A First Principles Study On The Effects Of Doping And Extreme Conditions On The Ideal Strength Of Materials

In this dissertation,the first-principles calculation method and Ab-Initio Molecular Dynamics method based on density function theory are used to study the effects of the different types of doping on the ideal strength of transition metal borides and nitrides,also we analyze the effects of extreme high pressure on the ideal compression strength of various carbon and nano-carbon structural materials.Finally,we study the tensile,compressive and shear strength of diamond under extremely high temperature and high pressure by using our recent improved first-order molecular dynamics method,we find some physical phenomena which are different from room temperature and atmospheric pressure.The research background and basic theories for this study are introduced in chapter one and two.In chapter three,we perform first-principles calculations to determine the stress-strain relation and the ideal indentation strength of the interstitial boron solid solution WB_3+xthat has been proposed as an alternative structural model for the recently synthesized?nominal?WB₄samples reported in experiments.Transition metal borides?TM_n?offer a cost-effective and versatile alternative to traditional superhard materials makes it an important research object of new superhard materials,which has aroused wide interest in the world.In the transition metal boron chemical family,an experimentally synthesized?nominal?WB₄material Vickers scale measurement hardness reached 46.3 GPa.It is the kind of material in the most promising material to become superhard,but because the boron element is not sensitive to X-ray,making the experimentally synthesized?nominal?WB₄material so far its exact structure is still very controversial.Very recently,based on first-principles structural search and high resolution tansmission electron microscopy?HRTEM?images,an interstitial boron solid solution model WB_3+x?x<0.5?was proposed,where boron atoms occupy interstitial positions in the WB₃?P63/mmc?lattice,seems to be able to perfectly explain the various phenomena observed in the experiment.However,Our calculations find that the ideal indentation strength of WB_3+xsuffers an unusually large reduction(about 50%for WB_3.25)compared to that of WB₃due to the interstitial boron solution.The calculated ideal indentation strength of WB_3.25?10 GPa?is considerably lower than that of ReB₂?27.6 GPa?,suggesting that the?asymptotic?Vickers hardness of WB_3+xshould be well below that of ReB₂.This result is in direct contradiction to the experimental findings that experimentally synthesized?nominal?WB₄has a hardness higher than that of ReB₂.Moreover,the calculated XRD spectra of WB_3+x+x should exhibit peak splitting near 2?=34^?as boron solution content x increases,which is not observed for the synthesized WB₄compound.These contrasting results offer compelling evidence demonstrating that the proposed WB_3+xstructure is incompatible with key properties of the synthesized?nominal?WB₄samples.The structural determination of the synthesized W-B compound is thus reemerging as an open question that deserves immediate attention.By our calculations and summary of previous experiments and theory calculations.We suggest the possibility that WB₄may not actually exist in the synthesized sample.More experimental investigations are needed to clarify this issue.Our results also show that increasing boron content in TMB_ncompounds by?interstitial?doping may not always enhance their hardness.In chapter four,we show by first-principles GGA+U calculations that Mn-substitution doping produces a ferromagnetic ground state of MnFe₃N with the Mn atoms replacing Fe atoms at the cubic corner posi-tions.?^?-Fe₄N produced in a nitriding process is widely used to improve the wear,fatigue,and corrosion resistance of iron and steel surfaces.In recent years,Fe₄N also attracted great interest for its large saturation magnetization and high spin-polarization ratio?SPR?high Curie temperature,which have been extensively studied experimentally and theoretically to explore potential applications in high density magnetic recording and spintronic devices.It has been expected to improve the mechanical properties and magnetic properties by?^?-Fe₄N substitution doped with the transition metals.The experimental results showed that the manganese-substitution-doped can improve the magnetic properties of?^?-Fe₄N,which contradictory opposition with the previous calculation works confirmed that the manganese-substitution-doped make the structure showed antiferromagnetic and the overall magnetic moment of the structure weakened.Here we demonstrate by first-principles calculations that the ferromagnetic state with enhanced magnetization in MnFe₃N is driven by the electron correlation effect not previously considered.This is because the on-site Coulomb repulsion poten-tials,especially U_Mn,can stabilize more strongly the Mn d electron states when Mn occupies the cubic corner position in MnFe₃N,while in ordinary GGA calculations this effect is neglected.Our calculation explains re-cent experimental observations that in Mn-doped Fe₄N Mn atoms mainly substitute the cubic-corner Fe atoms.The resulting MnFe₃N phase exhibits enhanced magnetization that is important to device applications.Our first-principles stress-strain calculations show that,although Fe₄N exhibits the largest Young's modulus along the[001]direction,its largest tensile strength occurs in the[111]direction,the same phenomenon is observed in MnFe₃N.Moreover,both Fe₄N and MnFe₃N have the largest shear strength of about 20 GPa in their?011?plane,which is 35%higher than those in all of their other sliding planes.This large shear strength is achieved by a remarkable nonlinear stress response that undergoes a steep rise at large strains,a phenomenon highly unusual for a crystalline solid.Our results suggest that the?011?planes of Fe₄N and MnFe₃N possess the highest scratching hardness and wear resistance,which make them suitable for applications in high density magnetic recording heads and otherapplications.In particular,it calls for experimental efforts to grow high quality single phase Fe₄N and MnFe₃N films in the[011]crystal directions.Under pressure and at low tem-peratures,dynamical instability with soft phonon modes start to appear in Fe₄N and MnFe₃N at about 14 and21 GPa,respectively.But the strong phonon anharmonic interactions in these materials can lift the dynami-cal instability even at the room temperature,and the structures of Fe₄N and MnFe₃N can remain dynamically stable up to pressures of about 30 GPa,which is consistent with high-pressure x-ray diffraction experiments.In this work,we have unveiled the simultaneous presence of several unusual mechanisms in Fe₄N and its substitution-doped derivative structure MnFe₃N,including strong electron correlation,large nonlinear stress response,and strong lattice anharmonicity,which produce fundamentally interesting and practically useful magnetic and mechanical properties.The obtained results are expected to have important implications for optimal design of these materials in wide ranging applications in electronic and spintronic devices.In chapter five,we report first-principles calculations based on density functional theory to predicate the highest pressure that can be generated in two-stage Diamond anvil cell?ts-DAC?with various super-hard car-bon structures and nano-carbon with various grain boundaries as the materials of the second stage indenters.Diamond anvil cell?DAC?is currently the only high pressure apparatus that can generate static pressures in the multi-megabar range?=100 GPa?,creating environment conditions deep in the earth in laboratories.The highest pressure in simple DAC is limited by the maximum compression strength of the diamond anvil,which is about 500 gigapascal?GPa?as predicated by the First-principles calculations in the[111]compression di-rection of diamond.Recent experimental observation of the molecule hydrogen transformation to metallic hy-drogen in DAC at 495 GPa has reached this static pressure limit.Very recently,a series of successful attempts has been made to achieve static pressure over 1 TPa?=1000 GPa?in a two-stage DAC?ts-DAC?.In order to comprehensively study the basic principles of the two-stage DAC.The second indentation stage in ts-DAC is simulated by compressing the material of the second indenter with a compression pressure P_cwhile maintain-ing a constant surrounding pressure P_sgenerated in the first stage indentation vertical to P_c.Our results show that the indentation strength of the second indenter in ts-DAC with uniform distribution of bond lengths and angles,such as cubic diamond,hexagonal diamond,and nano-twinning diamond,have much higher max-imum compression strength than those of carbon structures with uneven distributions of bond lengths and angles,such as M-carbon,C₄₆and amorphous carbon etc.Grain boundaries in nano-carbon generally reduce the compression strengths of the second indenter.Because of their grain boundaries can induce local atomic rebonding near grain boundaries under compression which triggers early structural phase transformation of the nano-carbon structures and lowers their compression strengths.The maximum compression strengths of all the carbon and nano-carbon structures we studied are limited by phase transformations induced by the compression pressure to structures with lower maximum compression strengths.Compared to the cubic dia-mond,actually hexagonal diamond has lower maximum compression strength than cubic diamond due to its specific stacking order,the neighbouring buckled carbon layers in hexagonal diamond will form an extra bond under compression which lead to its early structural phase transformation caused by phonon instability.The structural phase transformations are initiated by phonon instability under compression pressures with?cubic?diamond possessing the highest maximum compression strength at phonon instability.When the surrounding pressure P_sis above 300 GPa,the maximum compression strength of diamond?at phonon instability?reaches1 TPa.Our results explain experimental results and offer insights into the fundamental mechanisms for the two-stage Diamond anvil cell?ts-DAC?and mechanisms for the compression deformation of the super-hard carbon structures under extreme high pressure conditions.In chapter six,we report ab initio molecular dynamical?AIMD?calculations of the compression,tensile and shear strengths of diamond under high temperatures and pressures?HTHP?.Diamond,currently known as the hardest material in nature,as the benchmark for superhard materials,has been the most widely studied for its mechanical properties.However,for diamond at extremely high temperature and high pressure condi-tions,to our knowledge,no studies have been reported on mechanical strength of diamond under such HTHP conditions and how?shear?structural deformation affects the phase diagram of diamond,such as shear in-duced early melting and different deformation patterns,etc.For these issues we need to conduct a systematic study.The mechanical strengths of diamond reduce by two thirds as temperature reaches 3000 K,while its elastic constants remain almost unchanged,consistent with the experimental observations.The calculations show that in compression,the diamond structure undergos mainly brittle graphitizing at temperature below1500 K,while in shearing,plastic deformations with locally mixed sp²and sp³bonding develop even at room temperature.The results explain the conflicting experimental reports on the deformation pattern of diamond,whether it is dominated by brittle cracking or plastic deformation,under indentation at low temperatures?<1000 K?.At high pressures,directional shear-induced early melting of diamond is found in its?111?[11?2]easy sliding shear direction,at least about 1000 K below the melting temperature of diamond predicted pre-viously.These findings shed lights on deep understanding of the mechanical behavior of diamond in HTHP extreme conditions,such as in the laser heated diamond anvil cell,in the core center of the earth and other carbon-rich giant planets.