Nanotechnology (basic introduction and application)
The nanotechnology phrase originating from two words consists of the Greek numerical prefix nano referring to a billionth and the technology world [1-3]. As an outcome, nanotechnology or nano-scaled technology is commonly considered to be at a size under 100 nm (a nano-meter is 10-9 m) [1-2]. Nano-scale sciences (or nano-science) evaluate the phenomena, properties, and responses of materials at atomic, molecular, and macro-molecular scales, and generally at sizes in limited 1-100nm [1, 4-6]. Nanotechnology is considered as an emerging technology because of the feasibility to development of well-established products and to generate new products with totally new properties and functions with great potential in a wide range of applications [1, 7-8]. Remarkable applications of nano-sciences and nano-engineering lie in the fields of pharmaceutics, processed food, cosmetics, chemical engineering, high-performance materials, electronics, material science, precision mechanics, optics, energy production, and environmental sciences. Over 50,000 articles and more than 2,500 patents have been published annually about the nanotechnology and its application and progress in the different fields. Nano-materials with unique properties such as: carbon nano-particles, nano-tubes, nano-fibers, and nano-composites allow completely new applications to be found. The range of commercial products available today is very wide, including metals, ceramics, polymers, smart textiles, cosmetics, sunscreens, electronics, paints and varnishes. However new methodologies and instrumentation have to be developed for increase our knowledge [1- 2 and 7-10]. Another important term is known as nano-structured materials in which at least on constituent must be in nano-scale. In the field of advanced materials both terms are being employed to develop new generation of advanced materials and composites. One of the sub-groups of advanced materials is refractory materials used in different industries [8-10].
According to the ASTM C-71, the refractories are a “non-metallic materials having those physical and chemical properties that lead to them applicable for structures or as components of systems that are exposed to environments above 1000 °F (538°C) [11-14]. Also, some references mentioned that refractories are in-organic non-metallic material which can withstand high temperature without changing in their chemical or physical properties while remaining in contact with molten slag, metal and gases [11-13 and 14-18]. As well as, based on the operating condition, they should to have high resistant thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the thermal expansion coefficient[18- 22]. It is clear that refractories have an important role in glassmaking, metallurgical, and ceramic industries, where they are generated into a variety of shapes to line the interiors of furnaces or kilns or other devices for processing the materials at high temperatures [23-25]. Some of the technological and scientific inventions and progresses would not have been possible without refractory materials. Producing 1Kg of any metal without utilize of refractory is almost quite impracticable [26-27]. The history of using refractory materials dates back to since mankind start to develop metallurgical process. The first refractor raw material was clay. Up to the nineteenth century, refractory products were made of natural ores, such as magnesite, dolomite stones and clay. It was at the end of the eighteenth century and beginning of nineteenth century that the basis of modern metal beneficiation, the development of Portland cement and of modern glass processes started to inflict higher requirements to the refractory industry [11-17]. The most important materials used in the manufacturing of refractories are according to Fig. 1 [11-18].
In recent years, with the changing trends in steelmaking, the high performing shaped refractories are on an increasing demand. The higher campaign lives and the mutability of the newer steelmaking operations are decided by the accessibility and performance of such shaped refractories with superior high-temperature mechanical strength, erosion and corrosion resistance. The selection of refractories is often being utilized according to the conditions dominating in the application zone [22-32]. According to Fig. 2, it is observed that attention to improving the performance of refractories materials as well as the production of new refractory products, as a strategic product in various industries, has been highly respected by researchers.
Generally, refractories are classified according the basis of chemical composition, method of manufacture, physical form or according to their applications (Fig. 3) [14-20].
Classification based on Chemical composition
a) Acidic refractories
These types of refractories are used in region that slag and atmosphere are acidic. They have high resistance to acids but corroded by alkalis. The main raw materials belong to the RO2 category, such as SiO2, ZrO2 and etc.
b) Neutral refractories
These types of refractories are used in region that slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The main raw materials belongs to, but not confined to, R2O3 category. The common examples of these materials are Al2O3, Cr2O3 and carbon(C).
c) Basic refractories
These types of refractories are used in region that slags and atmosphere are basic; they have high resistance to alkaline materials but corroded by acids.
The main raw materials belong to the RO category to which magnesia (MgO) is a very common example. Other examples include (Mg.Ca (CO3)2 and (MgO-Cr2O3).
Classification based on manufacturing methods
a) Dry press process.
b) Fused cast.
c) Hand molded.
d) Formed (normal, fired or chemically bonded).
e) Un-formed (monolithic- plastic, ramming and gunning mass, castables).
Classification based on physical form
These types have determined shapes and size. These divided into standard shapes and special shapes. Standard shapes have dimension that are conformed by most refractory producer and are usually appropriate to furnaces or kilns of the same types. Special shapes are specifically made for special furnaces or kilns.
These types are without clear form and are only given shape upon application. These types are known as monolithic refractories. The common examples castables are, plastic masses, gunning masses, ramming masses, fettling mix, mortars and etc.
Monolithic refractory phrase is the name usually given to all un-shaped refractory products, the word “monolithic” extracted from the word monolith which means ‘big stone’[33-35]. Monolithic refractories are specific batches or blends of dry granular or cohesive plastic materials utilized to form nearly joint free linings. These materials are un-shaped refractory products which are installed as some form of suspension that finally harden to create a solid shape. Most monolithic formulations are made of three constituent such as: large refractory particulates (an aggregate), fine filler materials (which fill the inter particle voids) and a binder phase (that gels the particulates together in the green state), Fig. 4 [33, 35-38]. Monolithic refractories display a great range of mineral compositions and vary greatly in their physical and chemical properties. Some of them have low melting point (low refractoriness) whereas others approach high purity brick compositions in their ability to tolerate severe environments. Monolithic refractories are replacing the conventional type fired refractories at a much faster rate in many applications including those of industrial furnaces [33-33, 39-40].
These refractories are used to advantage compare to brick construction in different type of furnaces. Their use enhanced fast installation. Utilize of monolithic refractories often eliminates difficult brick laying tasks, which may be accompanied with looseness in construction. Protect of furnaces is very importance because substantial repairs can be made with a minimum loss of time [35-39]. Sometimes, monolithic refractories linings of the same composition as firebrick provide better insulation, lower diffusion and enhanced spalling resistance to the effects of repetitive thermal shock. Other major benefits of monolithic refractory linings are as follows [33-42]:
- Removing joints which is an inherent weakness.
- Easier and faster application.
- Better properties than pressed (sintered or tempered) bricks.
- Simpler transportation and handling.
- Better volume stability.
- Possibility to install in hot standby state.
- Higher mechanical resistance to vibration and impact.
- Matching shrinkage and expansion to the application.
Different methods are used in the placement of monolithic refractories such as ramming casting, spraying, gunning, sand slinging and etc. Heat setting monolithic refractories have a very low cold strength values and rely on relatively high temperatures to progress a ceramic bond [81-83]. Furnaces wall having the usual temperature drop across its thickness, the temperature in the cooler part is generally not enough to progress a ceramic bond. However with the use of a proper insulating material as backup, the temperature of the lining can be high enough to progress a ceramic bond throughout its entire thickness. To the installation and curing, monolithic refractories need a carefully controlled dry-out program. This causes the filler, binder and aggregate to fire creating a strong material [34, 41-43].
Types of monolithic refractories
Usually the monolithic refractories are divided according to Fig. 5 [33, 36-38].
a) Castable refractories
Materials with hydraulic setting in nature are name of castables. These refractories are containing cement binder (commonly aluminate cement), which creates hydraulic setting properties when blended with water. By heat-up temperature, the material and binder either transforms or volatilizes simplifying the generation of a ceramic bond. The most common binder used in castables is high alumina cement. Other binders are consisting of hydrate able alumina and colloidal silica. These materials are installed by casting and are also known as refractory concretes. Insulating castables are specialized monolithic refractories that are used on the cold surfaces of applications. These monolithic castables are composed of lightweight aggregates such as vermiculite, bubble alumina, perlite and expanded clay. The main function of castables is to create thermal insulation. Also, they are generally had low density and low thermal conductivity. The castables are classified according to following [33-37]:
ü Conventional Castable.
ü Low Cement Castables (LCC).
ü Ultra Low Cement Castable (ULCC).
ü No Cement Castable (NCC).
ü Light weight Castables.
ü Self-Flowing Castables (SFC).
ü Insulating Castable.
b) Plastic refractories
Plastic refractories are used to form refractory monolithic linings in different types of furnaces. These refractories are suitable for making quick, economical emergency repairs and they are easily rammed to any shape or contour. Plastic refractories are consisting of refractory aggregates and adhesive clays which are prepared in stiff plastic condition at the proper consistency for use without more preparation. During utilization, the blocks are tasked into pieces and are rammed or casted into place with pneumatic rammer. These refractories can also be poured into place with a mallet. These refractories suitable for many important applications due to the high melting point (high refractoriness), the range of compositions, and the ease with which plastic refractories are rammed into place make them. Also, they have often highly spalling resistant. Plastic refractories can consist of all the, clay-graphite, fireclay, high alumina, high alumina graphite and chrome types adapted for many various operating situations. Specific gunning types are also accessible. These are in granulated shape and are produced at the proper consistency, ready to use. Some examples of plastic refractories are [33, 36-41]:
ü Heat setting super duty fireclay plastic,
ü Super duty heat setting plastics with graphite,
ü Plastics in the 50 % alumina class,
ü Heat setting 60 % alumina class plastics,
ü Air setting high alumina plastics in 80 % alumina class,
ü Phosphate bonded high alumina plastics with alumina content ranging from 70 % to 90 %,
ü Phosphate bonded alumina chrome plastics,
ü And silicon carbide based phosphate bonded plastics.
c) Ramming mixes
Ramming mixes composed essentially of ground refractory aggregates, with a semi-plastic bonding matrix. These refractory materials are like to plastic refractories but are much harder. They need some sort of form to maintenance them when formed. The grain sizes are carefully classified and the final product is usually rendered dry and then mixed with a little content of water just before utilization. Other ramming products are rendered in wet state and are ready for use immediately upon opening. Ramming mixes are placed with pneumatic rammer in layers of 25 mm to 40 mm. Steel making, burner blocks, ports and similar applications used of High purity ramming mixes based on mullite grain. Ramming mixes consist of 80wt. % alumina content have good shrinkage resistance and thermal spalling at high temperatures. Some ramming mixed such as, stabilized high alumina air setting, have good thermal spalling resistance at high temperatures and volume stability up to their temperature limit. Also, phosphate-bonded alumina-chrome ramming mixes typically have very high strength at high temperatures and very good resistance to acid and neutral slags consist of coal ash slags. Alumina-graphite ramming mixes have mixture of high alumina grain and slag inhibitors which give them well slag resistance to acidic and slightly basic slags. In steel making industry, the dry ramming mixes based on high purity MgO and a sintering aid are useful. magnesite ramming mixes of exceptional purity and stability are used firstly as lining materials for coreless type induction kilns. Magnesia-Chrome fused grain ramming mixes can create special strength and density [33, 32-37].
d) Gunning mixes
The install method of more monolithic refractories is gunning. The constitution material of gunning mixes are different particles sized of refractory aggregate, a bonding compound, and may contain plasticizing agent to enhance their stickiness when pneumatically placed onto a kiln surface. These refractory materials are sprayed on application surfaces using a gun device. Usually gunning refractory mixes are supplied dry. In order to application, they are pre-damped in a batch mixer, and then continuously poured into a gun device. Water is added to the mix at the nozzle to achieve the proper consistency. Typically, gun mixes are including high alumina, siliceous, fireclay, dead burned magnesite and chrome types. Magnesite and hot gun mixes are not pre-damped and are placed in a batch pressure gun. Gun mixes should provide good coverage in a variety of applications [33, 34-40]. Some types of gunning mixes are:
ü Fireclay gunning mixes of multipurpose hard fired fireclay and standard calcium-aluminate cement compositions.
ü Fire clay gunning mixes with high purity calcium-aluminate bonding system.
ü Gunning mixes based on vitreous silica.
ü High purity alumina mixes which combine high fired alumina aggregate.
ü High purity calcium aluminate binder.
ü Basic refractory gunning mixes with magnesia content ranging from 60 % to 95 % with or without a phosphate bond.
e) Patching refractories
These type refractories materials are like to plastic refractories though have a very soft plasticity let them to be casted into place [33-39].
f) Coating refractories
These types of refractories materials are used to maintain refractory linings usually against chemical attack. Coating refractories are usually intended to coat just the working surface of a lining. They tend to be justly thin layers [33-39].
Generally, mortars are neither classified as refractory brick nor monolithic refractories. They are very fine refractory materials, which become plastic when mixed with water. These are used to bond the brickwork into solid unit, to provide cushion among the slightly irregular surfaces of the brick, to fill up spaces created by a deformed shell, and to make a wall gas-tight to prevent penetration of slag into the joints. Mortars should have good water keeping properties and must not foul. In this way, premature penetration of water in the refractory bricks after draying, causing the mortar to dry out can be avoided. Different types of refractory mortars are consisting of [33, 38-42]:
- Mortars with ceramic bonding (bonding starting at 800 °C)
- Mortars With chemical bonding
- Mortars with hydraulic bonding (bonding starting at 20 °C)
Also, the important properties of the mortars are consisting of:
ü Composition and characteristics of the mortar materials
ü Grain size
h) Fettling mixes
Fettling mixes are also granular refractory materials, with function like to gunning mixes, but are applied by shoveling into the kilns needing patching [33-36].
i) Tap-hole mixes
Tap-hole mixes are resin bonded. In these mixes the higher strength which is normally desired for monolithic refractory products, is not that important. Some criteria are necessary for all tap-hole mixes. These criteria are consisting of: correct consistency, setting and carbonization at the right time, precisely controllable PLC, and above all drilling capability [33-37]. Therefore, today, the use of monolithic refractories has become widespread because of their great benefits to shaped products. Therefore, research on the selection of raw materials, additives and formulations of this type of refractories has been considered as an important issue in recent researches (Fig. 6)
APPLICATION OF NANOTECHNOLOGY IN REFRACTORIES
Nanotechnology is mainly defined by size and comprises the visualization, characterizations, production and manipulation of structures which are smaller than 100 nm [1-3]. The structures the dimensions of which range from 100 nm down to approx. 0.1 nm exhibit special mechanical, optical, electrical, and magnetic properties which can differ substantially from the properties of the same materials at larger dimensions. Therefore, nanotechnology is a very active research field and has applications in a number of areas [1-6]. The advancements in nanotechnology have an impact almost in all fields such as materials, instrumentation, electronics, healthcare, defense, sensors, energy, manufacturing, environments etc. Up to recently, the application of nanotechnology in refractory material was very limited. Currently, significant attention has been paid to the application of nanotechnology in the development of refractories products [44-55]. Nanotechnology has been introduced to refractories. It has been reported that the performance of the refractories was appreciably improved for the well dispersion of nano-sized particles in the microstructure and reaction activity. Several efforts have been made by various researchers to improve the properties of refractories (bricks and castable) by using Nano-particles. The application of nanotechnology is aimed at obtaining the following unique properties of brick and castable refractories [52-56]:
- Ultra-high compressive strength
- Relatively high tensile strength and ductility
- More efficient cement hydration
- Increased aggregate-paste bond strength
- High corrosion resistance
- Control of cracks and self-healing.
In the past fifty years, refractories research and development based on conventional technical concepts and techniques seem to have had outstanding outcome for industries. If refinement of refractory technology is to be expected, the introduction of new technologies is indispensable. Nano materials as well Nano-structured materials improve several properties of refractory materials like mechanical resistance, thermal conductive, and carbon oxidation protection.
The first papers on nanotechnology in refractories causing a big interest appeared in UNITECR (The Unified International Technical Conference on Refractories) in 2003 . The first attempt by researchers was to modify the matrix (binding phase) of advanced refractory materials with nano-sized additives (Fig. 7) [56-74]. There have many articles in the field of application of nanotechnology in refractories [75-110]. Tamura and his co-worker from Nippon steel company published a technical report about technological philosophy and perspective on nanotech refractories . P.R. Rauta and N. Sahoo  presented a paper titled: Properties enhancement of refractory bricks by incorporation of nano materials. Kuznetsov et al  studied the application of Nanomaterials in refractory technology in a short review in 2010. In this short review they presented of prospects for using nanomaterials in various refractories and future directions of their use for improving the operating properties of refractories. They suggested that the possibility of controlling material properties at the atomic level makes it possible to create new innovative refractory materials and technology. There are much more publications in this field, but our focus will be on monolithic refractories [75-110].
APPLICATION OF NANOTECHNOLOGY IN MONOLITHIC REFRACTORIES
Recently Nanotechnology was introduced to the monolithic refractories, and nowadays it is an important tool included in many research projects. Several research groups have been working on the addition of different types and content of additives in monolithic refractories, and some of them have focused their investigations on the use of Nano-oxides, due to the reported benefits of adding these particles to ceramic bodies. According to the above, in this section, the results of carried out activities by various researchers using nanotechnology in monolithic refractories have been expressed. It is observed that the use of nanotechnology has been heavily used by researchers in recent years and they have achieved very interesting results. V.C. Pandolfelli group studied and published many papers in this regards [99-103], Nouri-Khezrabad et al. published a few papers about nano-bonded refractory castables [99-101]. They indicated that Calcium aluminates cement (CAC) contents higher than 3 wt% in refractory castables could have some drawbacks in the various processing steps (mainly drying) and also in their refractoriness when in contact with SiO2. A comparison of different binding systems for refractory castables was shown (Table 1)  .The benefits of replacing CAC or hydratable alumina by colloidal binders were also discussed. The advantages of colloidal silica/alumina as a refractory binder were also highlighted [99-101].
Finally, the challenges for suitable use of colloidal binders were discussed and the future direction of nano-structured refractory castables was outlined [99-101and 109-110]. They also studied development of nano-bonded refractory castables with enhanced green mechanical properties as well as rheological performance of high alumina nano-bonded refractory castables containing carboxylic acids and other materials as additives [100-101, 109-110]. They showed that applying colloidal silica suspensions as a binder source for refractory castables has been of great interest in recent years. Nevertheless, the resultant low green mechanical strength of these castables has hindered their application in relevant areas. The current paper provides a novel engineering route to improve the mechanical properties of colloidal silica bonded castables before firing. In order to attain this target, small amounts of calcium aluminate cement (CAC) and/or hydratable alumina (HA) were used as gelling agents. A splitting tensile test showed that although using HA resulted in a more significant increase in the modulus of rupture of the samples when compared to CAC, mixing both (CAC+HA) had the most positive impact on the green mechanical properties of the castables, leading to strength levels as high as the reference cement-bonded system. The results indicated that CAC enhanced the hydration of HA based on the accelerated dissolution of the gel layer formed on HA particles. The dual additive system (HA+CAC) on the siloxane bond formation showed that improved green strength was mainly related to a more