Showing papers on "Crystallization published in 2016"
TL;DR: These experiments provide the first mechanistic insight into the mechanism of imine-linked 2D COF formation, which is distinct from that of boronate-ester linked COFs.
322 citations
TL;DR: The formation of hydroxyl radicals, either by irradiation with ultraviolet light or with the Fenton reagent, can speed up the formation of the crystallized zeolite by about a factor of 2.Radically faster synthesis Zeolite synthesis normally proceeds under basic conditions that allow the oxide bridges between aluminum and silicon atoms to break and reform.
Abstract: In the hydrothermal crystallization of zeolites from basic media, hydroxide ions (OH(-)) catalyze the depolymerization of the aluminosilicate gel by breaking the Si,Al-O-Si,Al bonds and catalyze the polymerization of the aluminosilicate anions around the hydrated cation species by remaking the Si,Al-O-Si,Al bonds. We report that hydroxyl free radicals (•OH) are involved in the zeolite crystallization under hydrothermal conditions. The crystallization processes of zeolites-such as Na-A, Na-X, NaZ-21, and silicalite-1-can be accelerated with hydroxyl free radicals generated by ultraviolet irradiation or Fenton's reagent.
268 citations
TL;DR: A novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.
Abstract: The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.
261 citations
TL;DR: In this paper, a change in nucleation mechanism, from heterogeneous nucleation to surface nucleation (or in extreme cases, homogeneous nucleations), was proposed, which reflected that nucleation is the rate determining step in the overall crystallization of ideally confined polymers.
257 citations
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TL;DR: In situ electron microscopy is used to show how gold and silver nanocrystals nucleate from supersaturated aqueous solutions in three distinct steps: spinodal decomposition into solute-rich andsolute-poor liquid phases, nucleation of amorphous nanoclusters within the metal-rich liquid phase, followed by crystallization of theseAmorphous clusters.
Abstract: Nucleation and growth of solids from solutions impacts many natural processes and are fundamental to applications in materials engineering and medicine. For a crystalline solid, the nucleus is a nanoscale cluster of ordered atoms, which forms through mechanisms that are still poorly understood. These mechanisms have important consequences on the morphology and nucleation rates of the resultant crystals but it is unclear whether a nucleus forms spontaneously from solution in a single step or through multiple steps. Using in-situ electron microscopy, we observe and quantify how gold and silver nanocrystals nucleate from a supersaturated aqueous gold and silver solution in three distinct steps: (I) spinodal decomposition into solute-rich and solute-poor liquid phases, (II) nucleation of amorphous gold nanoclusters within the gold-rich liquid phase, followed by (III) crystallization of these amorphous clusters. Our ab-initio calculations on gold nucleation suggest that these steps might be associated with strong gold-gold atom coupling and water-mediated metastable gold complexes. The understanding of intermediate steps in nuclei formation has important implications for the formation and growth of both crystalline and amorphous materials.
223 citations
TL;DR: Fast scanning chip calorimetry in its non-adiabatic version allows for heating and cooling at rates up to 106 K s−1, covering all polymer processing relevant rates as mentioned in this paper.
208 citations
TL;DR: In this article, the ionic liquid driven crystallization was exploited to produce a planar perovskite solar cell with a stabilized power output of 19.5% for the first time.
Abstract: Ionic liquids can retard the perovskite crystallization with the aim to form compact films with larger and more uniformly distributed grain size. The ionic liquid driven crystallization is exploited to prepared a record planar perovskite solar cell with stabilized power output of 19.5%.
208 citations
TL;DR: The mechanism of this crystallization is explored and it is found that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization.
Abstract: Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. We use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.
183 citations
TL;DR: It is found that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties.
Abstract: Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites. Here we show that a large-area, orientationally pure crystalline (OPC) methylammonium lead iodide (MAPbI3) hybrid perovskite film can be fabricated using a thermal-gradient-assisted directional crystallization method that relies on the sharp liquid-to-solid transition of MAPbI3 from ionic liquid solution. We find that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties. X-ray diffraction patterns reveal that the film is strongly oriented in the (112) and (200) planes parallel to the substrate. This film is structurally confined by directional crystal growth, inducing intense anisotropy in charge transport. In addition, the low trap-state density (7.9 × 1013 cm-3) leads to strong amplified stimulated emission. This ability to control crystal orientation and morphology could be widely adopted in optoelectronic devices.
173 citations
TL;DR: In this article, a novel strategy to synthesize structurally, mechanically, electrically and optically anisotropic graphene aerogels (AN-GAs) by using gaseous hydrogen chloride to in situ solidify ordered graphene oxide liquid crystals followed by chemical reduction, supercritical fluid drying and annealing in an Ar atmosphere in sequence.
Abstract: To overcome fatal shortcomings of organic phase change materials (PCMs), such as leakage during work, low thermal conductivity and shortage of multiple driving ways, we propose a novel strategy to synthesize structurally, mechanically, electrically and optically anisotropic graphene aerogels (AN-GAs) by using gaseous hydrogen chloride to in situ solidify ordered graphene oxide liquid crystals followed by chemical reduction, supercritical fluid drying and annealing in an Ar atmosphere in sequence. The confined pore space and aligned wall structure of the resulting AN-GAs have benefited crystallization of organic phase change molecules and thus highly efficient phase change composites (PCCs) are fabricated with long durability and good strength. The resulting PCCs can also be driven either by applying a small voltage (1–3 V) with high electro-heat efficiency (up to 85%) or by irradiating with weak sunlight (0.8–1.0 sun) with high photo-heat efficiency (up to 77%).
166 citations
TL;DR: In this article, surface functionalization was used to regulate the interactions between a polyethylene glycol (PEG) and internal/external surfaces of a support of SBA-15 and yield desirable thermal properties of PEG confined in porous supports.
TL;DR: Results suggest that SSPVD is a promising method to significantly optimize perovskite CH3NH3PbI3 solar cell efficiency and improve surface coverage and moisture stability in a non-vacuum solution.
Abstract: In this work, an alternative route to fabricating high-quality CH3NH3PbI3 thin films is proposed. Single-source physical vapour deposition (SSPVD) without a post-heat-treating process was used to prepare CH3NH3PbI3 thin films at room temperature. This new process enabled complete surface coverage and moisture stability in a non-vacuum solution. Moreover, the challenges of simultaneously controlling evaporation processes of the organic and inorganic sources via dual-source vapour evaporation and the heating process required to obtain high crystallization were avoided. Excellent composition with stoichiometry transferred from the powder material, a high level of tetragonal phase-purity, full surface coverage, well-defined grain structure, high crystallization and reproducibility were obtained. A PCE of approximately 10.90% was obtained with a device based on SSPVD CH3NH3PbI3. These initial results suggest that SSPVD is a promising method to significantly optimize perovskite CH3NH3PbI3 solar cell efficiency.
TL;DR: In this paper, the authors developed a new perspective whether crystalline structures influence the adsorption of heavy metals on magnetic iron oxides synthesized by applying surfactant sodium dodecyl sulfate (SDS)-templated strategy.
TL;DR: In this paper, the physicochemical properties of binder gel in alkali-activated fly ash/slag exposed to high temperatures were investigated, and the porosity under high temperatures had an inverse relationship with the strength.
TL;DR: A first-principles mathematical model was developed which was able to fit the experimental receiver concentration-time profiles for concentration regimes both below and above amorphous solubility, providing support for the contention that the nanodroplet phase does not directly diffuse across a membrane but, instead, rapidly replenishes the drug in the aqueous phase that has been removed by transport across the membrane.
Abstract: Recent studies on aqueous supersaturated lipophilic drug solutions prepared by methods including antisolvent addition, pH swing, or dissolution of amorphous solid dispersions (ASDs) have demonstrated that when crystallization is slow, these systems undergo liquid-liquid phase separation (LLPS) when the concentration of the drug in the medium exceeds its amorphous solubility. Following LLPS, a metastable equilibrium is formed where the concentration of drug in the continuous phase corresponds to the amorphous solubility while the dispersed phase is composed of a nanosized drug-rich phase. It has been reasoned that the drug-rich phase may act as a reservoir, enabling the rate of passive transport of the drug across a membrane to be maintained at the maximum value for an extended period of time. Herein, using clotrimazole as a model drug, and a flow-through diffusion cell, the reservoir effect is demonstrated. Supersaturated clotrimazole solutions at concentrations below the amorphous solubility show a linear relationship between the maximum flux and the initial concentration. Once the concentration exceeds the amorphous solubility, the maximum flux achieved reaches a plateau. However, the duration for which the high flux persists was found to be highly dependent on the number of drug-rich nanodroplets present in the donor compartment. Macroscopic amorphous particles of clotrimazole did not lead to the same reservoir effect observed with the nanodroplets formed through the process of LLPS. A first-principles mathematical model was developed which was able to fit the experimental receiver concentration-time profiles for concentration regimes both below and above amorphous solubility, providing support for the contention that the nanodroplet phase does not directly diffuse across the membrane but, instead, rapidly replenishes the drug in the aqueous phase that has been removed by transport across the membrane. This study provides important insight into the properties of supersaturated solutions and how these might in turn impact oral absorption through effects on passive membrane transport rates.
TL;DR: In this paper, a hot-press setup was proposed to facilitate the synthesis of solid electrolytes by combining in situ electrochemical impedance spectroscopy (EIS) with simultaneous temperature and pressure-monitoring.
Abstract: Rechargeable solid-state lithium ion batteries (SSLB) require fast ion conducting solid electrolytes (SEs) to enable high charge and discharge rates. Li7P3S11 is a particularly promising lithium solid electrolyte, exhibiting very high room temperature conductivities of up to 17 mS·cm–1 and high ductility, allowing fast ion transport through the bulk and intimate contact to high surface electrodes. Here we present a novel hot-press setup that facilitates the synthesis of solid electrolytes by combining in situ electrochemical impedance spectroscopy (EIS) with simultaneous temperature- and pressure-monitoring. While a high room temperature conductivity in the order of 10 mS·cm–1 is readily achieved for phase pure Li7P3S11 with this design, it further enables monitoring of the different steps of crystallization from an amorphous Li2S–P2S5 glass to triclinic Li7P3S11. Nucleation, crystallization and—at temperatures exceeding 280 °C—decomposition of the material are analyzed in real time, enabling process opti...
TL;DR: In this paper, the microcellular structure, thermal properties, and crystallization behaviors were characterized using scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and wide angle X-ray diffraction.
Abstract: Poly(lactic acid) (PLA) and northern bleached softwood kraft (NBSK) or black spruce medium density fiberboard (MDF) fibers were melt compounded using a co-rotating twin screw extruder and subsequently microcellular injection molded. Poly(ethlylene glycol) (PEG) was used as a lubricant. The microcellular structure, thermal properties, and crystallization behaviors were characterized using scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and wide angle X-ray diffraction. Results show that cellulosic fibers, acting as crystal nucleating agents, increased the crystallization temperature and the crystallinity and decreased the crystallization half time. The dissolved N 2 , the shear stress, and biaxial stretching during foaming also enhanced the crystallinity of PLA. Compared to PLA/PEG, a finer and more uniform cell structure was achieved in the cellulosic fiber composite foams. The improved foam morphology was attributed to the cell nucleating effects of the fibers and the increased melt strength by the addition of cellulosic fibers and by the gas- and fiber- induced crystallization.
TL;DR: In this article, a thin film of antimony sulfide (Sb2S3) was obtained by rapid thermal evaporation (RTE) rather than conventional chemical bath deposition or atomic layer deposition based methods.
TL;DR: By adjusting additive concentration, film morphologies were optimized and the fabricated p-i-n planar perovskite-PCBM solar cells showed improved power conversion efficiences (an average of 3-4% absolute efficiency enhancement) compared to the nonsodium based devices.
Abstract: Thin-film p-i-n type planar heterojunction perovskite solar cells have the advantage of full low temperature solution processability and can, therefore, be adopted in roll-to-roll production and flexible devices. One of the main challenges with these devices, however, is the ability to finely control the film morphology during the deposition and crystallization of the perovskite layer. Processes suitable for optimization of the perovskite layer film morphology with large grains are highly desirable for reduced recombination of charge carriers. Here, we show how uniform thin films with micron size perovskite grains can be made through the use of a controlled amount of sodium ions in the precursor solution. Large micrometer-size CH3NH3PbI3 perovskite grains are formed during low-temperature thin-film growth by adding sodium ions to the PbI2 precursor solution in a two-step interdiffusion process. By adjusting additive concentration, film morphologies were optimized and the fabricated p-i-n planar perovskite...
TL;DR: In this article, a CH 3 NH 3 Cl precursor for antisolvent precipitation at low temperature and fabricated high quality perovskite films with desired morphology, crystallinity and optical properties.
TL;DR: Chen et al. as mentioned in this paper used a small-molecule 1,3,5-trichlorobenzene (TCB) as a template for polymer epitaxy under a temperature gradient crystallization process.
Abstract: Conducting polymers are potential candidates for thermoelectric (TE) applications owing to their low thermal conductivity, non-toxicity and low cost. However, the coil conformation and random aggregation of polymer chains usually degrade electrical transport properties, thus deteriorating TE performance. In this work, we fabricated poly(3-hexylthiophene) (P3HT) films with highly oriented morphology using 1,3,5-trichlorobenzene (TCB), an organic small-molecule, as a template for polymer epitaxy under a temperature gradient crystallization process. The resulting P3HT molecules, which were confirmed to be highly anisotropic by a combination of scanning electron microscopy, atomic force microscopy, polarizing microscope, polarized Raman spectroscopy, and two-dimensional-grazing incidence X-ray diffraction (GIXRD) analysis, not only markedly reduced the conjugated defects along the polymer backbone, but also effectively increased the degree of electron delocalization. These combined phenomena produced an efficient, 1D path for carrier movement and therefore resulted in enhanced carrier mobility in the TCB-treated P3HT films. The maximum values of the electrical conductivity and Seebeck coefficient were 320 S cm−1 and 269 μV K−1, respectively. Consequently, the maximum TE power factor and ZT value at 365 K reached 62.4 μW mK−2 and 0.1, respectively, in the parallel direction of the TCB-treated P3HT film. To the best of our knowledge, these are the highest values reported for pure P3HT TE materials. The method of using organic small-molecule epitaxy to generate highly anisotropic polymer films is expected to be valid for many conducting polymers. A process for stretching twisted polymer chains into ordered fibers can benefit production of low-cost materials that harvest heat losses. Most thermoelectric energy converters are made from inorganic semiconductors. Conductive polymers, however, are an attractive alternative because of their simple manufacturing requirements and intrinsic thermal insulating properties. Lidong Chen from the Shanghai Institute of Ceramics and co-workers have developed a way to reduce the defects that often plague plastic thermoelectrics with a dissolvable organic template. The team co-crystallized the conductive polymer poly(3-hexylthiophene) with small, trichlorobenzene-based molecules that solidify into needle-like structures under a temperature gradient. The aromatic stacking forces within the trichlorobenzene crystals lock the polymer chains into elongated positions that persist after template removal. Experiments on the resulting poly(3-hexylthiophene) filaments revealed near-record thermoelectric conversion efficiencies for this material. Herein, we fabricated pure poly(3-hexylthiophene) (P3HT) film with highly oriented morphology using a small-molecule 1,3,5-trichlorobenzene (TCB) as the template for polymer epitaxy with a temperature gradient crystallization process. The strong π–π conjugated interactions as well as the very close matching between the repeat distance of the thiophene units in P3HT and the repeat distance of TCB molecules have successfully induced the epitaxy process, allowing the P3HT polymer chains to lock onto the lattice of TCB, forming highly ordered P3HT chains in the direction parallel to the fiber axis. As a result, the electrical conductivity in the direction parallel to the fiber axis was significantly improved. The maximum thermoelectric power factor and ZT value at 365 K reached 62.4 μW mK-2 and 0.1 in the parallel direction of the TCB-treated P3HT film.
TL;DR: A new series of highly CO2-selective, amorphous PEO-containing segmented copolymers prepared from commercial Jeffamine polyetheramines and pentiptycene-based polyimide are reported, making them attractive for hydrogen purification and carbon capture.
Abstract: Poly(ethylene oxide) (PEO)-containing polymer membranes are attractive for CO2-related gas separations due to their high selectivity toward CO2. However, the development of PEO-rich membranes is frequently challenged by weak mechanical properties and a high crystallization tendency of PEO that hinders gas transport. Here we report a new series of highly CO2-selective, amorphous PEO-containing segmented copolymers prepared from commercial Jeffamine polyetheramines and pentiptycene-based polyimide. The copolymers are much more mechanically robust than the nonpentiptycene containing counterparts due to the molecular reinforcement mechanism of supramolecular chain threading and interlocking interactions induced by the pentiptycene structures, which also effectively suppresses PEO crystallization leading to a completely amorphous structure even at 60% PEO weight content. Membrane transport properties are sensitively affected by both PEO weight content and PEO chain length. A nonlinear correlation between CO2 p...
TL;DR: It is shown how the exposure of plant-based fibers to HCl vapor results in rapid degradation with simultaneous crystallization, which serves as a starting point for the solid-state tuning of the supramolecular properties of morphologically heterogeneous biological materials.
Abstract: Despite the structural, load-bearing role of cellulose in the plant kingdom, countless efforts have been devoted to degrading this recalcitrant polysaccharide, particularly in the context of biofuels and renewable nanomaterials. Herein, we show how the exposure of plant-based fibers to HCl vapor results in rapid degradation with simultaneous crystallization. Because of the unchanged sample texture and the lack of mass transfer out of the substrate in the gas/solid system, the changes in the crystallinity could be reliably monitored. Furthermore, we describe the preparation of cellulose nanocrystals in high yields and with minimal water consumption. The study serves as a starting point for the solid-state tuning of the supramolecular properties of morphologically heterogeneous biological materials.
TL;DR: Inclusion of an effective solution crystallization inhibitor as a minor component in ternary dispersions resulted in prolonged supersaturation following dissolution, showing the feasibility of formulation strategies for ASDs where a major polymer component is used to achieve one key property e.g. release, while a minor polymer components is added to prevent crystallization.
Abstract: The aims of this study were twofold. First, to evaluate the effectiveness of selected polymers in inhibiting solution crystallization of celecoxib. Second, to compare the release rate and crystallization tendency of celecoxib amorphous solid dispersions (ASDs) formulated with a single polymer, or binary polymer combinations. The effectiveness of polymers, polyvinylpyrrolidone (PVP), hydroxypropylmethyl cellulose (HPMC) or HPMC acetate succinate (HPMCAS), in maintaining supersaturation of celecoxib solutions was evaluated by performing nucleation induction time measurements. Crystallization kinetics of ASD suspensions were monitored using Raman spectroscopy. Dissolution experiments were carried out under non-sink conditions. Pure amorphous celecoxib crystallized rapidly through both matrix and solution pathways. Matrix and solution crystallization was inhibited when celecoxib was molecularly mixed with a polymer, resulting in release of the drug to form supersaturated solutions. Cellulosic polymers were more effective than PVP in maintaining supersaturation. Combining a cellulosic polymer and PVP enabled improved drug release and stability to crystallization. Inclusion of an effective solution crystallization inhibitor as a minor component in ternary dispersions resulted in prolonged supersaturation following dissolution. This study shows the feasibility of formulation strategies for ASDs where a major polymer component is used to achieve one key property e.g. release, while a minor polymer component is added to prevent crystallization.
TL;DR: It is found that electrostatic interaction not only can induce an enhanced radiative decay T1 → S0 through the dipole-allowed S1 intermediate state, but also can hinder the nonradiative decay process upon crystallization.
Abstract: Room temperature phosphorescence (RTP) from pure organic material is rare due to the low phosphorescence quantum efficiency. That is why the recent discovery of crystallization induced RTP for several organic molecules aroused strong interests. Through a combined quantum and molecular mechanics CASPT2/AMBER scheme taking terephthalic acid (TPA) as example, we found that electrostatic interaction not only can induce an enhanced radiative decay T1 → S0 through the dipole-allowed S1 intermediate state, but also can hinder the nonradiative decay process upon crystallization. From gas phase to crystal, the nature of S1 state is converted to 1(π,π*) from 1(n,π*) character, enhancing transition dipole moment and serving as an efficient intermediate radiative pathway for T1 → S0 transition, and eventually leading to a boosted RTP. The intermolecular packing also blocks the nonradiative decay channel of the high-frequency C═O stretching vibration with large vibronic coupling, rather than the conventional low-frequ...
TL;DR: In this paper, the authors focused on how different types of oil phase, MCT (medium chain triglycerides), exert influence on the gelation process of beeswax and thus properties of the organogel produced thereof.
TL;DR: Mesoporous magnesium oxide modified diatomite (MgO-D) was developed through a simple precipitation method, and applied for simultaneous removal of nutrients from contaminated waters as mentioned in this paper.
TL;DR: Laser crystallization of hybrid perovskite solar cells using near-infrared (NIR) laser attains higher device performances than conventional thermal annealing and demonstrates inverted-type perovkiteSolar cells with 11.3 and 8.0% efficiencies on typical glass and flexible polymer substrates, respectively, without rigorous device optimization.
Abstract: Organic–inorganic hybrid perovskites attract enormous research interest for next generation solar energy harvest. Synergistic crystalline structures comprising organic and inorganic components enable solution processing of perovskite films. A reliable crystallization method for perovskites, compatible with fast continuous process over large-area flexible substrates, is crucial for high performance solar cell production. Here, we present laser crystallization of hybrid perovskite solar cells using near-infrared (NIR) laser (λ = 1064 nm). Crystalline morphology of CH3NH3PbI3 (MAPbI3) perovskite films are widely controllable with laser irradiation condition while maintaining film uniformity. Photothermal heating effectively assisted by interfacial photoconversion layers is critical for phase transformation without beam damage of multilayered device structures. Notably, laser crystallization attains higher device performances than conventional thermal annealing. Fast laser crystallization with manufacture lev...
TL;DR: In this article, a self-assemblable aryl amide nucleator was used to promote Stereocomplex (SC) formation and controlled crystal morphology of a high-molecular-weight (HMW) PLLA/PDLA blend.
Abstract: Stereocomplex (SC) crystallization has been an effective method to improve the heat resistance of poly(lactic acid) (PLA). However, the preparation of SC-type PLA material is still a challenge because SC crystallization is much less prevailing than homocrystallization in the high-molecular-weight (HMW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends. In this study, we have successfully promoted SC formation and controlled crystal morphology of a HMW PLLA/PDLA blend by using a self-assemblable aryl amide nucleator, N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA). Crystallization kinetics, polymorphic crystalline structure, crystal morphology and superstructure of BTCA-nucleated PLLA/PDLA blends were investigated. During the nonisothermal melt crystallization and isothermal crystallization at different temperatures (80–170 °C), the crystallization rate of PLLA/PDLA blend is significantly promoted and the fraction of SCs is enhanced with incorporating small amount of BTCA. SCs a...
TL;DR: Results showed that the stabilization of the α-subcell crystals in the lattice of the carrier lipid is a key parameter for forming stable solid lipid dispersions, contributing to a better understanding of SLN as a function of the bioactive lipid.