Fabricate Hydrophobic Materials Of Transition Metal Diborides By Surface Modification

Hydrophobic material refers to one material whose static contact angle between the solid surface and water is greater than 90°.The most notable feature of this type of material is:self-cleaning,anti-adhesion,and has been widely used in many areas of the national economy.Although there are many kinds of hydrophobic materials,their functions are relatively single.Especially,multi-functional hydrophobic materials are required in extreme environmental conditions.Therefore,the research and development of multifunctional hydrophobic materials with electrical conductivity,high hardness and good thermal stability not only has scientific significance,but also has great application prospects.Transition metal borides(TMBs)have high hardness,high thermal stability and excellent electrical conductivity,and have been widely used in many fields.However,due to its high surface energy and difficulty in surface modification,hydrophobic properties have not yet been found in the system compounds.In recent years,through material preparation methods,a micro-nano co-constructed rough structure is prepared on the surface of the material,and the surface is modified with low surface energy substances,which can greatly enhance the hydrophobic properties of the material,and it has become a new method for preparing hydrophobic materials.However,the high melting point of TMBs leads to high preparation temperature,and the traditional template method(failure above 1000°C)and other simple surface morphology construction methods have failed for TMBs.There is an urgent need to develop new methods for constructing the surface morphology of TMBs.In addition,because TMBs are not traditional hydrophobic materials,there is few researching on the free energy of each crystal face of TMBs.In order to design TMBs with low surface energy exposed surfaces,we first need to reveal the surface energy of each crystal plane of TMBs.In this paper,transition metal diborides(TMBs,TiB₂,ZrB₂,HfB₂)are used as the research object,using density functional theory(DFT)to calculate the surface energy of each crystal plane of TiB₂,ZrB₂,and HfB₂ to guide the design of hydrophobic surface structure;using high pressure and high temperature methods to prepare bulk materials;preparation of nano-scale surface structures with hydrophobic characteristics by surface modification methods.The hardness,conductivity,oxidation resistance and hydrophobic properties of the samples after surface modification under different P-T conditions prepared under high temperature and high pressure were systematically studied,and the research results as following:1.The physical property parameters of bulk materials(TiB₂,ZrB₂ and HfB₂)which prepared under high pressure and high temperature were measured.Under the condition of 5 GPa pressure,the TiB₂,ZrB₂ and HfB₂ samples with the highest density were obtained by adjusting the temperature and holding time.It is found that TiB₂ is the most density at 1600°C and 300 min,ZrB₂ is the most density t at 1600°C and 30 min,and HfB₂ is the most density at 1600°C and 180 min,and their mechanical and electrical properties are as follows:TiB₂(25.0 GPa,1.21×10^-7(?)·m);ZrB₂(17.5 GPa,4.87×10^-8(?)·m);HfB₂(21.5 GPa,1.04×10^-7(?)·m).We can find that TiB₂,ZrB₂ and HfB₂ are harder than hard materials such as Al₂O₃ and WC;the conductivity of TiB₂ and HfB₂ is close to that of iron,and the conductivity of ZrB₂ is close to that of Cu.We conducted thermogravimetric analysis on the starting materials of TiB₂,ZrB₂ and HfB₂,and measured the oxidation onset temperatures of these three samples respectively:466°C,632°C and 763°C,which is better than the oxidation resistance of organic hydrophobic materials.2.Density functional theory(DFT)calculated the surface energy of each crystal plane of TiB₂,ZrB₂,and HfB₂,and found that the basal plane(001)_TM,(001)_B has a higher surface energy than(100)_B,(101)and(110).(100)_TM has the highest surface energy.Using hydrochloric acid to preferentially corrode high-energy sites,a nano-needle structure is formed on the surface of the sample.Using the TEM characterization method,it is found that the crystal orientation along the nanoneedle is[001],and there is a cap composed of amorphous boron at the upper end.Based on this,it is speculated that the hydrochloric acid corrosion process:the TM layer on the top of the base plane(001)is first destroyed by H⁺or Cl^-,and then the H⁺or Cl^-in the solution passes through the boronphene like subunit layer composed ofB atoms to corrode the internal TM.After the internal TM layer is destroyed,the boronphene like subunit layer whose surface is not corroded cannot maintain the structure and is transformed into an amorphous boron layer.Amorphous boron closes the pores in the boronphene like subunit layer.When the amorphous boron layer reaches a certain thickness,it is difficult for H⁺or Cl^-to pass through and continue to corrode the sample along the[001]crystal direction,while in(101),(110)crystal plane,this process does not occur,resulting in hydrochloric acid along the[001]direction,and along the[010],[110]direction corrosion rate difference,resulting in the appearance of nano-needle structure on the surface of TMBs,and in this process,the(101)and(110)crystal planes with low surface energy are exposed.3.Through the surface modification method,the nano-needle surface morphology structure is realized in the TMBs,and excellent hydrophobic properties are obtained.The hydrophobicity test results show that the contact angles of TiB_2,ZrB₂,and HfB₂are 132.0°,116.8°,and 114.0°,respectively.And their intrinsic contact angles are respectively:89°,84.8°,and 84°.After surface modification,TiB₂,ZrB₂,and HfB₂ have realized the transition from hydrophilic to hydrophobic,and the hydrophobicity is better than other inorganic materials(Mg,Al,BN,diamond,etc.).The hydrophobic properties of TMBs are derived from the lower surface energy crystal planes exposed after surface modification and the formation of nanometer-scale surface structures on the surface.The work in this paper not only prepares a new type of multifunctional hydrophobic material,but also has important significance for developing new functional materials based on TMBs.