The particle size distribution effect on the drying efficiency of polymeric fibers containing castables

Refractory castables present several placing methods, defined mainly by the application requirements and material characteristics. Considering the same chemical composition, the particle size distribution (PSD) is the key property related to the large differences in their rheology, creep and corrosion resistance. It also plays an important role on their fluids permeation and drying behaviors. Therefore, it is reasonable to consider that the benefits promoted by polymeric fibers, added as drying agents, would be affected by PSD changes. In this work, the permeability and drying behaviors of fiber containing refractory castables were correlated to their PSD. Typical pumpable, self-flowing and vibrated formulations were tested in combination with polypropylene fibers. Permeability measurements and explosion tests were associated to the maximum paste thickness (MPT) and interparticle separation (IPS) parameters and to the fine/coarse particles ratio. The different classes of castables presented distinct needs of drying additives and the fibers’ efficiency was strongly dependent on castables PSD.     

Microstructure and refractory properties of spinel containing castables

The bauxite-based and kaolin-based refractory castables investigated were carefully prepared. They are composed of 90 wt.% well-graded (coarse, medium, and fine) bauxite or kaolin aggregates, 10 wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) either preformed or in situ. The castable batches were cast into cubes (25 mm × 25 mm × 25 mm), left at 100% relative humidity for 24 h cured for 7 days under water, and dried at 110 °C for 24 h. The samples were then subjected to firing at 1550 °C for a soaking time of 1 h.     

Effects of andalusite and kyanite addition on the microstructure,thermo-mechanical properties and corrosion resistance of ultralow-cement bauxite-corundum castables

Ultralow-cement bauxite-corundum refractory castables are increasingly attracting attention because of the advantages in performance and cost. To improve its volume stability and high-temperature performance, andalusite and kyanite were incorporated into this type of refractory, respectively, and a comparative investigation of their effects on microstructure, thermo-mechanical properties, and slag corrosion resistance was carried out in this work. The expansion effects induced by the mullitization of these additives not only suppressed the sintering shrinkage but also made the matrix structure more compact and uniform, mainly owing to the generation of many new and strong mullite bonds. This improvement resulted in a significant increase in the refractoriness under load and creep resistance. This effect was far more pronounced for the andalusite-added castables, for which the RUL increased by 62 °C and the creep rate at the stationary stage decreased by 26%. Besides, adverse effects on microstructure and properties appeared when adding a higher amount of kyanite due to the excessive expansion. Moreover, the corrosion mechanisms of these refractory castables by refining slag were discussed, and the active role of andalusite addition in the corrosion resistance was also proposed.     

ZnO and MgO as inducers of spinel-like phase formation on alumina-based castables

Environmental issues regarding Cr⁶⁺ formation lead to replacing chrome-containing refractories with greener alternatives. MgO-containing compositions have been extensively investigated for this purpose, however, few studies evaluated the likelihood of using other chemical elements as inducers of spinel-like phase formation in refractory castables. In this study, the addition of zincite in alumina-based castables was evaluated and compared with its MgO-counterpart. In-situ elastic modulus, assisted sinterability and differential scanning calorimetry pointed out that the gahnite (ZnAl₂O₄) formation took place at lower temperatures (～ 1100 °C) than MgAl₂O₄ (～ 1300 °C). On one hand, this feature induces anticipated strengthening of the Zn-containing compositions, giving rise to the possibility of firing these compositions at lower temperatures. On the other, the faster kinetics of gahnite formation led to a significant Kirkendall effect, changing the morphology of the pores created during sintering, which became preferentially located at the interface of alumina aggregates, negatively affecting some mechanical properties of the castable.     

SiO2微粉和Al2O3微粉加入量对莫来石-刚玉浇注料性能的影响

以莫来石M45(10～5mm,5～3mm)、电熔莫来石(3～1mm,≤1mm)作骨料,刚玉粉(≤0.074mm)、电熔莫来石粉(≤0.074mm)、纯铝酸钙水泥、Al2O3微粉(d50=2.11μm)和SiO2微粉(d50=1.07μm)作基质,制备了脱硫枪用莫来石-刚玉浇注料,并分别研究了SiO2微粉加入量(分别为2%、3%、4%、5%)和Al2O3微粉加入量(分别为2%、4%、6%)对浇注料体积密度、显气孔率、强度、烧后永久线变化率、抗热震性和抗渣性的影响。结果表明:仅加入SiO2微粉时,随着其加入量在2%～5%范围内增加,莫来石-刚玉浇注料的体积密度提高,显气孔率降低,强度增加,抗渣性变好,但抗热震性变差;在加入3%SiO2微粉的基础上再加入2%～6%的Al2O3微粉时,浇注料的各项性能随Al2O3微粉增加没有明显改善,但均比仅加入3%SiO2微粉时有明显改善。     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

Direct observation of the moisture distribution in calcium aluminate cement and hydratable alumina-bonded castables during first-drying: An NMR study

The drying behavior for various calcium aluminate cement and hydratable alumina-bonded refractory castables was investigated in the first-drying temperature range (100°C-300°C). Using a specialized high-temperature Nuclear Magnetic Resonance setup, we were able to directly and nondestructively measure the spatially and temporally resolved moisture distribution, while simultaneously measuring the temperature distribution as well. These measurements show that the drying front position is a linear function of time, which can be explained on the basis of a simplified model where only vapor transport is considered. Based on the measurements and the model, one can directly determine the permeability at high temperatures. Moreover, the results demonstrate that the drying front speed and temperature strongly correlates with the control of key material parameters (eg, water demand, binder content, etc). In particular, microsilica fume-containing low-cement castables displayed the highest vapor pressures, while regular castables generated the lowest vapor pressures reflecting the permeability of these materials.     

硅微粉对铝镁系浇注料相组成及性能的影响

利用X射线衍射分析和相图分析,研究了硅微粉对高纯铝镁系浇注料物相组成和不同温度下的物理性能及蠕变性能的影响。     

Synthesis and evaluation of Mg-Al hydrotalcite formation and its influence on the microstructural evolution of spinel-forming refractory castables under intermediate temperatures

The mechanical properties of in-situ spinel (MgAl₂O₄)-forming alumina-based castables under intermediate temperatures are of critical importance before the refractory lining system reaches normal operating conditions. The objective of this study is to elucidate the role of the hydrotalcite formed within a fine-grained castables matrix, in which no strength loss of the MgO-bonded alumina-based castables without silica fume was observed. Numerous fundamental studies were conducted to examine the factors influencing hydrotalcite formation within the blended pastes composed of MgO and Al₂O₃ nanopowders; dead burned or fused magnesia and Al₂O₃ nanopowder; dead burned magnesia and water-dispersed sol of fumed alumina by using: XRD and DSC-TG-EGA(MS). The XRD, FTIR and ²⁷Al MAS NMR analysis of the hydrotalcite calcination products revealed that the spinel begins to form at temperatures as low as 700 °C. Finally, the physical properties, phase composition and microstructure of the refractory castables bonded with the hydrotalcite decomposition-routed nanostructured spinel were evaluated.     

Effects of curing time on the pore structure evolution and fracture behavior of CAC bonded alumina-spinel castables

The present work investigated the pore structure evolution and fracture behavior of calcium aluminate cement (CAC)-bonded alumina-spinel castables treated at 110 and 1600 °C after curing at 25 °C for 1 day and 3 days. The pore structure and fracture behavior were characterized by mercury intrusion combined with micro-CT scanning and a wedge splitting test coupled with acoustic emission, respectively. The results showed that the hydration degree was enhanced by extending the curing time, and more hydrates were conducive to the generation of complex pore structures after drying. In comparison, the heat treatment at 1600 °C resulted in a substantial reduction in nano-sized pores, the 3 d sample obtained developed and complex micro-sized pores than 1 d sample. Therefore, specimens cured for 3 d had better hot modulus of rupture (HMOR) and thermal shock resistance compared to those cured for 1 d, which could be attributed to the induced tortuous microcrack propagation within the matrix and along the aggregate-matrix interfaces.     

Fabrication of aluminum nitride coatings by electrophoretic deposition: Effect of particle size on deposition and drying behavior

Electrophoretic technique was used to deposit micro- and nano-sized aluminum nitride coatings on stainless steel surfaces by using a well-dispersed stable suspension produced by addition of AlN powder plus a small amount of iodine to ethanol. Parabolic regime governed the deposition. Electrophoretic deposition for 240 s at 100 V resulted in formation of a uniformly dense film on the top, but a porous inhomogeneous layer at the bottom. This was attributed to fast deposition of coarse particles and/or agglomerates at large electric fields. After drying, micro-sized particles led to a uniform crack-free interface while nano-particles resulted in fragmented non-cohesive layers. Weight loss measurements revealed higher drying rates for micro-layer as compared to nano-cover. This seemed owing to the larger pore sizes and lower specific surfaces of the former. Stress inducement by lateral drying of small capillaries led to crack initiation from the edges and its propagation across the surfaces. This resulted in fragmentation of the samples due to their delamination. Effect of deposition rate on particles packability was also investigated.     

The phase composition and molecular motions of plasticized wheat gluten-based biodegradable polymer materials studied by solid-state NMR spectroscopy

The water plasticized wheat gluten (WG) materials were prepared by thermal processing and studied by dynamic mechanical analysis and solid-state NMR spectroscopy. The results indicate that the materials displayed a broad distribution of molecular motions and could be divided into different phases in terms of their mobility above the T_g. The rigid phase mainly consisted of proteins and starch with enhanced interactions between the two components via hydrogen bonding with water molecules. Lipid and water formed the mobile phase, however, lipid molecules were always more mobile than water. The intermediate phase consisted of plasticized starch and proteins (mainly proline and glutamine segments). The whole plasticized WG materials were heterogeneous at a scale of 20-30 nm, but the miscibility between proteins and starch was enhanced via increasing hydrogen bonding interactions with water molecules. Such strong hydrogen bonding acted as adhesion among these multi-components/phases over a wide range of temperature thus resulting in good mechanical properties of the materials. The results demonstrated that solid-state NMR techniques can provide valuable information of quantitative composition of phase structures with different mobility in a multi-component system and the chemical nature of each phase along with the interactions among these components/phases.     

骨料粒度级配对Al_2O_3-SiC自流浇注料流动性和流变特性的影响

为了进一步研究骨料的粒度级配对Al_2O_3-SiC自流浇注料流动性能和流变特性的影响,通过正交试验法研究了5~3、3~1和≤1 mm的棕刚玉骨料的粒度级配对浇注料的自流值、相对塑性黏度和相对屈服应力的影响。结果表明:1)试验浇注料的自流值和相对塑性黏度均随3种不同粒度棕刚玉骨料加入量的增加而减小,而棕刚玉骨料粒度对浇注料自流值和相对塑性黏度的影响程度则均随其粒度的减小而增大;2)试验浇注料的相对屈服应力随5~3 mm棕刚玉骨料加入量的增加略有减小,随着≤1 mm棕刚玉骨料加入量的增加略有增大,随着3~1 mm棕刚玉骨料加入量的增加变化不大,并且无规律;3)≤1 mm棕刚玉骨料对浇注料相对屈服应力的影响最大,其次为5~3 mm的,3~1 mm的影响最小。     

The influence of different drying methods on cement paste microstructures as reflected by gas adsorption: Comparison between freeze-drying (F-drying), D-drying, P-drying and oven-drying methods

Drying of cement pastes is required prior to microstructure investigation by means of gas adsorption technique. An ideal drying method, which would give reproducible results that could perfectly remove only the non-bound water and, at the same time, preserve the microstructure, unfortunately does not exist. The different drying methods used affect the microstructures in different ways. However, an effective water removal and, less damaging drying method between the common methods used would be still of outstanding importance for sample preparation. Many drying methods have been investigated in the past for such a purpose, and a good agreement for the best drying method does not exist. The so-called D-drying method is being used in many laboratories as the “best” method for drying cement pastes. The surface areas and pore size distributions results of the current work confirm that D-drying (D-Drying C^− t) is a relatively good preservation and effective drying method, and that Freeze-drying gives slightly better results compared to D-drying (C^− t) and other methods. However the short time versions of some of these methods indicate the presence of very few “micropores”, which are not present with prolonged drying times. The outgas level is also a very important variable affecting the gas adsorption measurements especially in the case of short duration drying conditions, as indicated by the results of this work.     

Large-scale fabrication of TiC@C powders and its effect on the properties of Al2O3-MgO-C castables

High performance carbon containing castables have always been pursued by researchers and steelmaking producers, unfortunately, poor water-wettability of graphite flakes was greatly limited their application in castables. To respond this, we proposed a large-scale and low-cost modified molten salt shielding synthesis technique for fabricating TiC coated graphite in air atmosphere using graphite flake and Ti powder as raw materials. Microstructure, wettability and oxidation resistance of TiC@C powders, and effect of TiC@C powders on the properties of Al₂O₃-MgO-C castables were investigated. The results demonstrate that TiC coated graphite was synthesized via modified molten salt shielding synthesis route in air atmosphere. A uniform and continuous TiC layer was formed on the surface of graphite, thereby significantly improving the water-wettability and oxidation resistance of graphite flakes. The castables with 5% TiC@C powders possessed lower apparent porosity, higher cold strength, good oxidation resistance, and slag resistance in comparison with the castables with graphite flakes, and slag resistance were also better than Al₂O₃-MgO castables. The as-fabricated TiC@C powders have good water-wettability and oxidation resistance, making them as a prime carbon source for producing carbon containing castables for steel ladle linings.     

The RNA-bound conformation of neamine as determined by transferred NOE experiments

The tRNA(Phe)-bound conformation of the aminoglycoside neamine, a member of the neomycin B family, has been investigated by transferred NOE experiments in aqueous solution. This is the first time that the bioactive conformation of an RNA-bound aminoglycoside has been determined by this method. In buffers without divalent Mg(2+) ions, a high degree of electrostatically driven unspecific binding of aminoglycosides to the RNA was observed. Careful optimization of experimental conditions yielded buffer conditions optimized for cryo-probe NMR experiments. In particular, addition of Mg(2+) ions to the solutions was necessary to reduce the amount of unspecific binding as monitored by one-dimensional NMR and surface plasmon resonance experiments. CD spectroscopy was used to probe the effect of aminoglycosides and buffer conditions on the double helical content of tRNA(Phe). Finally the tRNA(Phe)-bound conformation of neamine was determined by trNOE build-up curves and compared with the previously reported crystal structure of neomycin B complexed to this RNA. Although the aminoglycoside in the crystal structure contains several configurational errors, the overall shape of the crystallographically determined RNA-bound structure is identical to the RNA-bound conformation defined by the NMR experiments. Therefore, the crystal structure has been refined by trNOE data. This is particularly important in the context of aminoglycosides being discussed as lead structures for the development of new anti-RNA drugs.     

Quantitative determination of pore-structure change and permeability estimation under hydrate phase transition by NMR

Quantitatively characterizing the seepage features is critical important for multi-fluid flow in gas hydrate accumulations; however, limited researches concern water permeability during hydrate phase transition. In this work, nuclear magnetic resonance (NMR) measurement is employed to observe the in situ formation and dissociation of tetrahydrofuran (THF) hydrate in porous media. Results indicate that the relative free water and bound water consumption during hydrate phase transition can affect the seepage features of sediments. In addition, we investigate the growth habits of THF hydrate in quartz glass sand and find the growth pattern of the hydrate transforms from suspension to cementation when its saturation exceeds approximately 35%. The Tokyo model shows that the hydrate are heterogeneous distribution of pore-filling and likely to evolve in larger pores; The findings clearly show that NMR is an efficient and direct technique for investigating the seepage characteristics during hydrate phase transition as well as pore fluid distribution in sediments.     

The effects of high-temperature annealing on microstructure and performance of Cf/C-SiC composites modified by FeSi2

FeSi₂ modified C/C-SiC composites (C/C-SiC-FeSi₂) are fabricated by chemical vapor infiltration (CVI) combined with reactive melt infiltration (RMI) with FeSi75 alloy. The effects of high-temperature annealing (1600?°C, 1650?°C, 1700?°C) on the microstructure and performance of C/C-SiC-FeSi₂ are investigated. With the elevation of annealing temperature, the porosity of the composites and the content of SiC increase due to the evaporation of liquid Si and the further reaction of Si and C. The mechanical performance gradually decreases due to the catalytic graphitization of the carbon fiber, the high porosity and the thermal residual stress (TRS) caused by thermal mismatch of different phases. The coefficient of thermal expansion and thermal diffusivity slightly decrease with increasing annealing temperature for the increase of porosity. However, the friction performance of the heat treated materials at high braking speed are greatly improved attributing to the increase of SiC content and the capturing and storage function of pores on hard particles.     

Diffusional behavior of amino acids in solid-phase reaction field as studied by H pulsed-field-gradient spin-echo NMR method

The diffusion coefficients (D) of tert-butyloxycarbonyl-l-phenylalanine (Boc-Phe) in Merrifield network polystyrene gels, used as a solid-phase reaction field have been determined as a function of the amino acid concentration over the temperature range from 30 to 50 °C by means of the ¹H pulsed-field-gradient spin-echo NMR method. From these experimental results, it was found that the D value of Boc-Phe in DMF-d₇ solution, in DVB 1 and 2% cross-linked network polystyrene gels depends on the amino acid concentration. The D value of Boc-Phe·Cs(tert-butyloxycarbonyl-l-phenylalanine cesium) salt in the solid-phase reaction field under chemical reaction was determined at 50 °C. Further, it was found that the D value depends on the NMR observation time, that is the applied two field-gradient pulse interval. Details of its analysis were discussed.     

The structure and permeability of the disordered layer prepared by freeze-casting

Asymmetric ceramic membranes with disordered layer prepared by directional freeze-casting can be used as filter materials. However, there are few research achievements on the structural and permeability of disordered layers. Here, we obtained alumina asymmetric ceramic membranes with different disordered layer by adjusting the amount of alumina in the ceramic slurry. The results show that there are two kinds of structures in the disordered layer: spherical micropores and interconnected meshy pores, which lead to the change of airflow state. With the increase of the alumina content, the pore size of the disordered layer decreases and the network structure becomes complex, resulting in the decrease of the velocity of the inertial flow. The filter resistance of disordered layer increases obviously, but its proportion in the total filter resistance increases first and then decreases. The permeability is related to the pore size, specific surface area and channel complexity of the membrane.