Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

16.6 Nano- and Macrobioceramics for Drug Delivery and Radiotherapy 549

delivery matrices may also be used as antibiotic - releasing materials. Ferraz and

coworkers [52] developed a novel injectable drug - delivery system with a drug -

releasing capability for treating periodontitis. Their aim was to use HAp, loaded

with an antibiotic, as a local drug - delivery system; consequently amoxicillin (with

and without clavulanic acid) and erythromycin – both of which are used orally and

parenterally to treat periodontitis – were tested. The results showed that the micro-

spheres exhibited chemical compositions, porosities, and surface areas which

provided adequate conditions for the sustained - release proﬁ le of the drugs inves-

tigated. Bactericidal tests indicated that the released antibiotics had an inhibitory

effect on the bacteria present, while osteoblastic cells were seen to proliferate well

on the surface of the microspheres, with cell growth being enhanced by the pres-

ence of antibiotics. Ferraz et al . concluded that these microspheres might also be

considered as an alternative carrier system for antibiotics and to enhance bone

regeneration while treating periodontitis.

16.6.3

Microbioceramics for Radiotherapy

The partial surgical removal of an organ afﬂ icted with cancer generally results in

a total or partial loss of organ function, that may or may not be recovered postop-

eratively. It would, therefore, be desirable to develop a cancer treatment that could

destroy only cancerous cells, such that the normal healthy cells could regenerate

after treatment and organ function be maintained. Although radiotherapy shows

such potential, the irradiation is generally applied from an external source, often

causing the cancer to receive an insufﬁ cient dose, especially if it is deep - seated.

Moreover, the irradiation may cause severe damage to healthy tissues. In order to

overcome this problem, microsphere - assisted radiation therapy is currently under

development for a range of clinical applications, and has indeed shown a degree

of success.

An in situ microsphere - assisted radiation method has been clinically applied

using 17Y

2

O

3

– 19Al

2

O

3

– 64SiO

2

(mol%) (YAS) glass microspheres that have been

prepared using a conventional melt – quench method. Although the yttrium - 89

(

89

Y) in this glass is a nonradioactive isotope, neutron bombardment will activate

89

Y to form the β - emitter

90

Y, which has a half - life of 64.1 h. When radioactive glass

microspheres 20 – 30 μ m in diameter are injected into a target organ (e.g., a liver

tumor), they become trapped inside the small blood vessels in the tumor, blocking

the nutritional supply to the tumor, and delivering a large, localized dose of short -

range, highly ionizing β - rays. As the β - rays have a short penetration range of

approximately 2.5 mm in living tissue, only minimal radiation damage will occur

among the neighboring healthy tissues. These microspheres show a high chemical

durability and, after their administration, the radioactive

90

Y remains essentially

within the microspheres so that it does not affect any neighboring healthy tissues.

The radioactivity of

90

Y decays to a negligible level within 21 days after its initial

preparation; hence, the microspheres become inactive soon after the cancer treat-

ment. These microspheres have been used clinically to treat liver cancer in Canada,

550 16 Nanoceramics for Medical Applications

the USA, and China. Likewise, radioactive yttrium - containing resin microspheres

30 – 35 μ m in diameter have also been used clinically to treat liver cancer in

Australia, China, New Zealand, and Singapore [53] .

Kawashita and coworkers [53] have successfully prepared hollow Y

2

O

3

micro-

spheres of 20 – 30 μ m diameter. For this, an aqueous carboxymethylcellulose

sodium salt solution containing urease was atomized into an aqueous yttrium

nitrate solution containing urea, using a spray gun located 2 m above the yttrium

nitrate solution. The resultant solid materials were heat - treated at 1300 ° C. An

investigation of the structure and chemical durability of these microspheres

showed the outer surfaces to be smooth and dense, and the inner parts to have a

honeycomb structure. In SBFs at pH 6 and 7, the hollow Y

2

O

3

microspheres

showed a high chemical durability.

It has been accepted that cancer cells generally perish at a temperature of

approximately 43 ° C because their oxygen supply via the blood vessels becomes

insufﬁ cient; this is in contrast to normal cells, which do not suffer damage at

higher temperatures of approximately 48 ° C. A tumors is also more easily heated

than the surrounding normal tissues, as its blood vessels and nervous system

are poorly developed. It follows, therefore, that hyperthermia might represent a

very effective treatment for cancer, with few adverse side effects. A variety of

techniques has been employed to heat the tumors, including hot water, IR radia-

tion, ultrasound, and microwaves but, unfortunately, deep - seated tumors cannot

be heated either effectively or locally using these methods. Ferrimagnetic micro-

spheres 20 – 30 μ m in diameter (and more recently nanospheres) have been shown

to be valuable as thermoseeds for inducing hyperthermia in cancers, especially in

those tumors located deep inside the body. In this situation, the spheres are

implanted through the blood vessels and become entrapped in the capillary bed

of the tumor. The application of an alternating magnetic ﬁ eld close to the tumor

then causes the spheres to generate heat by their hysteresis loss; this results in

the tumor being heated locally and strongly to a point where the cancerous cells

are killed.

Kawashita et al . [54] prepared Fe

3

O

4

microspheres by melting powders in a high -

frequency induction thermal plasma, and also by precipitation from an aqueous

solution. The preparation in a high - frequency induction thermal plasma involved

pure Fe

3

O

4

powders being completely melted in a plasma ﬂ ame of argon gas,

which was produced by high - frequency induction heating. The solidiﬁ ed products

were sieved using a nylon mesh in order to obtain particles of 20 – 30 μ m in size.

These particles were then heat - treated to 600 ° C and allowed to cool under a

reduced pressure of 5.1 × 1 0

3

Pa. Fe

3

O

4

powders were added into 600 ml of a 1 wt%

aqueous solution of hydroﬂ uoric acid (HF) for the preparation of microspheres in

aqueous solution. Silica glass microspheres of 12.4 μ m average diameter were

soaked in Fe – HF solution at 30 ° C for 24 days, with vigorous stirring. The products

were then heat - treated at 600 ° C for 1 h, and allowed to cool in a CO

2

– H

2

gas

atmosphere. The microspheres prepared in a high - frequency induction thermal

plasma were shown to be composed mainly of Fe

3

O

4

, accompanied by a small

amount of FeO. The surfaces of the microspheres were smooth before heat treat-

16.7 Nanotoxicology and Nanodiagnostics 551

ment, but became slightly rough after treatment. This increase in surface rough-

ness might be attributed to the formation of α - Fe

2

O

3

and/or the crystal growth of

Fe

3

O

4

. The saturation magnetization of the microspheres was 92 emu g

− 1

. The heat

generation was as low as 10 W g

− 1

, under 300 Oe and 100 kHz. Microspheres, 20 –

30 μ m in diameter, composed of small crystals of Fe

3

O

4

and 50 nm in size, were

prepared by precipitation from an aqueous solution and subsequent heat treat-

ment. Notably, the β - FeOOH was seen to precipitate on the surface of silica glass

microspheres, while its layer thickness increased with the increasing soaking

period. The deposited β - FeOOH was transformed into Fe

3

O

4

by heat treatment

above 400 ° C in a CO

2

– H

2

atmosphere. The silica glass microspheres soaked in

Fe – HF solution showed ferrimagnetism, with a saturation magnetization of

53 emu g

− 1

, a coercive force of 156 Oe and a heat generation of 41 W g

− 1

, under

300 Oe and 100 kHz, under the same magnetic ﬁ eld, which was fourfold higher

than for pure Fe

3

O

4

microspheres prepared in a high - frequency induction thermal

plasma.

16.7

Nanotoxicology and Nanodiagnostics

Today, nanomaterials and nanotechnologies represent a rapidly expanding disci-

pline, with major potential to improve the lifestyle of humankind, notably by

offering new solutions to many engineering and biomedical problems.

One major concern has arisen, however, regarding possible risks related to the

impact of synthetic nanoparticulate matter on human health. However, at present

insufﬁ cient data are available to make any accurate predictions concerning the

long - term consequences of using these materials. Nevertheless, unintentional

nanopollution is amply present in the current environment, and the use of nano-

materials for medical applications, as described here, is expected to increase as

their beneﬁ ts are recognized. Yet, it is somehow pertinent to mention that, there

is no reason why unintentional pollution should differ from the intentional

pollution, as far as its impact on health is concerned.

Gatti et al . [55] , in their European project “ Nanopathology ” (QOL - 2002 - 147),

succeeded in developing a novel diagnostics tool which showed the presence of

microsize and nanosized inorganic particles (including nanoceramics in patho-

logical tissues) by using an environmental scanning electron microscope equipped

with an X - ray microprobe of an energy - dispersive system. This study, which

included approximately 700 tissues affected by cryptogenic pathologies, showed

that in many cases where phlogosis (the inﬂ ammation of external parts) was

dominant, then microsized and nanosized inorganic particles were present, and

their elemental chemistry had been clearly identiﬁ ed and reported.

Among the pathologies investigated by Gatti et al . were cryptogenic granuloma-

tosis and different forms of cancer, including lymphomas, carcinomas of the lung,

colon and liver, and leukemia. Deep - vein thrombosis was also investigated, to

demonstrate the possible thrombogenic activity of environmental pollutants. In

552 16 Nanoceramics for Medical Applications

most cases, an inﬂ ammatory component was evident (e.g., in granulomas) when

comparatively large particles (usually > 10 μ m) were identiﬁ ed. Particles the size of

an erythrocyte or smaller were commonly found in thrombi, while particles down

to 10 nm in size were identiﬁ ed in the cell nuclei. From a chemistry standpoint,

Gatti et al . [55] identiﬁ ed a very wide variety of elemental compositions. As result

of their investigations, the group advised that the use of biodegradable nanoceram-

ics such as calcium phosphates should be seriously considered for biomedical

applications, due to their non - nanotoxicity. However, they suggested that extreme

care be taken with all other synthetic nanobiomaterial medical applications, until

a better understanding and a more comprehensive combat strategy is available

regarding these materials.

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Self - healing of Surface Cracks in Structural Ceramics

Wataru Nakao , Koji Takahashi , and Kotoji Ando

555

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

17

17.1

Introduction

Self - healing is the most valuable phenomenon to overcome the integrity decreases

that are caused by the damages in service. Thus, self - healing should occur auto-

matically as soon as the damages occur, and the healed zone should have high

integrity as it was before damaging. When proposing self - healing materials, one

must know the nature of the damage and the service, conditions of the materials.

In the case of the structural ceramics, the severest damage is surface cracks, which

is possible to be introduced by crash, fatigue, thermal shock, and corrosion during

their service time. Over the past 30 years, ceramics have become the key materials

for structural use at high temperature due to their enhanced quality and good

processability. Structural ceramics are also expected to be applied in the corrosion

environments such as air, because of its chemical stability. Thus, self - healing

of surface cracks in the structural ceramics is an important issue to ensure the

structural integrity of ceramic components.

In this chapter the mechanism and effects of self - healing of surface cracks in

structural ceramics are introduced. Apart from this, the fracture manner of ceram-

ics is also discussed. This will help the readers to understand the self - healing

phenomena in ceramics and its beneﬁ ts. The history of crack healing is also

included in the text. Furthermore, new methodology to ensure the structural

integrity using crack - healing effect and advanced ceramics having self – crack -

healing ability are mentioned. Finally, it is assayed the self - healing ceramics can

be applied to blades and nozzles of high - temperature gas turbine.

17.2

Fracture Manner of Ceramics

Ceramics tend to have brittle fracture that usually occurs in a rapid and cata-

strophic manner. Brittle fracture is usually caused by the stress concentration at

556 17 Self-healing of Surface Cracks in Structural Ceramics

the tip of the ﬂ aws. For brittle fracture under pure mode, I, loading, under

which crack is subjected to opening, the failure criteria is that the stress intensity

factor, K

I

, is equal to the fracture toughness, K

IC

. The stress intensity factor is an

indicator of the magnitude of stress near a crack tip or the amplitude of the elastic

ﬁ eld. The value of K

I

can be obtained from the liner elastic mechanics

as - follows:

KYa

I

=⋅ ⋅

σπ

(17.1)

where a is the crack length, σ is the tensile stress applying to crack perpendicularly,

and Y is a dimensionless parameter that is determined from the crack and loading

geometries. Thus the stress at fracture is given by

σ

π

c

IC

=⋅

⋅

1

Y

K

a

(17.2)

From the fracture criterion, one can understand that the fracture strength

of ceramic components is not intrinsic strength but is fracture toughness and

crack geometry.

In general, ceramic components contain ﬂ aws at which the stress concentration

causes brittle fracture before their end - use. These ﬂ aws have particular ﬁ gurations

and sizes and are introduced mainly during manufacturing. Figure 17.1 shows a

schematic diagram of the ﬂ aw populations that could exist in ceramics. In this

example, the severest ﬂ aws are surface cracks, perhaps resulting from machining.

The next severest ﬂ aws are voids and pores introduced by sintering. Voids and

pores would become the main fracture of origin, if the surface cracks are much

smaller or removed, such as healed. The variation in the fracture sources leads to

the large strength, distribution in ceramics.

There are few scenarios that can generate or introduce surface cracks during

service. One possibility is by contact events, for example, impact, erosion, corro-

sion, and wear. Contact events may cause high stresses to the vicinity of the contact

site, leading to crack formation. Sudden changes in temperature can also lead to

stresses, known as thermal stress or thermal shock . The introduced surface cracks

would be more severe than the pores and voids. As a result, these crackings cause

Size (sensitivity)

Frequency

Embedded flaws

(voids and pores)

Surface cracks

due to machining

Figure 17.1 Populations of the strength - controlling ﬂ aws.

17.3 History 557

large strength decrease to the component. If these scenarios occur again, it is

possible that the component fractures catastrophically.

As mentioned above, ceramics usually fracture when K

I

= K

IC

but, in studies of

fracture [1 – 3] , it is sometimes found that crack growth can occur at lower value of

K

I

The mechanisms have been analyzed to describe the slow crack growth behav-

ior, including chemical reaction kinetics and interfacial diffusion. At the stressed

crack tip, it is found that environmental species, for example, moisture, reacts

and breaks the bonds at the crack tip resulting in stress corrosion cracking. The

kinetics depends on the concentration of the environmental species. Actually it

was found that the crack growth velocity in toluene is less than that in air [2] . This

implies that the presence of moisture enhances the behavior. Furthermore, at high

temperature, it is also found that the localized creep damage can give rise to the

slow crack growth.

17.3

History

In 1966, the study on the strengthening behavior of ceramics by heat treatment

was reported by Heuer and Roberts [4] . Then, Lange and Gupta [5] reported the

strengthening of ZnO and MgO by heat treatment, and used the term “ crack

healing ” for the ﬁ rst time in 1970. Now, we can ﬁ nd more than 200 reports on the

strengthening effects by heat treatment for cracking ceramics. The crack - healing

mechanisms in these reports can be roughly categorized into

1. re - sintering

2. relaxation of tensile residual stress at the indentation site

3. cracks bonding by oxidation.

Re - sintering [5 – 11] , that is, diffusive crack - healing process, is an older crack -

healing concept and commences with a degradation of the primary crack. This

regression generates regular arrays of cylindrical voids in the immediate crack tip

vicinity. Also, some studies [9 – 11] on the model and the kinetics of diffusive crack

healing in single crystalline and polycrystalline ceramics have been proposed.

However, as this crack healing requires the high crack - healing temperature, grain

growth might also be generated. In some cases, the strength decreases than it

before heat - treatment, although large strength recovery due to the crack healing

is attained. The relaxation of the tensile residual stress at the indentation site

leads to strength recovery. However, this phenomenon does not heal cracks. The

crack bonding by oxidation has been ﬁ rst reported by Lange [12] . He investigated

the strength recovery of the cracked polycrystalline silicon carbide (SiC) by heat

treatment in air at 1.673 K, and reported that the average bending strength of

the specimens heat - treated for 110 h was 10% higher than that of the unheat -

treated specimens. The same phenomenon in polycrystalline silicon nitride (Si

3

N

4

)

was reported by Easier et al . [13] . The heat treatment temperature required for

this crack - healing mechanism by oxidation is less than that required for the

558 17 Self-healing of Surface Cracks in Structural Ceramics

re - sintering crack healing. The other important aspect is that the cracks healed by

this mechanism are ﬁ lled with the formed oxides. A further mechanism and

method for crack - healing has been proposed. As an example, Chu et al . [14] pro-

posed the crack - healing method using penetrating glasses. They succeeded to

repair cracks in alumina (Al

2

O

3

) and have found that the repaired part becomes

even stronger than the base alumina.

As mentioned above, many other investigators have shown their interest on the

crack healings of ceramics. In the ﬁ eld of ceramic nanocomposites, there are many

reports also available on crack healing. The original impulsion for research in

crack healings of ceramic nanocomposites originates from the works of Niihara

and coworkers [15 – 17] . They observed that the strength of the alumina containing

5 vol% of submicrometer - sized SiC particles can be enhanced by annealing at

1573 K for 2 h in Argon. Since the original report, various mechanisms have been

proposed to explain this phenomenon. Nowadays, this mechanism is conﬁ rmed

to be driven by the oxidation of the dispersed SiC particles. Thompson et al . [18]

observed that the partial healing of indentation cracks occurred when 5 vol%

0.15 μ m SiC particles reinforced alumina were annealed at 1573 K for 2 h. Chou

et al . [19] have also investigated the crack length and the lending strength of

alumina/5 vol% 0.2 μ m SiC particles nanocomposite after annealing at 1573 K for

2 h in Argon or air, concluding that the crack healing occurs by the oxidation of

SiC particles. The similar conclusion was also derived by Wu et al . [20] . However,

Chou et al . [19] noted that a uniform reaction layer was not formed between the

crack walls, because the lower SiC content (only 5 vol%) results in a small quantity

of the formed oxide.

Ando and coworkers observed that the similar crack healing in mullite

(3Al

2

O

3

2SiO

2

) [21 – 23] , Si

3

N

4

[24 – 27] and alumina [28 – 31] based composites con-

taining more than 15 vol% SiC particles can recover the cracked strength com-

pletely. They found that the healed zone is mechanically stronger than the base

material and proposed the following requirements to obtain a strong healed zone:

1. Mechanically strong products (compared to the base material) should be formed

by the crack - healing reaction.

2. The volume between crack walls should be completely ﬁ lled with the products

formed by the crack - healing reaction.

3. The bond between the product and crack wall should be strong enough.

Crack - healings reports can be classiﬁ ed into three generations. as shown in

Table 17.1 .

First generation, that is, the crack healing driven by the re - sintering is only to

recover the cracked strength. Second generation, that is, the crack healing driven

by oxidation of less than 10 vol% SiC can be triggered by damage and occur under

service conditions, but the strength recovery is inadequate. Third generation, that

is, the crack healing proposed by Ando et al . can be attained with all the require-

ments. Consequently, the third generation crack healing is conﬁ rmed to be “ true ”

self - healing for structural ceramics.

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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