解説

先進セラミックス研究センター年報（2019）. Vol. 8, 23‑27

Synthesis of Fumed Titanium Dioxide with a High Content of
Rutile Structure by Novel Natural Dropping Thermal Treatment
Yukiya Yamashita, Masayoshi Fuji
Advanced Ceramics Research Center, Nagoya Institute of Technology
3-101-1 Honmachi, Tajimi, Gifu 507-0033, JAPANA natural dropping thermal treatment is an effective method to calcine materials with very short heating time. It has been
applied to the transformation of titanium dioxide from anatase to rutile structure. In particular, the novel thermal treatment is anindispensable method for the fumed titanium dioxide (Fumed TiO2) to give high rutile structure content inhibiting the sintering /grain growth. Meanwhile, the relationships between conversion ratio of the rutile structure content and the morphology of theagglomerate before the thermal treatment (Precursor) has been investigated.
Keywords: Natural dropping thermal treatment, Fumed TiO2, Agglomerate

1．Introduction

A novel thermal treatment method; natural dropping
thermal treatment has been reported as one of very
simplified calcination process. It has advanced as part ofthermal treatment process with very short heating timearound 1 second. It has been applying to synthesize the
new fumed TiO 2 with high rutile structure content
maintaining the high dispersibility. The fumed TiO2 isapplied to various application fields such as a referenceof photocatalytic materials, TiO 2 electrode of a dye
sensitive solar cell and UV cutting agent for cosmetics
by the unique properties; high purity, fine primaryparticle size, high anatase contents and excellent
dispersibility [1]-[7]. Comparatively, it is reported thatthe anatase structure is not preferred to use for thecosmetics because of the high photocatalytic activities[8]. It is well known that the anatase structure transforms
to the rutile structure at high temperature and long
heating time. Some studies are reported for the
additional thermal treatment for the fumed TiO 2 .
However, in these investigations, the drastic increase of
the primary particle size and rapid decrease of thesurface area are observed [9]-[10]. In generally, thesintering of the fumed TiO2 are not affordable by theconventional thermal treatment method by using amuffle furnace and results a loose of the highdispersibility.
The natural dropping thermal treatment has been
investigating to improve this issue by reducing thethermal treatment time as short as possible. In theinvestigations, the effect of the morphology for the

fumed TiO
2 before the thermal treatment (Precursor)
㸬1RYHOQDWXUDOGURSSLQJWKHUPDO
was investigated. This paper introduces examples of the
WUHDWPHQW
novel natural dropping thermal treatment applying to the
fumed TiO2 as the new calcination method.
2．Novel natural dropping thermal treatment

The natural dropping thermal treatment is consistedfrom very simple thermal treatment method and applied
to the fumed TiO2. In this process, the fumed TiO2 isdirectly dropped without a sieve or through sieves from
top to the bottom of the electric furnace at several
temperature under air atmosphere. The heating time isvery short around 1 second. Fig.1 shows typicalexperimental apparatus for the thermal treatment.
TiO2

Sieve
(Vibration)

Electrical
Tubular
FurnaceOven cloth
Conical beaker

Fig. 1 Outline of the natural dropping thermal treatment

3．Thermal treatment of the fumed TiO2

The natural dropping thermal treatment has been
7KHUPDOWUHDWPHQWRIWKHIXPHG
applied to the pure fumed TiO2 (P25) to increase the
7L2
rutile structure content inhibiting
the sintering / the grain― 23 ―

KHUPDO

growth [11]. It is reported that just passing 0.5 mm meshsieve, the rutile structure content drastically increases to94%, although only very slight change of the rutile
structure content; from 16 % to 18 %. without the sieve
were observed.
Fig. 2 shows X-ray diffraction (XRD) patterns of thethermally-treated P25 at several temperatures throughthe 0.5 mm mesh sieve. The XRD data indicates that thecontent of the rutile structure (%) from the anatase issignificantly affected by the temperature rise.

furnace are examined by using the terminal velocity ofthe powder calculated with Stokes ʼ s equation. The
relationship between rutile structure content and particle
residence time at 1450 ℃ is appeared in Fig. 4.

'U\3000
Rutile (110)
2500
2000
Intensity

KHUPDO

Synthesis of Fumed Titanium Dioxide with a High Content of Rutile Structure by Novel Natural Dropping Thermal Treatment(d)1450 Υ

Anatase (101)Fig. 4 Illustration of thermal treatment based on particle

1500

residence time(c)1200 Υ

1000(b)800 Υ
500(a)Original
0
23

24

25

26

27

28

29

2 Theta

Fig. 2 XRD patterns of thermally-treated P25

Fig. 3 confirms the content of the rutile structure of
the thermally-treated P25 at several temperatures using
several mm mesh sieves. The same contents of the rutile
structure; 94 % are obtained by passing through from 0.5mm and 1.0 mm mesh sieves at 1450 ℃.

KHIXPHG

KHIXPHG

Fig. 3 Rutile conversion ratio for P25

Comparatively, the content of the rutile structure isstayed at 73 % with through 1.4 mm sieve.
The thermally-treated P25 showed good dispersibility
due to many pores in the agglomerate, sponge-like
agglomerate although it demonstrates large particle in
the particle size distribution analysis. The particle
residence time during the heating unit in the electrical

From 0.00 to 0.48 seconds of residence time, the rutile
structure content is increased related to the prolonging
the residence time. After 0.48 seconds, the rutile
structure contents sustained at 94% up to around 1.00
seconds. This result suggests that the crystal
transformation from the anatase to the rutile structure of
P25 is mainly proceeded before 0.48 seconds and after
0.48 seconds the applying heat energy is not mainly used
to the transformation but sintering or grain growth.
This result is caused from the deviations of
agglomerate density of P25. When some small
agglomerates have higher bulk densities than whole
original P25, the terminal velocity of the densified
agglomerates become higher than the normal ones andthe residence time also becomes too short to the crystal
transformation.
As described above, the novel natural dropping
thermal treatment is very effective method fortransformation from anatase to rutile structure of fumed
TiO2 (P25) with very short heating time. It is confirmed
it is difficult to achieve 100% of rutile structure for apure P25 due to existing small portion of densified
agglomerates.
4．Dry-type of surface modification withmetallic alkoxide

For the synthesis of the fumed TiO2 (P25) with 100%
rutile structure content, surface modification of P25 withmetallic alkoxides has been reported before the natural
dropping thermal treatment as other approach [12]. In
this study, P25 precursors are produced by the surface― 24 ―

Yukiya Yamashita, Masayoshi Fuji

modification with metallic alkoxides; Tetraethoxysilane(TEOS; TE(2); 2wt% and TE(5); 5wt%), andTitaniumtetraisopropoxyde (TTIP; TT(2); 2wt% andTT(5); 5wt%) by a dry-type of surface modification
process. Next the produced precursors are thermallytreated by the natural dropping method. P25 modified
with TE(2), TT(2) and TT(5)) by the dry-type ofmodification process achieves 100% of rutile structure
at 1400℃ although TE(5) maintains low rutile structure
content. A precursor modified with TEOS by the wettype process; W-TE(2) shows low content of rutile
structure. The results are shown in Fig. 5.

The content of rutile (%)

TE(5)
TT(2)

60
TT(5)

TE(2)

40

W-TE(2)

20

0
1000

1100

1200

1300

1400

5．Mixing of external additive

As another approach to synthesize 100% rutile
structure content for P25, mixing with solid statedexternal additive has been reported [13]. In thisinvestigation, P25 is mixed with fumed SiO2 modified
with dimethylsilyl group (R972), fumed Al2O3 (AluC)
and Calcium stearate to synthesize the suitableprecursors improving the agglomerate conditions andthen the precursors are thermally-treated by the natural
dropping method. The rutile structure conversation ratio
is demonstrated in Fig. 7.
0L[LQJRIH[WHUQDODGGLWLYH

100

80

potential, -42.0mV. The high dispersibility of thethermally-treated TE(2) is cased from both the porous
agglomerate structure; sponge-like agglomerate and theelectro charge repellence of the SiO2 layer coated on theP25 surface.
As described above, the dry-type of the surface
modification with small portion of TOES is very useful
method to synthesize 100% of rutile structure content
controlling0L[LQJRIH[WHUQDODGGLWLYHthe agglomerate of the precursor for thenatural dropping thermal treatment

1500

Temperature (Υ㸧100Fig. 5 Rutile conversion rate for thermally-treated P25

P25 with Calcium Stearate

Rutile Structure Content (㸣)0L[LQJRIH[WHUQDODGGLWLYH

It is confirmed that the results are caused from theagglomerate conditions of the precursors.
The precusor TE(2) shows smaller agglomerate than
precursors TE(5) and W-TE(2). The precursor W-TE(2)seems to make rigid and densified agglomerate. The
SEM photos of precursors are demonstrated in Fig. 6.

80
P25 with R972
60

Original P25

40

20

0
900

1000

1100

1200

1300

1400

Temperature (Υ㸧Fig. 7 Rutile conversion ratio for thermally-treated P25 mixed
with external additiveFig. 6 SEM photos of precursors

The terminal velocity of W-TE(2) becomes faster thanthe others due to the high densified small agglomerate.
These agglomerate conditions are relatively well fit to
the results of the conversion ratio from the anatase to therutile structure. The thermally-treated TE(2) with 100%
rutile structure showed very excellent dispersibility inthe water dispersion due to strong negative zeta

The thermally-treated P25 with R972 and AluC show
100 % of rutile structure at 1350 ℃ and 1300 ℃
respectively. These temperatures are lower than that ofmetallic alkoxides modified ones, TE(2), TT(2) andTT(5). The thermally-treated P25 with calcium stearateshows 100 % rutile content at 1200 ℃. This temperature
is more than 100 ℃ lower than the addition of R972 andAluC, and more than 250 ℃ lower than the thermallytreated original P25.
Fig. 8 indicates the sedimentation test results of thedispersion in water versus storage time for these
thermally-treated P25 having 100 % of the rutile― 25 ―

Synthesis of Fumed Titanium Dioxide with a High Content of Rutile Structure by Novel Natural Dropping Thermal Treatment

structure. The other types (Chloride process) of TiO2
(CR-EL (7 m2/g) Ishihara Sangyo Co. Ltd.) with therutile structure and similar surface area, and the original
P25 without thermal treatment are also illustrated.

Fig. 9 TEM images of the thermally-treated P25 with theexternal additives

Fig. 8 Sedimentation results of TiO2 dispersion

The thermally-treated P25 with R972 demonstrates an
excellent dispersibility with very similar to the original
P25. Regarding the thermally-treated P25 with calcium
stearate, it demonstrates low dispersibility similar to
CR-EL even though the thermal treatment temperature is
the lowest; 1200℃. All thermally-treated P25 show
larger particle size distribution patterns than CR-EL. The
sedimentation results are very different from particle
size distribution results. For the estimation of thesedimentation velocity, the result of CR-EL is relatively
good consistency with one of the particle size
distribution. Comparatively, the results of the thermallytreated P25 are quite different from the sedimentation
results. This difference suggests that the thermallytreated P25 have much lower powder densities than CREL. Additionally, the thermally-treated P25 with R972 at
1350℃ shows -47.8mV of zeta potential althoughoriginal P25 at 1400℃ demonstrates -18mV. This strong
negative charge seems to result to high dispersibility ofthermally-treated P25 with R972 due to electros charge
repellence of the SiO2 on P25 surface similar to thesurface coating with TEOS TE(2). TEM photos of thethermally-treated P25 with the external additives areillustrated in Fig. 9.
The thermally-treated P25 with R972 and AluC
contain small particle and many pores in the
agglomerate. The thermally-treated P25 with calcium
stearate shows no small particles and supports thesintering / grain growth.
The Carr Cohesion values is applied as an index of thepowder agglomerate [14]. The Carr Cohesion tendencyof these precursors are as follows; P25 with calcium

stearate (28) < P25 with AluC (49) < P25 with R972 (55)< original P25 (69). The Carr Cohesion values show
relatively good relationships between the temperatureachieve 100 % rutile structure. The agglomerate with the
low Carr Cohesion value passes through the sieve as
smaller agglomerate than the agglomerate with highCarr Cohesion value. The remarkable sintering / grain
growth of the thermally-treated P25 with calcium
stearate would be resulted from an additional heat sourceby the burning of an alkyl-group of the calcium stearate
under air atmosphere.
As described in this section, the mixing with external
additive is quite effective method to synthesis the new
fumed TiO 2 (P25) with 100% of rutile structure
maintaining the high dispersibility for the natural
dropping thermal treatment. Additionally, a new
possibility is also reported that the sintering / grain
growth of the fumed TiO2 by burning of calcium stearate
with very short thermal treatment time.
6．Extended application of the natural
dropping thermal treatment

The natural dropping thermal treatment has been
applying to not only fumed TiO2 but also other fumedoxides; fumed SiO 2 and fumed Al 2O 3. There are nochanged of the crystal structure at 1450℃ and madeslightly large agglomerate due to partial sintering in both
fumed oxides.
It is confirmed that the natural dropping thermal
treatment is only applied to the fumed TiO 2 for the
crystal transformation. Otherwise, the morphology
change maintaining the original crystalline polymorphs
is possible to not only fumed TiO2 but also fumed SiO2
and fumed Al2O3.
7．Conclusion

We introduced examples of applying the novel natural
dropping thermal treatment to fumed TiO2 (P25). It is avery effective method for the crystal transformation― 26 ―

Yukiya Yamashita, Masayoshi Fuji

from anatase to rutile structure inhibiting the sintering /grain growth to maintain the high dispersibility. The
preferable agglomerate conditions have been
investigated by controlling the morphology of P25
precursors. The preferable P25 precursors are producedwith the dry-type of surface modification and mixing
with the external additives. The precursors modified
with TEOS and mixing with fumed SiO2 (R972) achieve
100 % of rutile structure content maintaining the highdispersibility with the sponge-like structure and thestatic electricity repellence of SiO2 on the P25 surface.
We anticipate future application of the natural
dropping thermal treatment to not only fumed TiO2 but
also various other materials.
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