The effect of the slurry formulation upon the morphology of spray-dried yttria stabilised zirconia particles

A study has been performed to determine how the characteristics of spray-dried granules prepared from aqueous yttria doped zirconia slurries can be affected by the spray-drying process parameters: dispersant amount, pH and binder type. First, the colloidal stability of aqueous zirconia suspensions as a function of polyacrylic acid content, pH and binder nature has been investigated in order to establish a stability map. The concentration of dispersant required to stabilise the zirconia suspensions decreases with increasing pH (for instance Γ_max=0.93 mg m⁻² at pH 4 and 0.1 mg m⁻² at pH 10). The addition of a binder may modify the state of dispersion. The stability map makes it possible to define regions of stable (dispersed) and unstable (flocculated) suspensions. Then, dried granules have been produced by means of a single droplet drying method. Granules prepared from a dispersed slurry (low Ratio Sediment Height—RSH <53%) result in hollow shapes that contain a single large open pore. Changing the nature of the binder from latex to an hydrosoluble compound (polyvinyl alcohol) has an effect on the wall thickness of the hollow granules. Conversely, granules obtained from a flocculated slurry (high RSH>62%) result in full granules. The sedimentation volume, which represents the state of dispersion of the suspension, is the major factor controlling the droplet drying mechanism.     

Aqueous processing of Ce0.8Sm0.2O1.9 green tapes

An aqueous tape casting of Ce_0.8Sm_0.2O_1.9 (SDC) ceramics was developed using poly(acrylic acid) (PAA) as dispersant, poly(vinyl alcohol) (PVA) as binder, poly(ethylene glycol) (PEG) as plasticizer, and deionized water as solvent. Surface properties of SDC powder with and without PAA dispersant were characterized by electrokinetic measurements. The rheology of the SDC slurries was evaluated with a rotary viscometer. The zeta potential measurement showed that the isoelectric point (IEP) for SDC powders in the absence of dispersant corresponds to a pH value of 3.66. The experimental results showed that the pH value greatly affects the rheology of the slurry. The optimum content to get a stable dispersed slurry is 2 wt% PAA in pH value range of 9–10. In presence of 2 wt% PAA dispersant, 55 wt% SDC powders exhibited shear thinning behavior, indicating that SDC slurry was homogenous and well stabilized. Homogeneous, smooth, and defect-free green tapes were successfully obtained by an appropriate slurry formula.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

Aqueous tape casting and crystallization kinetics of Ce0.8La0.2O1.9 powder

An aqueous tape casting of Ce_0.8La_0.2O_1.9 (LDC) ceramics was developed using PAA as dispersant, PVA as binder, PEG as plasticizer, and deionized water as solvent. Surface properties of LDC powder with and without PAA dispersant were characterized by electrokinetic measurements. The rheology of the LDC slurries was evaluated with a rotary viscometer. The zeta potential measurement showed that the isoelectric point for LDC powders in the absence of dispersant corresponds to a pH value of 4.02. The experimental results showed the pH value greatly affects the rheology of the slurry. The optimum content to get a stable dispersed slurry is 1.5 wt% PAA at pH value of 9–10. In presence of 1.5 wt% PAA dispersant, 5 wt% PVA binder, 5 wt% PEG plasticizer, and 55 wt% LDC powders exhibited shear thinning behavior, indicating that LDC slurry was homogenous and well stabilized. With an appropriate formulation homogeneous, smooth, and defect-free green tapes were successfully obtained. Moreover, the crystallization kinetics of LDC powders prepared by coprecipitation process also has been investigated in this study. The activation energy of crystallization was calculated on the basis of differential scanning calorimetry (DSC) at different heating rates. From non-isothermal DSC data presented values in the range of 343.3–379.1 kJ/mol and 2.282–2.030 for the activation energy of crystallization and the Avrami exponent, respectively, at specific temperatures ranging from 280 to 285 °C.     

Tape casting of Y-TZP with low binder content

The rheology of Y-TZP slurries with a low content of an acrylic/styrene binder is studied. The optimum amount of dispersant was found to be 0.3 wt.% for the starting slurry. After a 5 wt.% of binder addition the dispersant amount needed to achieve a minimum viscosity increases to 0.5 wt.%. Tapes obtained from slurries with a solid content of 45 vol.% shows the higher green density of 51 th.%. Higher solid contents do not yield to better green densities due to the high viscosity.     

Gelling behavior of MgAl2O4 slurries with a common dispersant

MgAl₂O₄ transparent ceramics were shaped by a commonly used polyacrylic acid (PAA), which acted as both dispersant and gelling agent. The spinel slurries were prepared by ball-milling MgAl₂O₄ powder, PAA, and water in an attrition mill. The gelling of slurries happened at room temperature in air atmosphere without any other organic additive. The gelling mechanism was the formation of chelates between Mg²⁺ and carboxyl groups (-COO⁻) of PAA. The frequency-based testing method was applied to investigate the gelling process of the as-prepared slurry. In addition, a novel in situ characterization method based on a modified indentation testing was invented to better understand the strengthening of the wet green body with time and to guide when demolding could be carried out. After sintering, transparent MgAl₂O₄ ceramics with high in-line transmittance were resulted.     

Effect of ball-milling parameters on the rheological properties of mold powder slurry

In this study, mold powder slurries with high solid loading and low viscosity were prepared during the ball-milling process for improving the homogeneity and mechanical properties of granules after spray-drying. The effect of ball-milling parameters, such as solid loading, binder/dispersant content, and ball-milling time, on the flowability, dispersibility, stability, and rheological behavior of mold powder slurries was systematically investigated by rheology observation and sedimentation tests. As these parameters varied, the slurry exhibited the shear-thinning behavior of a non-Newtonian fluid with a shear rate range of 0–50 s^?1, which was adequately described by the Herschel-Bulkley model. The optimal parameters that optimized the flowability, dispersibility, and stability of the slurry, along with its rheological behavior, were chosen as follows: solid loading, 60 wt%; modified sodium carboxymethyl cellulose binder content, 1.0 wt%; sodium tripolyphosphate dispersant content, 0.5 wt%; ball-milling time, 60 min.     

High solid loading,low viscosity photosensitive Al2O3 slurry for stereolithography based additive manufacturing

Photosensitive Al₂O₃-resin slurries with high solid loading, low viscosity used for stereolithography based additive manufacturing were prepared in this paper. The dispersion behavior and rheological behavior of the Al₂O₃-resin slurries were studied by rheology observation and sedimentation tests. The dispersant type, concentration and solid loading had significant effects on the rheological behavior and stability of the photosensitive Al₂O₃-resin slurries. A long term stability and homogeneity slurry was obtained when the dispersant and concentration were KOS110 and 5?wt%, respectively. The Al₂O₃ slurry prepared with a high solid loading up to 60?vol%, low viscosity of 15.4?Pa?s at 200?s^?1 was chosen for stereolithography based additive manufacturing.     

Fabrication of transparent yttria ceramics by alcoholic slip-casting

Y₂O₃ transparent ceramics were prepared from alcoholic slurries of Y₂O₃ nanopowders via a slip-casting method to avoid the hydrolysis issue. Polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and polyethylenimine (PEI) were used as dispersants to improve the rheological properties of the slurries. It was found that PEI is the most effective dispersant in ethanol. The adsorbed amount of PEI was evaluated by infrared absorption and rheology measurements. Y₂O₃ slurry with a solid loading of 20.8 vol% and a viscosity of <0.1 Pa s at the shear rate of 10 s⁻¹ was obtained using 1.5 wt% PEI. The slurry yielded a homogeneous green body, and finally resulted in a high-quality Y₂O₃ ceramic with the in-line transmittance of 80% at 800 nm.     

Si@S-doped C anode with high cycling stability using PVA-g-PAA water soluble binder for lithium-ion batteries

The nanostructured Si@S-doped C (Si/C) hybrid is synthesized via a mild hydrothermal process of glucose and the simultaneous polymerization of 3,4-ethylenedioxythiophene and poly(sodium-4-styrene sulfonate) on the surface of nano-Si powders, then followed by a calcination process. A new water soluble polymer of poly(vinyl alcohol) (PVA) grafted with poly(acrylic acid) (PAA) is synthesized via a free-radical polymerization of acrylic acid (AA) in a 1:1 weight ratio in PVA water solution. The Si/C anode using PVA-g-PAA binder, exhibits improved lithium storage properties and cycling stability than that of the parallel electrode with carboxymethyl cellulose binder, which exhibits an initial coulombic efficiency (82.0%), even a reversible capacity of 487 mAh g⁻¹ after 300 cycles with 81.2% capacity retention at 0.4 A g⁻¹. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48764.     

Additive manufacturing of SiC-Sialon refractory with excellent properties by direct ink writing

Additive manufacturing of SiC-Sialon refractory with complex geometries was achieved using direct ink writing processes, followed by pressureless sintering under nitrogen. The effects of particle size of SiC powders, solid content of slurries and additives on the rheology, thixotropy and viscoelasticity of ceramic slurries were investigated. The optimal slurry with a high solid content was composed of 81 wt% SiC (3.5 µm+0.65 µm), Al₂O₃ and SiO₂ powders, 0.2 wt% dispersant, and 2.8 wt% binder. Furthermore, the accuracy of the structure of specimens was improved via adjustment of the printing parameters, including nozzle size, extrusion pressure, and layer height. The density and flexural strength of the printed SiC-Sialon refractory sintered at 1600 °C were 2.43 g/cm³ and 85 MPa, respectively. In addition, the printed SiC-Sialon crucible demonstrated excellent corrosion resistance to iron slag. Compared to the printed crucible bottom, the crucible side wall was minimally affected by molten slag.     

Preparation of Cu(II)‐chelated poly(vinyl alcohol) nanofibrous membranes for catalase immobilization

Poly(vinyl alcohol) (PVA) nanofibers were formed by electrospinning. Metal chelated nanofibrous membranes were prepared by reaction between Cu(II) solution and nanofibers, and which were used as the matrix for catalases immobilization. The constants of Cu(II) adsorption and properties of immobilized catalases were studied in this work. The Cu(II) concentration was determined by atomic absorption spectrophotometer (AAS), the immobilized enzymes were confirmed by the Fourier transform infrared spectroscopy (FTIR), and the amounts of immobilized enzymes were determined by the method of Bradford on an ultraviolet spectrophotometer (UV). Adsorption of Cu(II) onto PVA nanofibers was studied by the Langmuir isothermal adsorption model. The maximum amount of coordinated Cu(II) (q_m) was 2.1 mmol g⁻¹ (dry fiber), and the binding constant (K_l) was 0.1166 L mmol⁻¹. The immobilized catalases showed better resistance to pH and temperature inactivation than that of free form, and the thermal and storage stabilities of immobilized catalases were higher than that of free catalases. Kinetic parameters were analyzed for both immobilized and free catalases. The value of V_max (8425.8 μmol mg⁻¹) for the immobilized catalases was smaller than that of the free catalases (10153.6 μmol mg⁻¹), while the K_m for the immobilized catalases were larger. It was also found that the immobilized catalases had a high affinity with substrate, which demonstrated that the potential of PVA‐Cu(II) chelated nanofibrous membranes applied to enzyme immobilization and biosensors. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of Concentration of Ammonium Poly(acrylate) Dispersant and MgO on Coagulation Characteristics of Aqueous Alumina Direct Coagulation Casting Slurries

Coagulation of aqueous alumina slurries prepared using various concentrations (0.43–1.04 wt% based on alumina) of ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC) of alumina. The slurries prepared at dispersant concentration below 0.84 wt% are not suitable for DCC at room temperature (∼30 ° C) as they undergo premature coagulation. Mixing the slurry with MgO at a low temperature of nearly 5 ° C slows down the reactions leading to coagulation and keeps the slurry viscosity low for a sufficient period of time. Coagulation of slurries prepared at a dispersant concentration of 0.92 wt% and above at room temperature requires MgO concentrations much higher than the equivalent amount required for reaction with the dispersant. This anomalous behavior at higher dispersant concentration is explained such that the Mg-poly(acrylate) formed by the reaction between ammonium poly(acrylate) and MgO formed a sheath over the remaining MgO particles and prevented them from further dissolution at room temperature. Faster coagulation could be achieved by heating the slurries after casting in closed molds. The Mg-poly(acrylate) acts as a binder and stabilizes the coagulated bodies as their strength and stability against oscillatory stresses increase with an increase in dispersant concentration.     

Gelation and Degelation of PVA in Aqueous BaTiO3 Slurries

Poly(vinyl alcohol) (PVA) gels favorably with boron oxide (B₂O₃) in aqueous slurries of BaTiO₃, resulting in a dilatant rheology, but the gelation and dilatancy can be efficiently reduced by the use of hydroxylated BaTiO₃. The degelation effect caused by the use of hydroxylated BaTiO₃ was studied, and the corresponding degelation mechanisms were clarified. Interestingly, the hydroxylated BaTiO₃ interacts better than the as‐received form with PVA and B₂O₃, and the hydroxylated form is more chemically stable and less soluble in the aqueous slurry. Based on these specific properties of hydroxylated BaTiO₃, especially its good chemical stability, the free concentrations of PVA and ^?B(OH)₄ in the aqueous slurry are efficiently diminished. The decreased free concentrations of PVA and B₂O₃ facilitate the reduction of the gelation opportunity between them, leading to a great improvement in the rheological behavior of the aqueous slurries.     

Tape casting of PLZST tapes via aqueous slurries

A well-dispersed aqueous slurry for tape casting of PLZST (Pb_0.97La_0.02Zr_0.66Sn_0.23Ti_0.11) was prepared. Properties of PLZST slurries with and without polyelectrolyte dispersant were characterized by zeta potential, sedimentation etc. The experimental results show that the pH value of the slurries is changing with ball-milling time, and that the polyelectrolyte dispersant has a significant effect on the isoelectric point of PLZST. TGA analysis indicates that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. The density of PLZST ceramics is tightly related to the plasticizer and the sintering temperature. However, the results demonstrated that it is possible to prepare a dense PLZST thick film through aqueous slurry tape casting.     

Reinforcing effect of graphene nanoplatelets in the electrochemical behaviour of manganese oxide-based supercapacitors produced by EPD

A simple procedure for the manufacture of metal oxide based supercapacitors consisting on the electrophoretic deposition (EPD) onto graphite paper of well-dispersed aqueous slurries of manganese oxide with graphene nanoplatelets is presented. EPD was performed using optimised slurries and depositing onto graphite paper electrodes. In order to improve manganese oxide conductivity graphene nanoplatelets (GNP) were incorporated in a concentration of 20 wt% over the MnO₂ content. The evaluation of the electrochemical properties of the so prepared deposits demonstrates that the addition of GNP improves both the capacity and specific energy, with values of 158 F g⁻¹ and 4 W h Kg⁻¹ respectively, which represents four times the values reported for the bare manganese oxide.     

Eco-friendly tape casting of borosilicate glass/Al2O3 sheets for LTCC applications

This work reports the innovative development of a borosilicate glass/Al₂O₃ tape for LTCC applications using an eco-friendly aqueous tape casting slurry. Polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) were the respective dispersants, while carboxymethyl cellulose (CMC) and styrene acrylic emulsion (SA) were the respective binders. The results showed that PVP was more suitable than PAA as the dispersant for the aqueous casting slurry, and that 1.5 wt% PVP would achieve well dispersion of CABS glass/Al₂O₃ powder in the aqueous slurry. Moreover, a small amount of 2.0 wt% CMC binder could yield smooth CABS glass/Al₂O₃ tapes crack free. A high-quality CABS glass/Al₂O₃ tape with a smooth surface was made from an aqueous slurry containing 1.5 wt% PVP dispersant, 2.0 wt% CMC binder, and 2.0 wt% PEG-400 plasticizer. The density, tensile strength, and surface roughness of the green tape were 2.05 g/cm³, 0.87 MPa, and 148 nm, respectively. The resulting CABS glass/Al₂O₃ composites sintered at 875 °C exhibited a bulk density of 3.14 g/cm³, a dielectric constant of 8.09, a dielectric loss of 1.0 × 10^?3, a flexural strength of 213 MPa, a thermal expansion coefficient of 5.30 ppm/^°C, and a thermal conductivity of 3.2 W m^?1 K^?1, thus demonstrating its broad prospects in LTCC applications.     

Partially lithiated ternary graft copolymer with enhanced elasticity as aqueous binder for Si anode

Partially lithiated ternary graft copolymers were synthesized through free radical graft polymerization of acrylic acid (AA), lithium acrylate (LiAA) and hydroxyethyl acrylate (HEA) onto polyvinyl alcohol (PVA). With optimized feeding molar ratio of AA/LiAA/HEA (2:1:2), the ternary graft copolymer with partial lithiation shows the best flexibility, elasticity and adhesion strength comparing to PVA-g-PAA and PVA-g-P(AA-HEA) when used as aqueous binder for Si anode. The Si anode using PVA-g-P(AA-LiAA-HEA) with the optimized molar ratio of AA-LiAA-HEA exhibits better cycling stability and rate performance, delivering a capacity of 2265 mAh g⁻¹ with a capacity retention of 82.4% after 200 cycles at 1A g⁻¹. Even at a high current of 10 A g⁻¹, the Si electrode still obtained a high capacity of 1300 mAh g⁻¹.     

A new processing aid for dry-pressing: A copolymer acting as dispersant and binder

PVX copolymers containing both carboxylate (COO⁻) and hydroxyl groups (vinyl alcohol) are synthesized with the objective to ensure the double function of dispersant and of binder for dry-pressed green parts.Carboxylate groups are responsible for strong adsorption of copolymers onto alumina surface and can promote sufficient electrostatic repulsive forces to achieve a good state of dispersion, similar to that obtained with a classical ammonium polymethacrylate (PMA-NH₄⁺) for a proportion of carboxylic groups in the copolymer larger than 35%. On the other hand, hydroxyl groups confer higher mechanical strength than PVA to the green pressed parts (4.2 MPa for 1.5 wt.% PV35 (35% COO⁻) versus 1.8 MPa for 1.5 wt.% PVA, with and without PEG as plasticizer, in the case of samples pressed at 120 MPa).The high density of the green parts obtained with soft PV35 copolymer and the good adsorption of this compound onto alumina surface lead to a high mechanical strength of the pressed compacts. After addition of 1.5 wt.% PV35, the amount adsorbed (0.7 wt.%) confers a high cohesion to the spray-dried granules and the non-adsorbed copolymer (0.8 wt.%) reinforces adhesion between granules through interdiffusion of the low T_g polymeric films.Then, synthesized PVX copolymers have proved to be efficient dispersants for aqueous suspensions of alumina and also efficient binders for dry-pressing, even for concentrations as low as 1.5 wt.%.     

Granule performance of zirconia/alumina composite powders spray-dried using polyvinyl pyrrolidone binder

Polyvinyl pyrrolidone (PVP) was used as a binder in spray-drying a slurry containing zirconia/alumina composite powder and its influence on granulation and granule deformability was compared with those of polyvinyl alcohol (PVA) and polyethylene glycol–hydroxyethyl cellulose cobinder (PEG–HEC). Although the most spherical solid granules were obtained from the slurry containing PEG–HEC, the granules containing PVP were the most deformable during compaction. It was apparent that a high-viscosity organic additive mixture added to the slurry resulted in highly spherical solid granules, and a low T_g of the mixture led to a high deformability. The flexural strengths of composites prepared from granules containing PVP, PEG–HEC, and PVA were 634, 578, and 468 MPa, respectively, which corresponds to the ascending order of T_g of the binders mixed with plasticizers.