1

Thermodynamics on SOG-SiRefining Processes

Institute of Industrial Science, The University of Tokyo

Kazuki Morita

The 3rd Workshop onReactive Metal Processing

2

Recent Research Fields

･Physical Chemistry of High Temperature Processing (Steelmaking and Waste Management)

･Microwave Processing for Recycling and Waste Treatment

･Thermodynamics and Processing on Solar Grade Silicon (SOG-Si) Production

Materials Production and Recycling Engineering Lab.

(Formerly Ferrous Metallurgy Lab.)

3

Contents

1. Introduction

2. Metallurgical refining for SOG-Si- Thermodynamic properties of impurities in molten Si alloys -

- Optimized Process Combined with Leaching Treatment -

3. Solidification refining of Si with Si-Al metls

- Segregation ratios of impurities between solid Si and Si-Al melt -

4. Conclusions and Future Work

4

Requirement for metallurgical refining process for SOG-Si

Depending on off-grade Si for semiconductor

Increase in solar cell production

90 91 92 93 94 95 96 97 98 99 1001011021031040

100200300400500600700800900

100011001200

Am

ount

of p

rodu

ctio

n (M

W)

YearFig. Amount of solar cell production by various processes.

Total

Poly Crystalline

Single Crystalline

AmorphousRibbon

00 01 02 03 04

5

Possibility of Low Cost SOG-Si Production

Can we remove impurities from MG-silicon by metallurgical refining processes ?

MG-Si SOG-Si

New Refining Process?

6

Ti

Fe

P

Al

Fig. Solar-cell efficiency versus impurity concentration for 4-Ω・cm p-base devices.

Fe, Ti, Al, P, B

Impurtiry Content(ppma)

Fe 1300 Ti 220 Al 3300 P 30 B 30

Table 1 Impurity contents in MG-Si.

Impurities and solar cell efficiency

7

-1600

-1200

-800

-400

0

0 500 1000 1500 2000 2500

4 Cu + O2= 2 Cu2O

2 Fe + O2 = 2 FeO

2 C + O2 = 2 CO

C + O2 = CO2

3/2 Fe + O2= 1/2 Fe3O4

2 Mn + O2= 2 MnO

Si + O2= SiO2

4/3 Ti + O2= 2/3 Ti2O3

4/3 Al + O2= 2/3 Al2O3

2 Mg + O 2= 2 MgO

2 Ca + O 2= 2 CaO

Temperature　(K)

Gib

bs e

nerg

y of

form

atio

n fo

r oxi

des　

(kJ/

mol

O2)

-1600

-1200

-800

-400

0

0 500 1000 1500 2000 2500

4 Cu + O2= 2 Cu2O

2 Fe + O2 = 2 FeO

2 C + O2 = 2 CO

C + O2 = CO2

3/2 Fe + O2= 1/2 Fe3O4

2 Mn + O2= 2 MnO

Si + O2= SiO2

4/3 Ti + O2= 2/3 Ti2O3

4/3 Al + O2= 2/3 Al2O3

2 Mg + O 2= 2 MgO

2 Ca + O 2= 2 CaO

Temperature　(K)

Gib

bs e

nerg

y of

form

atio

n fo

r oxi

des　

(kJ/

mol

O2)

Fe-FeO

Al-Al2O3

Mg-MgOCa-CaO

Si-SiO2

Fig. Ellingham diagram for some representative elements.

Strong Affinity of Silicon for Oxygen=

Difficulty inOxidation Treatment

8

Si Standard : solid Si standard : solid

GM solid0 - 0 - liquid

liquid solid

→Component X Si

LL T1

S+LS+L S

Si Si

(a) Small segregation coefficient

→Component X→Component X

→Component X

T=T1

Si

GM

(b) Large segregation coefficient

T1

T=T1

Fig. Relationship between Gibbs free energy and phase diagram below melting point of silicon.

P, B, C, etc.Most metallicimpurities

9

10-6 10-5 10-4 10-3 10-2 10-1 10010-1110-1010-910-810-710-610-510-410-310-210-1100101102103104

segregation coefficient

impu

rity

cont

ent (

ppm

w) after first

solidification

after secondsolidification

Impurity content of MG-SiRequired impurity content for SOG-Si

Al

NiCr

Fe

V

Ti BP

Fig. Removal of impurity in silicon by solidification refining.

10

Thermodynamic properties of impurities in molten silicon alloys

・PhosphorousEquilibrated in a controlled phosphorous partial pressure

・Titanium and IronEquilibrated with lead

・Aluminum, Calcium and MagnesiumEquilibrated with Oxides

11

Thermodynamics of P in Molten Silicon

Gas Inlet Tube

Porous Alumina Block

Molten Si-P Alloy

Graphite or Alumina Crucible

Graphite Holder

Mullite Tube

Fig. Schematic Cross Section of Experimental Apparatus.

Ar+P4(P2+P)

To Ribbon Heater Temperature Controller

Ar+P4

Thermocouple

Silicone Plug

Red Phosphorus Ribbon Heater

Ar

Red Phosphorus Temperature 398-461KArgon Flow Rate 190cc/min

Fig. Schematic cross section of the phosphorus vapor generator.

To Ribbon Heater Temperature Controller

Ar+P4

Thermocouple

Silicone Plug

Red Phosphorus Ribbon Heater

Ar

Red Phosphorus Temperature 398-461KArgon Flow Rate 190cc/min

Fig. Schematic cross section of the phosphorus vapor generator.

12

0.05 0.1

0.001

0.002

0[mass%P]

P P 1/

2 (atm

1/2 )

2

1750 1800 1850-80

-70

-60

-50

T(K)Fr

ee e

nerg

y ch

ange

(kJ/

mol

)

1/2 P2(g)=P(mass%, in Si)ΔG°= -139,000+43.4T (J/mol)

Fig. Relationship between equilibrium phosphorous partial pressure and phosphorous concentration of silicon at 1823K.

Fig. Temperature dependence of free energy change of phosphorous dissolution into silicon.

13

10-6 10-5 10-4 10-3 10-2 10-110-1610-1510-1410-1310-1210-1110-1010-910-810-710-610-510-4

[mass%P]

P(a

tm)

P

P

P

P2

P,

PP 2

1 2 3 4

0.01

0.02

0.03

0Time (ks)(V

/A)・

log(

[mas

s%P]

0/[m

ass%

P]t)(

m) Suzuki et al.

estimated

Yuge et al.

1823K 1867K

Ikeda et al.estimated

Fig. Relationship between equilibrium partial pressure of P, P2 and phosphorous content of silicon at 1823K.

Fig. Relationship between time for vacuum treatment and (V/A)･log([mass%P]0/[mass%P]t).

Hertz-Knudsen equation

iiii Xp

RTM

dtdy

⋅⋅⋅= 0

2γ

π

14

Fig. Schematic Cross Section of Experimental Apparatus.

Gas Inlet Tube

Porous AluminaBlock

Graphite Crucible

Graphite Holder

Mullite Tube

Graphite Lid

Molten Si-M(M:Fe,Ti)

Molten Lead

Fig. Phase diagram of Si-Pb system.

Thermodynamics of Ti and Fe in Molten Silicon

Si Pb

15

0 0.02 0.04 0.06-6

-5

-4

-3

-2

-1

XFe in Si

lnX F

e in

Pb-

lnX F

e in

Si-1

4.1X

Pb in

Si

-23.

8XSi

in P

b+4.

91

lnXFe in Pb-lnXFe in Si-14.1XPb in Si-23.8XSi in Pb+4.91=-3.56(±0.40)+3.17(±5.46)XFe in Si

γ°Fe(l) in Si=2.85×10-2

εFeFe

in Si=3.17

0 0.025 0.05 0.075 0.1-9

-8

-7

-6

XTi in Si

lnX T

i in

Pb-ln

X Ti i

n Si

-13.

2XPb

in S

i

-10.

8XSi

in P

b-1.

74

lnXTi in Pb-lnXTi in Si-13.2XPb in Si

= -7.71+3.97XTi in Si

γ°Ti(l) in Si=4.48×10-4

εTiTi

in Si=3.97

-10.8XSi in Pb-1.74

Fig. Relationsip between XTi in Si and ln XPb in Pb – ln XPb in Si at 1723K.

Fig. Relationsip between XFe in Si and ln XPb in Pb – ln XPb in Si at 1723K.

16

Ti and Fe are stable in molten silicon.

Removal of Ti and Fe by chemical reaction (i.e. oxidation, chlorination) is considered to be impossible.

Double solidification process is required.

17Fig. Schematic Cross Section of Experimental Apparatus.

Graphite Holder

Al2O3-Al6Si2O13 Crucible

Molten Si-Al Alloy

SiO2 Crucible

Molten Si-Ca Alloy

Molten SiO2 satd.CaO-SiO2 Slag

SiO2 Crucible

Molten Si-Mg Alloy

Molten MgSiO3, SiO2satd.MgO-SiO2-Al2O3 Slag

MgSiO3, SiO2 pellet

Thermodynamics of Al, Ca and Mg in Molten Silicon

Al

Ca

Mg

18

5.4 5.6 5.8

-1

-0.8

-0.6

-0.4

-0.2

0

-3.6

-3.4

-3.2

-3

-2.8

-2.6

104/T(K-1)

log γ A

l(l),

log γ M

g(l)

log γ C

a(l)

Ca

Mg

Al

172317731823T(K)

Fig. Temperature dependence of the activity coefficient of aluminum, calcium, and magnesium in molten silicon relative to pure liquid.

51.411300ln

55.114300ln

452.03610ln

Ca

Al

+−=

+−=

+−=

T

T

T

Mgγ

γ

γ

19

1 2 3 4 5 6

10

20

30

0

1000

2000

3000

0

P

Ca

Al

Fig. Relationship between vacuum time and impurity content of silicon at 1823K.

Time(ks)

mas

s ppm

P

mas

s ppm

Al,

mas

s ppm

Ca

0.739m240.3m2mass of Si : 1000kg

surface area

P, Al, Ca and Mg can be removed by vacuum melting.

Mg

20

Vacuum melting by electron beam

Vacuum

Plasma melting with water vapor

Directional solidification

MG-Si

ArgonSOG-Si

Removal of P, (Al, Ca) Removal of B, C

Removal of Fe,Ti,Al

First step purification Second step purification

Directional solidification

Removal of Fe,Ti,Al

Lower cost refining process is desired to be developed

NEDO SOG-Si Manufacturing Process(Operated in JFE Steel Co.)

21

Is there any way to reduce the times of solidification refining?

If the treatment of MG-Si can remove 90% of Ti and Fe, ･････

Possibility of Acid Leaching

22

Si-Ca-Fe or Ti dissolved at 1723K

Cooling to 1300K(CaSi2 formation)

Acid leaching withaqua regia

Chemical analysis

Acid Leaching Test with Ca Addition

Porous AluminaBlock

Graphite Crucible

Graphite HolderAr

23

Removal Fraction of Fe

24

Fig. Phase Diagram of the Si rich corner for the Ca-Fe-Si System [calculated by Thermo-Calc]

Eutectic PointXCa/XFe≒6.4

Secondary PhaseCaSi2 or FeSi2 ?

Consideration of Removal Behavior of Fe with the Ca-Fe-Si Phase Diagram

25

Fig.(a) SEM micrograph of Si-Ca-Fe alloy.(b) Microstructure of Si-Ca-Fe alloy.(c) Concentration profile by EPMA line analysis.

Si-3.26%Ca-0.424%Fe, 4.4K/min

26

Fig.(a) SEM micrograph of Si-Ca-Fe alloy.(b) Microstructure of Si-Ca-Fe alloy.(c) Concentration profile by EPMA line analysis.

Si-2.41%Ca-0.685%Fe, 4.4K/min

27

Vacuum melting by electron beam

Vacuum

Plasma melting with water vapor

Directional solidification

MG-Si

Argon

SOG-Si

Removal of P, (Al, Ca) Removal of B, C, (Al, Ca)

Removal of Fe,Ti,Al

First step purification Second step purification

Directional solidification

Removal of Fe,Ti,Al

NEDO SOG-Si Manufacturing Process(Operated in JFE Steel Co.)

P : 30 → < 0.1ppmw B : 5-10 → 0.1-0.3 ppmw

Acid leaching treatment with Ca additionRemoval of Fe and Ti

28

Vacuum melting by electron beam

Vacuum

Plasma melting with water vapor

Directional solidification

MG-Si

Argon

SOG-Si

Removal of P, (Al, Ca) Removal of B, C, (Al, Ca)

Removal of Fe,Ti,Al

First step purification Second step purification

Directional solidification

Removal of Fe,Ti,Al

NEDO SOG-Si Manufacturing Process(Operated in JFE Steel Co.)

P : 30 → < 0.1ppmw B : 5-10 → 0.1-0.3 ppmw

Acid leaching treatment with Ca additionRemoval of Fe, Ti, P and B

Reduction of processing time

29

Solidification refining of Si with Si-Al melt at low temperature.

Low eutectic temperature

Fig. Phase diagram of Si-Al system.

Removal of impurities by use of enhanced segregation at low temperature

Phase diagrams for the Si-Al system

MG-Si(98-99%)

Pure Al

Segregation ratiomeltAlSiini

Sisolidinii X

Xk

−

=

30

Segregation ratios of impurities between solid Si and Si-Al melt

·Thermodynamics of impurity elements in solid SiEvaluation of low temperature Si refining process

Segregation ratios of impurities between Si-Al melt and solid Si

Al, B, P Fe,Ti, etc.

Experimentally determined

Theoretically determined

31Fig. Diffusion coefficient of impurity elements in solid silicon.

Al, P, B ··· Considerably small diffusion coefficient in solid Si

Solidification method to attain the equilibrium

Diffusion coefficient of impurity elements in solid SiDetermination for segregation ratios of P and B

32

1 2

Solid Si

Si-Al melt

Solid Si

T2

T1

Tm,A

C1sC1

l C2l C2

s A

Component BDistance

a) b)

Tem

p.→

LS

L+S

Fig. Schematic diagrams of the temperature gradient zone melting.(a) Portion of phase diagram. (b) Temperature gradient in system.(c) Physical system comprising (A+B)molten zone traveling through solid A.

c)

Temperature Gradient Zone Melting(TGZM) methodDetermination for segregation ratios of P and B

Precipitated Si

33

Temperature Gradient Zone Melting MethodAl and P contents in solid silicon -

EPMA analysisAr atmosphere 24-72h

Temperature Gradient 10-40K/cm

1

2

3

4

5 6 8

10

11

97

12

Fig. 1. Schematic diagram of an experimental apparatus: 1-SiC heating element; 2-thermocouple connected to PID controller; 3-gas outlet tube; 4-stainless steel holder; 5-single crystalline silicon; 6-aluminum foil; 7-mullite tube; 8-thermocouple for measuring temperature of molten zone; 9-porous alumina boat 10-gas inlet tube; 11-alumina plate; 12-Sponge titanium

Distribution of P (and Al) between solid Si and Si-Al-P alloy

- Red phosphorus stuck Al foil

Distribution of B (and Al) between solid Si and Si-Al-B alloy

- Prepared Al-B foil

Determination for segregation ratios of P and B

Experimental method

34

Fig. Photo of a TGZM specimen after the experiment.

10mm

B A

Fig. Intensity profiles of Ka radiation of aluminum and phosphorus of the sample (accelerating voltage of 15kV, sample current of 200nA, sampling step of 10μm).

100 200 300 400 500 600

100

200

300

400

0Distance (μm)

Inte

nsiti

es o

f Kα r

adia

tion

ofal

umin

um a

nd p

hosp

horu

s P Al

(a) Single crystalline Si (d) Single crystalline Si(c) Si-Al zone

(b) Migrated region

Segregation ratio

Sample after TGZM experiment

Determination for segregation ratios of P and B

35

Segregation ratios of P and B

1200 1400 16000.01

0.05

0.1

0.5

1

5

Temperature (K)

Segr

egat

ion

ratio

P B

Segregation ratios are smaller at low temperature.Low temperature refining is effective.

Segregation ratio between solid Si and Si-Al melt

Segregation coefficient between solid/liquid Si.

36

si

si

li

li aRTGaRTG lnln 00 +=+

0

0

lnlnln si

li

fusi

li

si

i RTG

XX

kγγ

+Δ

==

Equality in chemical potential of impurity between solid Si and Si-Al melt

1)

2)

Activity coefficients in solid Si and Si-Al meltBased on the reported thermodynamic data

Derivation of segregation ratio of impurity elements between solid Si and Si-Al melt

37

Segregation ratios between solid Si and Si-Al melt< Segregation coefficients between solid/liquid Si

Segregation ratio between Si-Al melt and solid Si Element

1073K 1273K 1473K

Segregation coefficient between solid/liquid Si at

m.p. of Si(6-3)

Fe 1.7×10-11 5.9×10-9 3.0×10-7 6.4×10-6

Ti 3.8×10-9 1.6×10-7 9.6×10-7 2.0×10-6

Cr 4.9×10-10 2.5×10-8 2.5×10-7 1.1×10-5

Mn 3.4×10-10 4.5×10-8 9.9×10-7 1.3×10-5

Ni 1.3×10-9 1.6×10-7 4.5×10-6 1.3×10-4

Cu 9.2×10-8 4.4×10-6 2.5×10-5 4.0×10-4

Zn 2.2×10-9 1.2×10-7 2.1×10-6 1.0×10-5

Ag 1.9×10-8 1.7×10-6 6.6×10-6 5.0×10-5

Au 1.5×10-11 6.1×10-9 3.6×10-7 2.5×10-5

Ga 2.1×10-4 8.9×10-4 2.4×10-3 8.0×10-3

In 1.1×10-5 4.9×10-5 1.5×10-4 4.0×10-4

Sb 3.4×10-3 3.7×10-3 8.2×10-3 2.3×10-2

Pb 9.7×10-5 2.9×10-4 1.0×10-3 2.0×10-3

Bi 1.3×10-6 2.1×10-5 1.7×10-4 7.0×10-4

P 4.0×10-2 8.5×10-2 1.6×10-1 3.5×10-1

B 7.6×10-2 2.2×10-1 4.9×10-1 8.0×10-1

Al 1.4×10-4 4.9×10-4 1.2×10-3 2.8×10-3

3810-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

—

Contents of MG-Si — Allowable contents for SOG-S Contents after solidification refining with Si-Al melt Contents after ordinary solidification refining

Impu

rity

cont

ent (

ppm

w)

—

——

Fe Ti P B

Impurity contents of Si after low temperature solidification refining with Si-Al melt at 1273K and ordinary Si solidification refining of MG-Si.

Contents after purification; Ci = ki Ciini Initial content

(MG-Si)at 1273K

Si

39

Viewing the development of the solidification refining of Si with Si-Al melt at low temperature

Fig. Phase diagram of Si-Al system.

Si bathAl bath

Si-Al melt

Poly or Single Crystalline ?

・ Discussion of the Si solidification method from Si-Al melt

・ Refining test

40

Discussion for separating of solid Si from Si-Al melt by flotation

Holding Si-55.3at%Al alloy (liquidus temp. 1273K) at 1173K for 12hSi saturated, solid fraction = 0.168

Needle like Si grainsPreparation – Melting in the

induction furnace and rapid cooling

(ρsolid Si < 2.33g/cm3, ρAl-25.8%Si ～ 2.45g/cm3)

41

Separation of solidified Si from Si-Al melt using electromagnetic force

― Induction furnace

13

7

1211

54 6

8

109

2

Si-55.3at%Al alloy (liquidus, 1273K) was melted and held at 1323K in the induction furnace

Measuring the surface temp. of the melt by the infrared pyrometer

Cooling and Solidifying the sample by lowering from the position of induction coil

42

Solidified Si-Al alloy using induction furnace

Agglomeration of solidified Si by electromagnetic stirring

Bottom

1cm

43

Si-Al melt Solidified Si

Inductioncoil

J

B

J

BInduced flow

F

・Difference in induced swirl current to the perpendicular direction

Solidification of Si at lower position

・Downward bulk flow Agglomeration of Si grains to the bottom of the sample

Agglomeration mechanism of Si under the fixed alternative magnetic field

→ Solidification refining test

44

Fig. Schematic cross section of experimental apparatus.

Alumina block

Gas inlet tube

Induction coil

Graphite crucible

Quartz tube

Prism

Molten alloy

Melting and holding at 1323K or 1223K

Cooling and solidifying the sample by lowering from the coil

Measuring the surface temp.by infrared pyrometer

Experimental method

Crashing the Si agglomerated part (> 840μm), then acid cleaning

Sample No. Fe Ti Al B P

PrSi-1 4500 691 1280 56 36

PrSi-2 2160 248 1560 36 19

Synthesized MG-Si was alloyed to Si-55.3at%Al(liquidus, 1273K) or Si-64.6 at%Al(liquidus, 1173K) with pure Al

45

Sample No Source Fe Ti B P Al

SR-1 PrSi-1 13 (99.7%) 5.2 (99.2%) 0.81 (98.6%) 0.93 (97.4%) 599 (53.1%)

SR-2 PrSi-1 13 (99.7%) 2.7 (99.6%) 0.88 (98.4%) 1.2 (96.7%) 534 (58.1%)

SR-3 PrSi-2 20 (99.1%) 2.8 (98.9%) 0.71 (98.1%) 0.72 (96.3%) 575 (63.1%)

SR-4 PrSi-2 27 (98.8%) 4.5 (98.2%) 1.90 (94.8%) 1.0 (95.1%) 602 (61.3%)

SR-5 PrSi-1 47 (99.0%) 7.7 (98.9%) 0.98 (98.3%) 0.42 (98.8%) 538 (57.8%)

SR-6 PrSi-1 36 (99.2%) 5.6 (99.2%) 0.99 (98.2%) 0.66 (98.2%) 453 (64.5%)

Fe, Ti・・・ Well reduced but not as well as predicted valuesB, P・・・ Effectively removed

Impurity contents of refined Si in the test run (ppmw)

High ability for purification was confirmed.

46

Vacuum melting by electron beam

Reduction of SiO2 in arc furnace

Removal of P, (Al, Ca)

Removal of Fe,Ti,Al

Directional solidification

Alloying of silicon with Al

SOG-Si

Solidification refining with Si-Al melts

Pre-refined Si (5N except Al)

Proposal of refining process for SOG-Si

Acid cleaning

47

Conclusions and Future WorkThe optimum metallurgical refining process for SOG-Si was thermodynamically assessed.

Possibility of solidification refining of Siwith Si-Al melt was clarified. Solidification and separation of Si from the melt is the major problem to solve for the practical process.

48

Acknowledgments

Present data are based on two Doctoral Dissertations by Takahiro Miki (1999), now a research associate of Tohoku University, and Takeshi Yoshikawa (2005), now an assistant professor of Osaka University.

49

Periodical Journal Publication List 1Metallurgical Refining

・T.Miki, K.Morita and N.Sano :Metallurgical and Materials Transactions B, 27B(1996), 937-942, “Thermodynamics of Phosphorus in Molten Silicon”・T.Miki, K.Morita and N.Sano :Metallurgical and Materials Transactions B, 28B(1997), 861-867, “Thermodynamic Properties of Titanium and Iron in Molten Silicon”・T.Miki, K.Morita and N.Sano :Metallurgical and Materials Transactions B, 28B(1997), 861-867, “Thermodynamic Properties of Titanium and Iron in Molten Silicon”・T.Miki, K.Morita and M.Yamawaki :Journal of the Mass Spectrometry Society of Japan、47(1999), 72-75, “Measurements of Thermodynamic Properties of Iron in Molten Silicon by Knudsen Effusion Method”・T.Miki, K.Morita and N.Sano :Materials Transactions, JIM, 40 (1999), 1108-1116, “Thermodynamic Properties of Si-Al, -Ca, -Mg Binary and Si-Ca-Al, -Ti, -Fe Ternary Alloys”・坂田智浩、三木貴博、森田一樹 ：日本金属学会誌、66(2002), 459-465, “酸リーチングによる多結晶シリコン中鉄、チタンの除去”・K.Morita and T.Miki :Intermetallics, 11(2003), 1111-1117, “Thermodynamics on Solar-Grade-Silicon Refining”・G.Inoue, T.Yoshikawa and K.Morita :High Temperature Materials and Processes, 22(2003), 221-226, “Effect of Calcium on Thermodynamic Properties of Boron in MoltenSilicon”・T.Shinpo, T.Yoshikawa and K.Morita :Metallurgical and Materials Transactions B, 35B(2004), 277-284, “Thermodynamic Study of the Effect of Calcium on Removal of Phosphorus from Silicon by Acid Leaching Treatment”

50

Periodical Journal Publication List 2Solidification Refining

・T.Yoshikawa and K.Morita :J.Electrochem. Soc., 150(2003), G465-G468, “Solid Solubilities and Thermodynamic Properties of Aluminum in Solid Silicon”・T.Yoshikawa and K.Morita :Science and Technology of Advanced Materials, 4(2003), 531-537, “Removal of Phosphorus by the Solidification Refining with Si–Al Melts”・T.Yoshikawa and K.Morita :Journal of Physics and Chemistry of Solids, 66(2005), 261-265, “Thermodynamics of Solid Silicon Equilibrated with Si-Al-Cu Liquid Alloys”・T.Yoshikawa and K.Morita :Materials Transaction, 46(2005), 1335-1340, “Thermodynamic Property of B in Molten Si and Phase Relations for the Si-Al-B System”

・T.Yoshikawa and K.Morita :ISIJ International, 45(2005), 967-971, “Refining of Si by the Solidification of Si-Al Melt with Electromagnetic Force”・T.Yoshikawa and K.Morita :Metallurgical and Materirals Transactions B, 36B(2005), 731-736, “Removal of B from Si by the Solidification Refining with Si–Al Melts”・T.Yoshikawa, K.Arimura and K.Morita :Metallurgical and Materirals Transactions B, 36B(2005), 837-842, “B Removal by Ti Addition in the Solidification Refining of Si with Si–Al Melts”・T.Yoshikawa and K.Morita :Journal of Alloys and Compounds, 420(2006), 136-144“Activity Measurements of Al and Cu in Si-Al-Cu Melt at 1273 and 1373 K by the Equilibration with Molten Pb”