Freeze Granulation Research Articles

Effect of phase composition on thermal and electrical conductivity of additive-free silicon carbide ceramics

Dense (RD > 97 %) silicon carbide ceramics without any sintering additives were prepared by freeze granulation of silicon carbide powders, annealing of granulated powders and subsequent densification by field assisted sintering technique (FAST) at 1900°C. Thermal and electrical conductivity of prepared materials considering the mass fraction of SiC polytypes was investigated. Different ratio of alpha and beta silicon carbide has been achieved by adjusting dwell time during sintering. Thermal diffusivity, specific heat capacity, thermal conductivity, electrical conductivity, density, microstructure of additive-free SiC with various α/β content has been investigated. The electrical conductivity of SiC ceramics decrease from 104 S/m to 8 S/m as a content of α-SiC increase from 63 % to 94 %. Opposite trend, when thermal diffusivity increased from 47.3 to 69.9 mm2/s as a content of α-SiC increase from 63 % to 94 %, was observed. The highest thermal conductivity (λ=165 W/m.K) among the SiC ceramics sintered at temperatures ≤ 1900°C was achieved for additive-free SiC sintered at 1900°C under vacuum for 80 minutes.

A Review of Recent Techniques for Granulation

Granulation is the process of designing particles by combining primary power particles into massive multiparticle structures known as granules. Its primary purposes are to boost the flow of power, prevent segregation, improve the compaction characteristics of mix uniformity, and improve the flow and compressibility of powders. It also produces a combination that is uniform. Wet granulation and dry granulation are the two primary categories of granulation methods. The granulation process is contingent upon the description of the ingredient entity and its ability to surge appropriately, compact, and collapse. Choosing the appropriate granulation method necessitates a methodical examination of each ingredient in the combination and how they interact. Granule particle size is determined by the amount and feeding rate of the granulating liquid. New methods have been added to the granulation process to improve it: foam binder granulation, fluidized bed granulation, melt granulation, activated dry granulation, heat adhesion granulation, steam granulation, spray drying granulation, and freeze granulation. The major purpose of this page is to provide an overview of each advancement, including its limitations and relevance.

Full Density Powder Metallurgical Cold Work Tool Steel through Nitrogen Sintering and Capsule-Free Hot Isostatic Pressing

Vanadis 4E (V4E) is a powder metallurgical cold work tool steel predominantly used in application with demand for wear resistance, high hardness, and toughness. It is of interest to have a processing route that enables full density starting from clean gas-atomized powder allowing component shaping capabilities. This study presents a process involving freeze granulation of powder to facilitate compaction by means of cold isostatic pressing, followed by sintering to allow for capsule-free hot isostatic pressing (HIP) and subsequent heat treatments of fully densified specimens. The sintering stage has been studied in particular, and it is shown how sintering in pure nitrogen at 1150 °C results in predominantly closed porosity, while sintering at 1200 °C gives near full density. Microstructural investigation shows that vanadium-rich carbonitride (MX) is formed as a result of the nitrogen uptake during sintering, with coarser appearance for the higher temperature. Nearly complete densification, approximately 7.80 ± 0.01 g/cm3, was achieved after sintering at 1200 °C, and after sintering at 1150 °C, followed by capsule-free HIP, hardening, and tempering. Irrespective of processing once the MX is formed, the nitrogen is locked into this phase and the austenite is stabilised, which means any tempering tends to result in a mixture of austenite and tempered martensite, the former being predominate during the sequential tempering, whereas martensite formation during cooling from austenitization temperatures becomes limited.

Comparative sintering behaviour of MOX powders synthesized through the freeze granulation or dry-cogrinding routes

Two MOX powders containing 11 mol% Pu/(U+Pu) have been synthesized using the freeze granulation or dry-cogrinding routes. The sintering behaviour of green compacts has been investigated up to 1700 °C in an Ar/4.3 vol% H2 humidified with 1200 vpm H2O. The evolution of the sintering trajectory (grain size as a function of relative density) is identical up to approximately 94% relative density. Above this value, grain growth becomes significant in compacts made with the powder obtained by freeze granulation and densification becomes more modest. As for it, the sintering trajectory relating to the compacts shaped from the co-grinded powder continues up to approximately 97.5% relative density, without remarkable grain growth. Beyond this value, grain growth occurs and densification slows down. It has also been shown that the plutonium distribution is more homogeneous in the case of a sample sintered from the powder obtained by freeze-granulation (relative density around 97–99%).

Compactability and sinterability of alumina granules made by spray freeze granulation drying and spray drying

Compactability and sinterability of alumina granules made by spray freeze granulation drying and spray drying

Optimization of freeze granulation and sintering behavior of MgO granules for thermal interface materials

Heat management of high-performance secondary batteries, used in various applications that require high charging and discharging rates, such as electric vehicles and electronic devices, has recently been attracting increased attention. One of the most popular heat management technologies involves the use of thermal interface materials (TIMs) for heat dissipation. TIMs are composites of thermally conductive granulated fillers uniformly dispersed in a polymer matrix. In this study, the freeze granulation process is optimized to prepare MgO granules with high thermal conductivity as an alternative to commercial alumina fillers for TIMs. The heat dissipation characteristics of TIMs are directly related to their thermal properties, size distribution, shape, density, and filler content. Therefore, a suspension is optimized with high solid content and low viscosity for proper spraying. The size distribution and sintered granule density are analyzed for various spraying distances and pressures to optimize the process for producing high-quality TIMs. Finally, a TIM with MgO granules formed by freeze granulation dispersed in silicone-based resin is fabricated, with a high thermal conductivity exceeding 5.425 W/m∙K (with an interfacial thermal resistance of 0.343 K∙ cm2/W) and low density of 2.78 g/cm3.

Selecting an appropriate binder to prepare alumina granules via spray freeze granulation drying

Spray freeze granulation drying (SFGD) is a granulation technique, involving the freezing of sprayed droplets, followed by freeze drying. This technique affords soft granules, which is advantageous for producing sintered bodies with few defects. However, non-spherical and/or hard granules are sometimes formed by SFGD, and the sintered body fabricated from such granules shows reduced density and strength. The formation of such non-spherical and/or hard granules is expected to be avoided by selecting an appropriate binder. Therefore, in this study, four types of binders (water-soluble acrylic polymer, acrylic emulsion, and poly vinyl alcohol with low and high degree of polymerization) were tested to produce alumina granules via SFGD. The shapes and properties of the granules, as well as densities and strength of the sintered bodies, were evaluated. By choosing a suitable binder, the formation of non-spherical granules can be significantly reduced, and soft granules are obtained. These leads to a sintered body with high density and high strength. The acrylic emulsion binder was found to be the best performing binder among the four binders.

Formation of dry beads for bioactives encapsulation by freeze granulation

Solid beads formed by whey protein isolate (WPI) and various sugars/polyols with a wide range of glass transition temperatures showed potential as structures for encapsulation of Aronia berry bioactives. Whey protein isolate (WPI), Aronia extract, and carbohydrates (maltitol, sucrose, or trehalose) were mixed into water to form concentrated liquid feed dispersions with varied pH. Microstructures were imaged and physical properties including complex viscosities, surface tensions, particle size distributions, and centrifuge separation were measured to investigate the effects of carbohydrate type, WPI:sugar ratio, and Aronia polyphenols (PP) concentration on liquid properties. Feed dispersions were used to produce dry beads with an adapted freeze granulation method, where individual drops were pumped into liquid nitrogen for flash freezing and harvested for subsequent freeze-drying to remove water. Dry bead diameters, water contents, and water activities were measured prior to measuring hardness and glass transition temperatures. While formulating with different sugars did not meaningfully impact liquid feed characteristics that impact processing, compositional differences were found to influence characteristics of the final dried beads more notably. • Freeze granulated beads show potential as encapsulating structures. • Altering sweetener type did not strongly affect physical properties of liquid dispersions. • Beads made with trehalose had higher T g , indicating higher storage stability.

Porous silicon nitride–based drug delivery carrier

AbstractTetracalcium phosphate/monetite biocement was modified with the addition of 30 wt% highly porous silicon nitride/α‐tricalcium phosphate (α‐TCP) microgranules. The volume ratio of Si3N4 and α‐TCP in microgranules was 1:1 and showed good in vitro simulated body fluid bioactivity with precipitation of hydroxyapatite particles. The intention of addition of microgranules to the biocement was to have a carrier of drug, which can be released into the body in due time. Granules prepared by the freeze granulation of starting mixture of silicon nitride and calcium phosphate and subsequent sintering at 1100°C have a suitable pore structure for the foreseen use. The pore volume was almost 1000 mm3/g with the open porosity of 77 vol%. This porosity and the biocompatible composition of silicon nitride–based granules gave a chance to fabricate a suitable composite cement for dexamethasone (DMZ) drug release into the human body. An accelerated release of dexamethasone from composite cement was observed and the full amount of DMZ was released from the composite biocement after 10 days. The presented results are a good base to adjust the total drug release time by the mixing of an appropriate amount of drug infiltrated ceramic granules with the tetracalcium phosphate/monetite cement.

Freeze granulation and spray drying of mixed granules of Al2O3

This paper proposes a comparative study of two techniques of granulation of a submicronic alumina powder with high binder content slurries, by spray drying and freeze granulation for the preparation of mixed granules. First, the viscosity and flow index of the suspensions are given as a function of dispersant, solid and binder contents versus alumina content, and the data are analysed in order to give a predictive model in a wide range. Suspensions with varying viscosities (7–208 mPa.s), densities (1.31–1.76) and surface tensions (23–40 mN/m) were then granulated. The first observations reveal the importance of the content and the molar mass of the binder: when they are too high, the freeze granulation fails and filaments are produced instead of granules due to extensive stretching of the molecular chains of the binder during spraying. Then through a theoretical analysis of the phenomena leading to granulation, an original dimensionless number is proposed to describe the evolution of the granule size as a function of suspension formulation. This number is related to the Reynolds and Weber number and is able to predict the granule size over a wide range (20–500 μm for freeze granulation, and 5–30 μm for spray drying). Spray drying leads to smaller granules with various shapes, from full shape to hollow or donut-like, whereas freeze granulation leads to bigger but spherical granules with a microporosity, and a size easier to predict as no drying shrinkage is observed.

Freeze Granulation of Nanoporous UiO-66 Nanoparticles for Capture of Volatile Organic Compounds

Freeze Granulation of Nanoporous UiO-66 Nanoparticles for Capture of Volatile Organic Compounds

Comparative Study on Compaction and Sintering Behavior of Spray and Freeze Granulated Magnesium Aluminate Spinel Powder

Commercially procured magnesium aluminate (MgAl2O4) spinel powder with average particle size of 300 nm and irregular morphology were spray and freeze granulated. The granules were characterized for the texture, strength, granule size and its distribution. In order to evaluate the flowability of granules, rheological parameters, such as Carr index and Hausner ratio were measured. The granules were further subjected to quasi-static compaction under loading, unloading conditions and compaction curves were constructed. Compaction coefficients thus obtained were correlated with granulation conditions and other parameters including green densities. Compacted green samples were also subjected to flexural strength measurements following ASTM standards and fractographs were observed under FESEM. A Carr index of 13.7 and Hausner ratio of 1.16 were obtained for freeze dried granules indicating the superiority in flow behavior as compared to its spray granulated counterpart, thereby showing a better compaction behavior: compaction coefficient of 5.9 in combination with higher true and predicted green density values and lower granule strength of 1.22 MPa. The green samples exhibited a flexural strength of 11 and 12.5 MPa and compressive strength of 2.44 and 1.22 MPa for spray and freeze granulated sample, respectively. The above observations were also complemented by the fractographs.

Li2TiO3 pebble fabrication by freeze granulation & freeze drying method

Lithium titanate (Li2TiO3) ceramic is one of the candidate tritium breeding material for fusion reactor. Li2TiO3 in the form of spherical pebbles are kept inside the canisters of the fusion blanket. Prior to pebble fabrication, Li2TiO3 powder was prepared by using Li2CO3 and TiO2 as precursor materials by solid-state reaction method. To make the powder-fine, they were mixed and milled together inside the high energy planetary ball mill followed by calcination at 1000 °C for 5 h. An experimental setup based on the freeze granulation and freeze-drying method was developed in-house, where the green pebbles of 1.5–2 mm were prepared by dropping Li2TiO3 slurry into liquid nitrogen through the 0.6 mm nozzle opening. The pebble drying was performed by the sublimation process at – 60 °C under vacuum followed by the sintering of green pebbles at 950 °C, 1050 °C, and 1150 °C. Various characterizations of the powder and pebbles were carried out to ascertain its phase purity, surface morphology, porosity, pore-size distribution, etc. The results of those characterizations confirm the single-stage reaction, phase purity, desired dimension of pebbles, and their physical and thermal properties. The details of those characterizations along with the dependence of sintering temperatures with the pore size distribution and bulk density of pebbles are discussed in this paper.

Transparent cobalt‐doped yttrium aluminum garnet (Co2+:YAG) ceramics—An innovatory fast saturable absorber

AbstractHighly transparent Y3Al5O12 (YAG) ceramics doped with 0.025 and 0.05 at.% Co ions were prepared for the first time by the freeze granulation process and reaction sintering. Scanning electron microscopy (SEM) and X‐ray diffraction (XRD) were performed to analyze the microstructure and crystal structure of the samples. The absorption spectra of the Co2+:YAG ceramics were measured at room temperature, and significant absorption bands at 600 nm, as well as at 1535 nm, were observed. The nonlinear behavior of the received material was experimentally demonstrated. Due to the short relaxation time of investigated ceramics, we considered them as fast saturable absorbers. The influence of the postsintering annealing process was examined. The values of saturation intensities of YAG ceramics with different concentrations of cobalt were estimated by fitting experimental data to the theoretical model for fast saturable absorbers.

Comparison of alumina granules prepared by spray freeze granulation drying and spray drying

Comparison of alumina granules prepared by spray freeze granulation drying and spray drying

Freeze Granulated Zeolites X and A for Biogas Upgrading.

Biogas is a potential renewable energy resource that can reduce the current energy dependency on fossil fuels. The major limitation of utilizing biogas fully in the various applications is the presence of a significant volume fraction of carbon dioxide in biogas. Here, we used adsorption-driven CO2 separation using the most prominent adsorbents, NaX (faujasite) and CaA (Linde Type A) zeolites. The NaX and CaA zeolites were structured into hierarchically porous granules using a low-cost freeze granulation technique to achieve better mass transfer kinetics. The freeze granulation processing parameters and the rheological properties of suspensions were optimized to obtain homogenous granules of NaX and CaA zeolites 2–3 mm in diameter with macroporosity of 77.9% and 68.6%, respectively. The NaX and CaA granules kept their individual morphologies, crystallinities with a CO2 uptake of 5.8 mmol/g and 4 mmol/g, respectively. The CO2 separation performance and the kinetic behavior were estimated by breakthrough experiments, where the NaX zeolite showed a 16% higher CO2 uptake rate than CaA granules with a high mass transfer coefficient, 1.3 m/s, compared to commercial granules, suggesting that freeze-granulated zeolites could be used to improve adsorption kinetics and reduce cycle time for biogas upgrading in the adsorption swing technology.

Dense and homogeneous MOX fuel pellets manufactured using the freeze granulation route

AbstractMOX fuels (UO2‐PuO2) are used in light water nuclear reactors of several countries and are also potential candidates for the fast neutron reactors. Industrial MOX is currently manufactured by a dry route process, involving steps with fine powders. To reduce dusting, enhance MOX powder flow properties, and decrease manufacturing scrap rate, a new wet route process is investigated: the freeze granulation of concentrated water‐based powder suspensions having optimized rheology and dispersion properties. Highly flowable, dustless, and easy‐to‐press MOX granules have been elaborated. Sintering green compacts made of such granules gives highly dense and defect‐free pellets that have a very homogeneous U‐Pu spatial distribution, thus improving the fuel characteristics. Indeed, MOX fuels devoid of large Pu‐rich aggregates are thought to have a better behavior under irradiation in reactors by limiting/preventing the formation of the typical high burnup structure.

Porous granules by freeze granulation of Pickering emulsions stabilized with halloysite particles

Pickering emulsions stabilized with a natural silicate mineral, halloysite, were dried by freeze granulation in order to prepare porous granules. Different formulations were studied, by varying the pH of the suspension, the ionic strength and with the addition of organic binders (poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG)). To get a better understanding and control on the process, the objective was to correlate the formulation and the emulsion behavior to the morphology and size of the granules. This study highlights the predominant role of electrostatic interactions on the emulsions. Their stability is improved if the dispersion of solid particles in the continuous phase is relatively limited, with medium zeta potential absolute values. Freeze granulation of emulsions prepared at different ionic strength and pH led to similar granules, with an internal porosity attributed to the low solid fraction. The addition of organic additives PVA and PEG led to a better conservation of the structure, with fewer broken fragments and a larger porosity, attributed to a higher cohesion of these emulsions during the spray freezing and the lyophilisation steps.

Fabrication of homogenous pellets by freeze granulation of optimized TiO2-Y2O3 suspensions

MOX fuels are used in nuclear reactors. To ensure the manufacture of MOX, wet processes are investigated. Freeze granulation of water-based powder suspension seems a promising way to yield homogenous and easy-to-press UO2-PuO2 granules. These granules are expected to form dense and homogenous pellets by uniaxial pressing. Granules and pellet properties are affected by suspension formulation and atomization conditions. Therefore, these conditions must be studied and optimized to produce granules with good processability and thus MOX pellets with compliant density, homogeneity and absence of defects. In this scope, key properties of water-based suspensions of UO2 and PuO2 surrogate powders, TiO2 and Y2O3 respectively, were firstly studied to assess their suitability for the freeze granulation process. These properties were compared to those of UO2 and PuO2 suspensions to verify and confirm the correctness in the choice of surrogate powders. Then, the freeze granulation process itself was investigated focusing on TiO2-Y2O3 suspensions.

Processing of sintered and CVD coated SiC/CNFs thin composite tubes

Silicon carbide (SiC) and its composite powders dispersed with 1–3 wt% carbon nano-fibres (CNFs) were spray freeze granulated to produce spherical granules with uniform composition. Powders were shaped to thin dense tubes for use in a harsh environment by cold isostatic pressing followed by pressureless sintering at 2150 °C for 1 h in argon and SiC coating by chemical vapour deposition (CVD) technique. The physical, microstructural and thermo - mechanical properties of the composite tubes were evaluated and compared with bare SiC tubes. Good dispersibility of CNFs and sintering additives in the matrix by spray freeze granulation processing technique resulted in more than 98.5% theoretical density of the composite tubes with superior mechanical properties. SEM & EBSD analysis show grains are platy and equiaxed in nature with many CNFs trapped at the triple points hindering the SiC grain growth which is also confirmed by TEM analysis. The thermal conductivity value of the composite was also found higher than bare SiC at all temperatures when measured up to 1000 °C due to uniform distribution of CNFs in the matrix.