for rheometers. Compared to classic coating procedures with liquid materials, powder coating is still a relatively young procedure. First industrial applications date back to the mid-1960s. Since then these applications have considerably increased, which can be mainly attributed to the desire for more environment-friendly coating systems. The powder coating process is dependent on one end on pneumatic conveyance behavior, namely the transport as a dense fluidized phase of powder and the associated bulk solid values, on the other end on bulk polymer behavior to determine and keep constant the curing point at which the applied powder bonds and produces an (ideally) smooth surface. This multivariant process is an ideal object of quality control for the MCR rheometers and two accessories: the Powder Cell and a disposable Plate–Plate Geometry with Peltier heated hood (P-PTD200, H-PTD200). In this work we will show the influence of both artificial aging behavior as well as flow aids on the powder flow properties (collapsed and fluidized) as well as the melting and polymerization steps and show a way to optimize processes utilizing this complex system.

Torque measurements were performed with the powder cell mounted on an Anton Parr rheometer using glass beads of two different sizes and different impeller geometries. A discrete element method simu...

The Anton Paar Powder Cell was used to measure the torque necessary to rotate an impeller in beds of glass beads, sand and alumina powders aerated between no aeration to the minimum for fluidization. Measured torque values depend on the material tested, on the air flow rate applied, on the impeller depth and on the height of the impeller blade. The effect of the impeller depth is linear for low impeller depth and is less than linear at high depth values. A model was developed for the interpretation of the experimental results based on the idea that the material is shearing on the surface described by the impeller rotation. The model allows to estimate an effectiveness of the impeller in the torque determination and also to predict the torque for the impeller at the at deepest positions at which the wall effects have to be considered. © 2017 American Institute of Chemical Engineers AIChE J, 2017

A powder rheometer equipped with a cylindrical impeller was studied to assess whether it could be suitable to derive quantitative physical parameters of powder flow properties. The equipment used in was the Powder Cell unit purposely developed by Anton Paar to fit on Anton Paar MCR Rheometers and to carry out torque measurements on aerated and not aerated powders. In this instrument, the state of consolidation of the powder can be adjusted by changing the impeller depth and the aeration rate, and very low consolidation stresses can be achieved. A cylindrical impeller geometry was tested on free flowing and easy flowing powders. A simple model was developed to relate the measured torque with the effective angle of internal friction in the powder measured with a standard powder shear tester. The results indicated the clear relationship between the effective angle of internal friction and the measured torque. The inversion of the calculation procedure highlighted that the Couette system does not always allow estimating the effective angle of internal friction reliably. In fact, small experimental error in the torque measurement may lead to large changes of the estimated effective angle of internal friction.

Various compositions of the solid solution system (100 − x ) Bi 0,5 Na 0,5 TiO 3 – x Bi 0,5 K 0,5 TiO 3 ( x = 0, 10, 25, 50, 75, 90, 100) were prepared by the mixed oxide route. The formation reaction was analyzed by thermogravimetry coupled with mass spectroscopy and differential scanning calorimetry. In situ high temperature X-ray diffraction up to 770 °C indicated emerging and vanishing of phases during the calcination. Intermediate phases such as alkalipolytitanate (Na/K) 2 Ti 6 O 13 and bismuth titanate Bi 2 Ti 2 O 7 were identified as forming the perovskite phase. The formation reactions were proposed based on the data obtained. Furthermore the microstructure and the dielectric behavior of the sintered samples were observed by scanning electron microscopy, impedance spectrometry and polarization measurements.

The influence of Ti-nonstoichiometry in BNT was investigated using XRD, SEM/EDX and electrical measurements. It is shown that nonstoichiometric compositions of BNT derived materials tend to maintain stoichiometry by forming a secondary phase like TiO 2 or by the evaporation of volatile compounds like Bi 2 O 3 and Na 2 O. Due to this ability, variations of the Ti-content can be used to control the grain size of the material with little impact on electrical properties. The data also implies that the used processing temperatures lead to evaporation approximately 2 mol% of A-site cations during preparation since the sample with 1 mol% Ti-deficiency still shows TiO 2 secondary phase.

The electromechanical behavior of a lead-free ceramic multilayer actuator is described. The actuator is made of an Nd-doped [Bi0.500Na0.335K0.125Li0.040]TiO3 ceramic with inner electrodes of an Ag/Pd-alloy. For electromechanical characterization, the actuator is under constant mechanical stress, and the strain over the applied electric field is recorded. The mechanical stress ranges from 0.1 to 80 MPa. The maximum applied electric field is 8 kV/mm. Starting with 0.1 MPa mechanical stress, the actuator exhibits a maximum strain of

The main aim of this paper is to characterize experimentally the flow behaviour of the green sand that is used for casting of sand moulds. After the sand casting process is performed, the sand moulds are used for metal castings. The rheological properties of the green sand is important to quantify as they can be used to evaluate whether the casting process will be successful. In addition, the properties can potentially be implemented in a computational fluid dynamics model which can be used as a tool to optimize the process. The rheological experiments are carried out on a MCR 502 rheometer with a new module for characterizing granular materials. The new module enables viscosity measurements of the green sand as function of the shear rate at different flow rates, i.e. 0, 2, 4, 6, 8, 10, 12 and 15 L/min. The results show generally that the viscosity decreases with both the shear- and flow rate. In addition, the measurements show that the green sand flow follows a shear-thinning behaviour even after the full fluidization point.

The synthesis of Bi 0.5 Na 0.375 K 0.125 TiO 3 (BNT–BKT or NBT-KBT) by the mixed oxide route was investigated using thermogravimetry, X-ray diffraction, scanning electron microscopy and electron microprobe analysis. The results show the formation of a paraelectric secondary phase with the composition (K,Na) 2 Ti 6 O 13 during sintering. Thermogravimetric measurements indicate that the main loss of volatile components mainly occurs during the calcinations step, exceeding the theoretical loss by more than 2%. These losses, most likely caused by evaporation of K 2 O induce a high number of Schottky-vacancies. These vacancies highly affect the electrical properties of the material. The formation of the secondary phase leads to a shifted composition of the ceramic to approximately Bi 0.5 Na 0.4 K 0.1 TiO 3 .

Bismuth sodium titanate (BNT)-derived materials have seen a flurry of research interest in recent years because of the existence of extended strain under applied electric fields, surpassing that of lead zirconate titanate (PZT), the most commonly used piezoelectric. The underlying physical and chemical mechanisms responsible for such extraordinary strain levels in BNT are still poorly understood, as is the nature of the successive phase transitions. A comprehensive explanation is proposed here, combining the short-range chemical and structural sensitivity of in situ Raman spectroscopy (under an applied electric field and temperature) with macroscopic electrical measurements. The results presented clarify the causes for the extended strain, as well as the peculiar temperature-dependent properties encountered in this system. The underlying cause is determined to be mediated by the complex-like bonding of the octahedra at the center of the perovskite: a loss of hybridization of the 6s2 bismuth lone pair interacting with the oxygen p-orbitals occurs, which triggers both the field-induced phase transition and the loss of macroscopic ferroelectric order at the depolarization temperature.

A piezoelectric multilayer device based on bismuth–sodium–titanate ceramics (BNT) co-fired with Ag/Pd inner electrodes is described and its dielectric and piezoelectric properties are measured. The ceramic powder and tape was made by a water-based preparation process. After printing with Ag/Pd paste for inner electrodes these tapes were stacked to a multilayer device with 50 active layers. This device exhibited a large and temperature independent strain around 0.19% between 25 °C and 150 °C. The large strain is due to a field-induced phase transition. The low temperature dependence results from the broadening of the nonpolar phase by doping.

The phase diagram of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 was completed and investigations on polarization and strain in this system were carried out. (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (Td), the temperature of the rhombohedral–tetragonal phase transition (Tr–t) and the Curie temperature (Tm) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO3-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.

The electromechanical behaviour of a lead-free ceramic multilayer actuator is described. The actuator is made of a Nd-doped bismuth-alkali-titanate ceramic with inner electrodes of an Ag/Pd-alloy. For electromechanical characterization the actuator is under constant mechanical load and the strain over the applied electric field is recorded. The mechanical load ranges from 0.1 to 80 MPa. The maximum applied electric field is 8 kV/mm. Starting with 0.1 MPa mechanical load the actuator exhibits a maximum strain of less than 0.1%. With increasing load the strain increases up to 0.17 % at 5 MPa. With higher load the strain decreases and reaches 0.125% at 80 MPa. Reducing the mechanical load leads to a steady increase in strain, which results in a doubling of the strain at 0.1 MPa compared to the initial value. The strain characteristic contains an electrostrictive part, a linear piezoelectric regime as well as some creep behaviour.