| REB2C2(RE=Sc,Y and lanthanides)compounds have a feature of layered crystal structure,which is similar to that of MAX phases.In the crystal structure of REB2C2,boron and carbon atoms form two-dimensional(2-D)networks which alternate with 2-D sheets of RE atoms along the z direction.And strong B-C covalent bonds in the B/C layer coexist with weak RE-B and RE-C bonds between RE layer and B/C layer.The layered structure of REB2C2 ceramics and the weak bonds between RE layer and B/C layer make it possess excellent mechanical properties similar to the MAX phase,such as machinability,high damage tolerance,anisotropy,and excellent high temperature mechanical properties.Considering to the fact that REC2 can react with water and the importance of carbide derived carbon(CDC),it is easy to speculate that REB2C2 materials may possess high chemical activity.In all REB2C2 materials,YbB2C2 is rather difficult to synthesize.Thus,this work takes REB2C2 materials as the research object,mainly taking YbB2C2 ceramic as an example.Based on the successful preparation of high-purity REB2C2 ceramics,a systematic study of its mechanical properties and chemical activity is carried out.The main research work is as follow:(1)YbB2C2 material was synthesized by boro/carbothermic reduction method with a mixture of Yb2O3,B4C,and graphite powders as raw materials.The ideal preparation condition is:the molar ratio of Yb2O3:B4C:C(graphite)is 0.7:0.8:6.0;the sintering temperature is 1950?,the holding time is 1.5 hours,and the heating rate is 10?/min;the atmosphere is high-purity argon(Ar).After that,the Raman spectrum and active vibrational modes of YbB2C2 were systematically studied by a micro-Raman spectroscope and the first-principles calculations,respectively.In addition,a new exploration of the "boron/carbothermic reduction method" to synthesize REB2C2 materials was carried out by changing the raw materials(replacing RE2O3 powder with REH2,REBO3 and RE(OH)3 powder,or using carbon black instead of graphite)and by adjusting the molar ratio of the raw materials.As a result,Yb0.38Y0.62B2C2 solid solution,YB2C2 and LaB2C2 materials were successfully prepared.(2)The anisotropy of YbB2C2 compound is calculated and predicted by the first-principles calculations.YbB2C2 compound is easier to compress along z-axis direction than along x-axis direction,and shows easy sliding in the x-y plane.The anisotropy of YbB2C2 compound is verified by ?//(the loading direction parallel to the forming direction)and ??(the loading direction perpendicular to the forming direction)compressive strength of the as-prepared porous YbB2C2 samples.Porous YbB2C2 ceramics have a high porosity in the range of 69.89-58.11%and a high compressive strength in the range of 19.49-63.44 MPa.YbB2C2 compound remains stable in a flowing argon atmosphere at a high temperature of 2250? under normal pressure,but it can produce a small number of impurities such as YbB6,C and unknown phase under hot pressing(1980?,30 MPa).(3)YbB2C2 compound shows high activity,for example,it can react with water at room temperature.The solid reaction products between YbB2C2 and H2O are oxygen-containing graphite crystal(C(O),the atomic ratio O is about 7%),YbB6 and Yb3(OH)3n(BO3)(3-n)sol.The initial C(O)is cabbage-like microsphere consisting of a number of thin lamellae.C(O)microsphere gradually disintegrates and can change into layered graphite composed of multiple smaller layers with the reaction undergoing.The gaseous reaction products of the hydrolysis of YbB2C2 include H2(about 90.55%),CH4(about 9.02%),C2H2,C2H4,C2H6,C3H6,C3H8,C4H8,CO and CO2.The reaction process of the hydrolysis of YbB2C2 can be divided into four stages according to the reaction rate:preparation period,initial reaction period,later reaction period and ending period.The Yb3(OH)3n(BO3)(3-n)sol was believed to play an important role in the reaction process.The fact that YbB2C2 can react with water to produce C(O)indicates that it has the similar property to that of carbide employed to prepare CDC by supercritical water method.According to the first-principles calculations,the high activity of YbB2C2 compound comes from the weak bonds:Yb is likely to form metal-like bonds with weak bonding to C and B atoms;the B-C bonds in B/C layers can be separated to longer and shorter groups,the longer one,i.e.,B-CL(L refers to‘long')is weak.The hydrolysis mechanism of YbB2C2 can be demonstrated by a molecular cell model:Yb3+acts as a cathode and(B2C2)3? acts as an anode.At the cathode side,six H2O molecules are reduced to six H atoms,and six OH-ions are produced.At the anode side,four B0.5-ions transfer two electrons to H2O molecules and change into four B atoms,and four C-ions transfer four electrons to H2O molecules and change into four C atoms.The hydrolysis reaction depends on the charge transfer from(B2C2)3-to H2O with Yb3+as a bridge((B2C2)3-? Yb3+?H2O)in the molecular cell.In the next step,the as-produced OH-diffuses to the anode and is oxidized to form O atom.The hydrolysis of YbB2C2 is exothermic,for example the as-prepared porous YbB2C2 sample emitted so much heat that it could burn a piece of paper when the sample was wetted by water.Similar to YbB2C2 compound,YB2C2 compound can also react with water at room temperature,which indicates that other REB2C2 materials may also have the similar high activity.The hydrolysis of REB2C2 is not only different from that of REC2,but also different from that of carbide employed to prepare CDC by supercritical water method.Compared with REC2,other carbides and MAX phases,REB2C2 shows higher activity.(4)The reactions of REB2C2 with H3BO3,KOH and HF solutions are more severe than its hydrolysis reaction.The reaction products between YbB2C2 and H3BO3 solution are HBO2,YbB6,C(O),B7O,unknown phase,H2,CH4,C2H2,C2H4,C3H6,CO and CO2.The reaction products between YbB2C2 and KOH solution are YbB6,C(O),amorphous phase,unknown phase,residual YbB2C2,H2,CH4,C2H4 and CO2.The abundant H+ ions in the H3BO3 solution and the rich OH-ions in the KOH solution are the main reasons for the difference between these two reaction products.The reaction mechanisms of YbB2C2 with H3BO3 and KOH solution are similar to its hydrolysis mechanism,and both can be understood by the molecular cell model. |
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