Fabrication And Properties Of Inorganic Solid Buoyancy Materials Based On Hollow Ceramic Microspheres

Buoyancy materials are very important engineering materials and they were widely used in the manufacture of deep-sea submersibles,marine oil and gas resources exploitation,construction of artificial island on the sea and other marine exploration fields.There is a huge demand for heat-resistant buoyancy materials with the further exploitation of marine,for example,detecting deep-sea hydrothermal vents,constructing sea orbiter and building trans-media aircraft.At present the main buoyancy materials used at home and abroad are resin-based buoyancy material,which were fabricated by adding hollow spheres into the resin matrix.However,due to the poor heat-resistance,the resin-based buoyancy materials were unsuitable for the application in high temperature.In this paper,fabrication and investigation of inorganic solid buoyancy materials were carried out.The inorganic solid buoyancy materials were prepared by binding hollow ceramic microspheres with different high-temperature binders.Firstly,inspired by the idea of preparing abrasives with vitrified bond and diamond particles via a low temperature sintering process,heat-resistant inorganic solid buoyancy material was prepared by bonding hollow glass microspheres with boro-alumino-silicate glass powder.The effects of types and content of hollow glass microspheres,and sintering schedule on the properties of samples were investigated.Two different modes of failure under compressive loading for different samples were observed and the failure mechanisms were analyzed.Beside,the compressive properties of the samples at high temperatures were tested and analyzed.The bulk density and compressive strength of samples were 0.23-0.91g/cm³ and4.94-28.03MPa,respectively,when the samples containing different types of hollow glass microspheres sintered at 700-800°C for 2 h.The high-temperature compression test indicated that the sample still maintain the original mechanical properties at500-550℃.The test shows that the thermal conductivity and elastic modulus of the samples decreases with the increase of the pore volume fraction.Thermal conductivity values of the samples were close to the values calculated with the Landauer expressions.Elastic modulus values of the samples were close to the values calculated with the Hashin-Shtrikman upper bound model.Secondly,in order to reduce the effect of softening of boro-alumino-silicate glass binder on the properties of the sample at high temperature,inorganic buoyancy materials with high-strength were prepared by bonding the hollow glass microspheres with phosphate binder.The influence of additives B₄C on the properties of buoyancy material was studied.It was found that the phosphate binder lost its moisture and a certain amount of voids were appeared at elevated temperature,which adversely affected the mechanical properties of the samples.B₄C’s oxidation can make volume compensation to the appeared voids.When the amount of additives of B₄C increased from 5 to 15 wt%and calcined at 700-900°C for 2h,the compressive strength,elastic modulus and bulk density of buoyancy material is 10.80-20.38 MPa,0.81-1.23 GPa and 0.525-1.198 g/cm³,respectively.In addition,the butane flame was used to simulate high-temperature conditions and the heat-resistance test was carried out.The result indicated that the prepared inorganic buoyancy material possess good heat resistance.Besides,experimental values were compared to calculated values using three analytical models of the elastic modulus as a function of the pore volume fraction:the Hashin-Shtrikman upper bound,Pabst and Gibson-Ashby model.Elastic modulus values of the samples were close to the values calculated with the Hashin-Shtrikman upper bound model.The buoyancy material containing hollow glass microspheres cannot be used at higher temperatures（above 600℃）for long time due to the low softening temperature of glass hollow microspheres（about 600℃）.Finally,inorganic buoyancy materials with high temperature resistance were prepared by bonding alumina hollow spheres with high-temperature composite binders.The effects of sintering schedule,sintering agent with low melting point and chopped mullite fibers on the properties of the samples were analyzed.The results showed that when weight ratio of hollow alumina spheres with particles size of 0.2-0.5:0.2-1.0:1.0-2.0mm=2:1:2,adding 40wt.%spherical silica powder,5 wt.%B₂O₃ and 6 wt.%chopped mullite fibers,calcinated at1500℃for 4h,the bulk density,compressive strength,porosity of sample was1.30g/cm³,11.97MPa,and 62.5%,respectively.