Hydroxyapatite (HA) was synthesized by using a hydrothermal method with Ca(NO3)2·4H2O and H3PO4. We use x-ray diffraction and field-emission scanning electron microscopy to investigate how pH, reaction temperature, hydrothermal-reaction time, and calcium-ion concentration affects the microstructure and the growth of HA crystals. In addition, we discuss the growth mechanism. The results show that the crystals grow more completely and that the aspect ratio tends to increase with increasing hydrothermal-reaction time, reaction temperature, and calcium-ion concentration. The pH of the system strongly impacts the growth of HA crystals. With increasing pH, the HA crystal grain size and aspect ratio decrease significantly. By using 1 mol/L calcium-ion concentration, pH = 10, and a hydrothermal reaction at 200 °C for 8 h, we obtain high crystallinity and crystal clear of the growth polarity with hexagonal 60–100-nm-long columnar HA, 30–40 nm in diameter. The mechanisms producing this growth may be the effect of growth conditions on ion concentration, thereby changing the HA crystal growth rate along the different crystal axes.
Melamine–urea–formaldehyde resins/n‐dodecanol microencapsulated phase‐change materials (microPCMs) were prepared by in situ polymerization method in this work. In the microPCMs, the n‐dodecanol was used as phase‐change material (PCM) for thermal energy storage, and the melamine–urea–formaldehyde resins (MUF) acted as shell materials. The Fourier‐transform infrared spectroscopy (FT‐IR) indicated that the PCM n‐dodecanol is encapsulated in the MUF shell controlling pH of n‐dodecanol emulsion at 4.5, and there is no chemical reaction between them. The thermal properties of the microcapsules were measured by differential scanning calorimeter (DSC). The morphology and particle size of the microPCMs were surveyed by scanning electron microscopy (SEM) and laser particle diameter analyzer. The results showed that styrene‐maleic anhydride copolymer (SMA) anionic emulsifier is suitable for the preparation of MUF microPCMs. The cooling and melting phase‐change enthalpy were 130.5 and 135.2 J/g, and the cooling and melting phase‐change temperature were 20.9 and 19.11°C when core–shell ratio reached 3:1. In addition, increasing amount of SMA can enhance the phase‐change enthalpy.
In mass concrete, shrinkage resulting from temperature drop and drying leads to cracking, which can seriously affect the strength and durability of cement-based materials. Fortunately, expansion agents can deter or prevent these effects, especially MgO expansion agents (MEAs). In this study, the effects of four MEAs of different activity on the expansion properties, strength, and hydration of cement paste were explored. The different expansion phenomena between the high activity and low activity MgO was especially explained by the hydration model and dynamic theory. The results indicate that when the other conditions were the same, higher curing temperature and dosage could improve the expansion to some extent. Moreover, the hydration of high activity MgO and the expansion behavior occurred mainly in the early hydration stage, while the hydration of low activity MgO and the expansion behavior had a high contribution rate in the later stage, and the final expansion of cement mixed with low activity MgO was larger.
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