論文 / 著書情報
Article / Book Information
題目(和文)

微構造の3次元可視化による焼結の熱力学的駆動力の解明

Title(English)

Clarification of thermodynamic driving force for sinteringfrom 3-Dvisualization of the microstructure

著者(和文)

大熊学

Author(English)

Gaku Okuma

出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第10999号,授与年月日:2018年9月20日,
学位の種別:課程博士,
審査員:若井 史博,東 正樹,尾中 晋,安田 公一,吉田 克己,西山 宣正,田中諭

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第10999号,
Conferred date:2018/9/20,
Degree Type:Course doctor,
Examiner:,,,,,,学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Science Tokyo Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

系・コース：
Department of, Graduate major in

学生氏名：
Student’s Name

材料
材料
大熊 学系
コース

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

指導教員（主）：Academic Supervisor(main)

指導教員（副）：Academic Supervisor(sub)

（工学）

若井史博
西山宣正

要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）Sintering is a common process during which nanoparticles and microparticles
are bonded, leading to the shrinkage of interstitial pore space. Understanding
morphological evolution during sintering is a challenge, because pore structures
are elusive and very complex. A topological model of sintering is presented
here, providing insight for understanding 3-D microstructures observed by
X-ray microtomography. Three stages of sintering are distinguished by using
Euler characteristics, which is given as the number of closed pores minus thegenus. The random packing of particles is expressed as vertices, edges, faces,and cells topologically. The genus is the number of holes, which is equivalent to
the number of pore channels connecting voids inside each cell. The elementary
processes in morphological transformation of pore structure is the creation andannihilation of pore channels and those of closed pores. Although the Euler
characteristic vs relative density curve was studied only for viscous sintering ofsilicate glass particles here, I believe the result is general and may explain thesintering behavior of many ceramic and metallic particles. These results may
open the way to control internal defects formed during sintering, because it iscrucial to understand the evolution of heterogeneous pore structures for
improving the mechanical reliability of products. The interfacial topology
provides a description of stages of sintering, and the foundations to identify therange of applicability of the methods for determining the sintering stress, that is
the thermodynamic driving force of sintering.
Sintering stress and bulk viscosity of spherical soda lime glass were derived
as functions of relative density from microtomographic images in viscous
sintering of glass particles. In the initial stage of sintering, the sintering stress isestimated from the sintering force acting among particles. The mixed method
gives fairly good estimate of sintering stress in the initial and the intermediate
states. The surface energy method is valid in the final stage of sintering. I havedemonstrated also that the sintering stress obtained by the surface energy
method depends on the pore size distribution. The decrease of sintering stress inthe final stage results from pore coarsening. The sintering stress is the driving
force for shrinkage, but can generate internal tensile stress when the shrinkageof the sample is constrained. The presence of internal stress is the origin of
microstructural instability, so that pore coarsening occurs in the constrainedsintering. Overall, we have demonstrated the feasibility of combining
microtomographic images with constitutive parameters to predict the sintering
behaviors of specific powder compacts. This approach can be applied to study
the relationship between powder processing and sintering mechanics from thepoint of view of the nature of initial particle packing.
Sintering stress and bulk viscosity of CAS glass were also measured by thediscontinuous sinter forging experiment for comparison with experimental
values from the microstructural evolution during viscous sintering of sphericalsoda lime glass particles by X-ray microtomography. The bulk viscosity andshear viscosity increased with increasing relative density. The sintering stressincreased with relative density. The decrease of sintering stress in the final stage,
which had been reported during sintering of crystalline particles, was notobserved in viscous sintering. Even though the initial particle size and chemicalcomposition of glass particles were different, both techniques provided
consistent and reliable values when the bulk viscosity was normalized by theviscosity of glass and the sintering stress was normalized by the surface tension
and the initial particle size.
Lastly, The continuum mechanics is normally concerned with the behavior ofmatter on a macroscopic scale that is large compared with discrete particles.
However, although there is general agreement on the need for a unified
treatment of continuous media, there is no agreement as to the proper level atwhich this unification should take place. I introduce the concept of therepresentative volume element (RVE), which is a volume that is sufficiently
large to contain enough information at the micro scale in order to be
representative, but it should be much smaller than the macroscopic body. The
macroscopic properties in heterogeneous materials are defined as the averageover the RVE. The minimum size of RVE was defined as the edge length of
cubic volume elements above which the heterogeneity no longer affected thestandard deviation. The RVE size was estimated to be from 11 to 17 times larger
than the average initial particle size. Alternatively the RVE size was definedfrom the value of normalized standard deviation. The RVE size increased when
the desired accuracy was higher. The RVE size was dependent on relative
density, and varied with microstructural evolution. The relationship between
RVE size and relative density was dependent on the physical property ofinterest.

備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of 2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800
Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository Website (T2R2).