万小涵
,
张广清
,
John Sharp
,
Oleg Ostrovski
,
俞乐
有色金属工程
doi:10.3969/j.issn.2095-1744.2015.04.003
碳热还原/氮化合成氮化硅在1 450～1 650℃、氮气分压700～1 100 kPa下进行.非晶二氧化硅与石墨粉末以n(C)/n(SiO2)=4.5比率混合、压片.样品中的氧、氮、碳含量通过LECO元素分析仪测得.反应物和生成物中各物质组成通过XRD定性分析.结果表明,通过增加氮气分压,氮化硅的稳定温度升高,二氧化硅的还原速率随着温度升高而变大.碳化硅的生成仍无法避免.与常压下的碳热还原/氮化反应一致,高压下的碳化硅和氮化硅的生成亦为连续反应.
关键词:
高压
,
Si3N4稳定性
,
连续反应
新型炭材料
Carbon nanotubes (CNTs) with sharp tips were synthesized by a hydrogen arc discharge method. Three unusual morphologies, i.e., a cone-shaped tip, a suddenly-shrinking tip, and a pencil point-like tip were observed. These novel tip structures are considered to be related to the addition of a small amount of silicon powder in the raw material, which may introduce structural defects in the CNTs. The field emission property of the sharp-tip CNTs was investigated, and a low threshold electric field of 3.75 V/m, a high field emission current density of similar to 1.6x10(5) A/cm(2), and a good emission stability were demonstrated. The superior field emission performance of the CNTs can be attributed to their good crystallinity and unique tip structures.
关键词:
Carbon nanotubes;Sharp tip;Field emission;electron source
Z.S. Ma
,
Y.C. Zhou
,
S.G. Long
,
C.S. Lu
材料科学技术（英文）
An inverse method for extracting the elastic–plastic properties of metallic thin films from instrumented sharp indentation has been proposed in terms of dimensional analysis and finite element modeling. A wide range of materials with different elastic modulus, yield strength, and strain-hardening exponent were examined. Similar to the Nix–Gao model for the depth dependence of hardness H, (H/H0)2=1+(h*H/h), the relationship between elastic modulus E and indentation depth h can be expressed as (E/E0)4=1+(h*E/h) . By combining these two formulas, we find that there is a relationship between yield stress σy and indentation depth h: σy=σy0?(1+(h*E/h))r(n)?(1+(h*E/h))[0.25-0.54f(n)], where σy0 is the yield strength associated with the strain-hardening exponent n, the true hardness H0 and the true elastic modulus E0. f(n)= (2(1-n))-1 is constant, which is only related to n, and h*H and h*E are characteristic lengths for hardness and elastic modulus. The results obtained from inverse analysis show that the elastic–plastic properties of thin films can be uniquely extracted from the solution of this relationship when the indentation size effect has to be taken into account.
关键词:
Minh-Quy LE
,
Seock-Sam KIM
材料科学技术（英文）
Finite element analysis was carried out to investigate the conical indentation response of elastic-plastic solids within the framework of the hydrostatic pressure dependence and the power law strain hardening. A large number of 40 different combinations of elasto-plastic properties with n ranging from 0 to 0.5 and σy/E ranging from 0.0014 to 0.03 were used in the computations. The loading curvature C and the average contact pressure pave were considered within the concept of representative strains and the dimensional analysis. Dimensionless functions associated with these two parameters were formulated for each studied value of the pressure sensitivity. The results for pressure sensitive materials lie between those for Von Mises materials and the elastic model.
关键词:
Finite element analysis
,
null
,
null
,
null
Yongli Huang
材料科学技术（英文）
The reverse analysis provides a convenient method to determine four elastic-plastic parameters through an indentation curve such as Young's modulus E, hardness H, yield strength σy and strain hardening exponent n. In this paper, mathematical analysis on a reverse algorithm from Dao model (Dao et al., Acta Mater., 2001, 49, 3899) was carried out, which thought that only when 20≤E*/σ0:033≤26and 0.3<n≤0.5, the reverse algorithm would yield two solutions of n by dimensionless function Π2. It is shown that, however, there are also two solutions of n when 20≤E*/σ0:033≤26 and 0≤n<0.1. A unique n can be obtained by dimensionless function Π3 instead of Π2 in these two ranges. E and H can be uniquely determined by a full indentation curve, and σy can be determined if n is unique. Furthermore, sensitivity analysis on obtaining n from dimensionless function Π3 or Π22 has been made.
关键词:
Elastic-plastic properties
,
null
,
null
,
null
,
null
Yongli Huang
材料科学技术（英文）
The reverse analysis provides a convenient method to determine four elastic-plastic parameters through an indentation curve such as Young's modulus E, hardness H, yield strength σy and strain hardening exponent n. In this paper, mathematical analysis on a reverse algorithm from Dao model (Dao et al., Acta Mater., 2001, 49, 3899) was carried out, which thought that only when 20≤E*/σ0:033≤26and 0.3<n≤0.5, the reverse algorithm would yield two solutions of n by dimensionless function Π2. It is shown that, however, there are also two solutions of n when 20≤E*/σ0:033≤26 and 0≤n<0.1. A unique n can be obtained by dimensionless function Π3 instead of Π2 in these two ranges. E and H can be uniquely determined by a full indentation curve, and σy can be determined if n is unique. Furthermore, sensitivity analysis on obtaining n from dimensionless function Π3 or Π22 has been made.
关键词:
Elastic-plastic properties
,
null
,
null
,
null
,
null
