State-of-the-art review of fabrication, application, and mechanical properties of functionally graded porous nanocomposite materials

Functionally graded porous (FGP) nanocomposites are the most promising materials among the manufacturing and materials sector due to their adjustable physical, mechanical, and operational properties for distinctive engineering applications for maximized efficiency. Therefore, investigating the underlying physical andmaterialistic phenomena of such materials is vital. This research was conducted to analyze the preparation, fabrication, applications, and elastic properties of functionally graded materials (FGMs). The research investigated for both porous and nonporous synthesis, preparation, and manufacturing methods for ceramics, metallic, and polymeric nanocomposites in the first section, which is followed by deep research of the development of elastic properties of the above-mentioned materials. Main nano-reinforcing agents used in FGMs to improve elastic properties were found to be graphene platelets, carbon nanotubes, and carbon nanofibers. In addition, research studied the impact of nano-reinforcing agent on the elastic properties of the FGMs. Shape, size, composition, and distribution of nanoreinforcing agents were analyzed and classified. Furthermore, the research concentrated on modeling of FGP nanocomposites. Extensive mathematical, numerical, and computational modeling were analyzed and classified for different engineering analysis types including buckling, thermal, vibrational, thermoelasticity, static, and dynamic bending. Finally, manufacturing and design methods regarding different materials were summarized. The most common results found in this study are that the addition of reinforcement units to any type of porous and nonporous nanocomposites significantly increases materialistic and material properties. To extend, compressive and tensile stresses, buckling, vibrational, elastic, acoustical, energy absorption, and stress distribution endurance are considerably enhanced when reinforcing is applied to porous and nonporous nanocomposite assemblies. Ultimately, the review concluded that the parameters such as shape, size, composition, and distribution of the reinforcing units are vital in terms of determining the final mechanical and materialistic properties of nanocomposites.


Introduction
Nowadays, the manufacturing sector is evolving rapidly, and raw material demand is proportionally increasing. To tackle this, nanocomposite material usage has started to gain a reputation in many engineering sectors [1]. Nanocomposites are noncrystalline materials that are accepted as composites, which include nanoparticles of a material of dimensions smaller than 100 nm [2]. Utilizing nanocomposites as nanobuilding blocks establishes brand-new materials with exceptional flexibility and enhanced physical features [3]. This study deeply analyzes different porous and nonporous nanocomposites and the main reinforcing units preferred to manufacture nanocomposites. The production process of a reinforced nanocomposite consists of adding reinforcement units (mainly as a form of nanofibers or nanotubes) to any composite materials [4]. To add, since different nanocomposite types have different structures, candidate reinforcing agents are required to be selected carefully to create a valuable chemistry between nanocomposites and reinforcing units. Nanocomposites are further divided into three main groups, which are [2,5] as shown in Table 1. Furthermore, processing and manufacturing methods of porous and nonporous nanocomposites (which will be discussed later in detail) play very important role in terms of acquiring demanded mechanical properties. Even though the main manufacturing methods are generally the same for every type of porous and nonporous nanocomposites, processing and synthesis procedures vary slightly and vastly for nonporous nanocomposites and porous nanocomposites, respectively.

Recent development of elastic properties of functionally graded porous (FGP) nanocomposites
In terms of materials sector, FGP materials can be cited as a prevalent example of development in the materials industry, which are specifically engineered to get utilized in many sectors [6]. This reputation of FGP nanocomposite materials is due to their incredible high surface area to volume ratio of consolidating phase. Toward the end of twentieth century, many initial prototypes of functionally graded material (FGM) were designed as thermal insulation coatings [7]. Nowadays, FGP nanocomposite materials are obtained via nanoscale addition of consolidation materials, which are generally carbon nanotubes (CNTs) and graphene platelets (GPLs) into metal, ceramic, or polymer matrices [8,9]. The addition of CNTs and GPLs considerably improves energy absorption properties of thin-walled rings, arches, beams, and plates [10,11]. Additionally, such materials have become commonly selected for a wide range of engineering applications such as lightness, electrical conductivity, energy absorption, and thermal management [12]. The unique physical and materialistic properties of FGP materials come from their specifically adjusted composition or microstructure shape toward specific operations. In an engineering aspect, developing of FGPs was specifically required to reduce the stress fluctuations observed in composite materials. Moreover, FGPs exhibit decreased transverse and in-plane stresses, minimized residual stress, elevated thermal resistance, minimized thermal conductivity, and elevated fracture toughness and resistance to interlaminar stresses [13,14]. However, detailed analysis performed by Yas and Rahimi [15] on FGP nanocomposites particularly on weight fraction, scattering patterns, size and geometry of platelets, and porosity allocation and coefficient revealed that the operation performance of GPLs is highly dependent on their geometry. Free vibration, buckling, and bending analysis of FG graphene nanoplatelets (GNPs)-reinforced nanocomposite under hygro-thermo-mechanical loads were presented by Yas and Rahimi [15]. Results highlighted that the assembly gets stiffer as the weight fraction of the GNPs increases, leading to an additional increase in the natural frequency and critical buckling stress. Elevated temperature and moisture decrease the stiffness, natural frequency, and critical buckling load [16]. Safaei et al. [17] conducted a research to investigate the effects of CNTs and porosity properties of CNT cluster/polymer porous nanocomposite sandwich plates (PNSPs). Additionally, mechanical and thermal stresses, geometry, elastic foundation parameters, and boundary conditions impact on the loading Authors highlighted that the functional grading of the core decreases deflection. Furthermore, the utilization of 5% volume fraction CNTs indicated negligible impact on the deflection of PNSPs due to the growth of CNT clusters.
1.2 Functionally graded graphene plateletreinforced composites (FG-GPLRCs) As previously mentioned, geometry is a vital issue in terms of free vibration and static bending performances of porous nanocomposites. Liu et al. [18] developed a design to investigate the weight fraction and geometry of FG-GPLRC spherical shells. Researchers created five distinctive models that have different GPL distributions, which are named as functionally ungraded, functionally grading type O (FG-O), functionally grading type X (FG-X), functionally grading type V (FG-V), and functionally grading type A (FG-A) and are displayed in Figure 1 [18].
As highlighted by the investigators, in terms of free vibration, the models FG-A ( Figure 1 As shown in Figure 2, a FG-GPLRC multilayer annular plate with an outer radius R a , an inner radius R b , and a thickness h is declared. The plate is reinforced of GPLs either uniformly distributed (U) or functionally graded (X and O) across the thickness. The dispersion type X with more amount of GPLs at the outer layers promotes higher linear frequency, followed by the types U and O. Contrarily, a conflicting tendency is detected for the nonlinear frequency ratio. The increase in temperature leads to an increase in nonlinear frequency ratio, however, decreased the linear frequency; those impacts were most considerable, consecutively, in type X, U, and O [19].
The impact of composition proportions on the elastic properties of functionally graded carbon nanofibers (CNFs)/   [19].
phenolic nanocomposites that were manufactured via combination of compression molding and powder stacking was scrutinized by Bafekrpour et al. [20]. Functionally graded nanocomposites (FGN) were designed to have eight layers with same thickness, two layers with 0 weight fraction, wt%, CNFs; two layers with 2 wt% CNF; two layers of 4 wt% CNF; and two layers of 16 wt% CNF. Four specimens were designed, which are FGN-1, FGN-2, FGN-3, and FGN-4 ( Figure 3). FGN-1 was designed to have 16 wt% CNFs at the top and bottom of the beam, and mass fraction was decreased toward the center. FGN-2, 16 wt% CNF at the center and 0 wt% CNF at the top and the bottom. FGN-3, 16 wt% CNF at the top, 0 wt% CNF at the bottom, FGN-4 was designed to have 16 wt% CNF at the bottom, and 0 wt% CNF at the top. Investigators utilized finite element as well as analytic modeling to review composition-related variances on boundary conditions, loadings, and elasticity properties of nanocomposites. The authors concluded the dependency of elastic properties of the structure on the CNFs content of the thickness of the assembly. The investigation used a high proportion of CNFs parts, which have proven to improve Young's modulus; however, the Young's modulus of the final complete nanocomposite assembly still remained low even though building parts of the assembly have high CNF content. Kumar et al. [21] mentioned the consolidation of the structure by using CNFs is dependent on CNFs aspect ratio, balanced dispersion, CNFs misalignment [22], end-effects, and interlaminar bonding strength. Thermal residual stresses occurred during manufacturing are reported to have an impact on the overall mechanical features of the assembly [20,23]. Bafekrpour et al. [20] studied the tensile stress-strain and deflection curves of distinctive functionally graded nanocomposites and nongraded nanocomposites. The results highlighted that functionally graded nanocomposite with the highest CNF content (16 wt%) showed the best flexural properties, especially, the highest stiffness, whereas the nongraded nanocomposite exhibited the highest fracture load. This was explained by the reduction of toughness when high content of CNFs was utilized. Moreover, Mishra et al. [24] concluded the vitality of morphology of nanoparticles on determining the elastic properties. Flexural strength and modulus are highly dependent on the direction of the exerted load. In addition to this, spherical nanoparticles offer higher flexural strength while nanorods give higher flexural modulus to the structure.

FGP graphene platelet-reinforced nanocomposites
Chen et al. [25] have studied especially the nonlinear vibration and postbuckling load of multilayer FG-GLPRC beams. Porosity and reinforcement distribution were kept constant in each layer while the porosity coefficient and reinforcement weight fraction were varied for every layer.  The study was carried out investigating three different porosity distributions depicted in Figure 4, which includes both uniform and nonuniform porosity distributions. E 1 and r 1 are the highest Young's modulus and mass density, respectively. E 2 and r 2 are the lowest Young's modulus and mass density of the structure, respectively. The straight lines located at the top and the bottom indicate the intersection of the E, r 1 and E 2 , r 2 with the corresponding pore size distribution of both uniform and nonuniform distributions. It is vital to emphasize the highest Young's modulus and mass density values for nonuniform porosity cases (1 and 2) correspond to locations where porosity is more uniformly scattered. Additionally, for the steady porous distribution case, the middle plane was found to be the lowest magnitude of Young's modulus and mass density and also the most vulnerable to stresses. Another study carried out by Xu et al. [26] investigated the acoustical characteristics of the FGP graphenereinforced nanocomposite plates. Different porosity and graphene distributions were used. Figures 5 and 6 have been added to illustrate both porosity and GPL distribution inside the nanocomposite assembly.
Investigation concluded the dependency of the acoustic properties on both porosity and reinforcement distribution such that the porosity considerably impacts stiffness, which has a direct influence on sound transmission loss values. Additionally, the impact of separation of reinforcement units (GPL) within the nanocomposite structure has been found to be controlling parameter in terms of acoustical features [26]. Moreover, different porous structures of cylindrical shells to find critical buckling values have been studied. Results concluded that the symmetrical distribution of the pores and GPLs through the thickness of the cylindrical shell proposes the optimal buckling values, whereas the physical size of the pores is inversely proportional with the buckling features of the structure [27]. Figure 7 obtained from an earlier study [28] illustrates the effect of distinctive porosity distributions (i.e., even and uneven) on deflection abilities of a square plate. Where w adim being the centralized deflection, which is the ratio of the central transversal displacement w, to the plate thickness h, (w adim = w/h). Moreover, load parameter, donated by P is defined by the formula q 0 a 4 /E × h 4 , where q is generalized nodal displacements vector, a is the square length, E is the Young's modulus, and a is the porosity volume fraction where 0 < a < 1. The research concludes higher displacement values for porous structures compared to nonporous structures. Additionally, even porosity distribution with higher a value led to the greatest displacement [28].
1.4 Effects on the reinforcement of particle stiffness, geometry, and size The main materials used in the reinforcement of nanocomposites are GPLs and CNTs. Even if both materials offer decent levels of reinforcement in terms of mechanically, elastically, and operational life, distinctions are present between the two reinforcement types. CNTs are proven to be more efficient than GPLs in terms of mechanical reinforcement in the case of symmetrical distribution, whereas GPLs propose more efficient reinforcement when random  distribution of reinforcement is the case. Furthermore, when the same physical magnitude and identical separation is utilized, CNTs offer considerably higher reinforcement than that of GPLs. This will cause a difference in bulk properties between CNT-reinforced and GPL-reinforced nanocomposites [29]. However, increasing the composition of soft/elastic micron/nanofillers enhances the impact toughness, however, decreases the Young's modulus. Contrarily, increasing the composition micron/nano hard/rigid filler enhances impact toughness as well as Young's modulus of polymerbased assemblies [30,31]. Additionally, the toughness and stiffness of the nanotube-reinforced polymer nanocomposites were obtained to be the functions of the elastic modulus of the nanotubes [32]. However, elastic modulus has been found to be increased slightly when the size of nanofillers utilized in the structure was decreased. Inversely, when the size of nanoparticles used in the structure decreased, the tensile strength of the structure has been found to be decreased [33]. Xu and Hoa [33] noted that the interfacial fracture toughness of carbon-fiber-reinforced epoxy/nanoclay nanocomposites was nearly doubled (85%) when four pieces of nanoclay were added to hundreds of epoxy by mass. Polit et al. [34] stated the impact of the weight dispersion pattern of GPLs on the stiffness, which was related to the porosity dispersion in metal foams. The places of the maximum shear stress and zero normal stress among the thickness are again related with porosity and GPLs load dispersion forms. Noteworthy variation in the buckling and fundamental frequency values was recorded when the amount of GPLs increased [35][36][37]. The characteristic and thickness proportion of GPLs significantly affect the operation performance of the beam. Feng et al. [38] studied the nonlinear static bending of multilayer functionally graded nanocomposite beams consolidated with GPLs. The study also discussed both the random and uniform distribution of GPLs. This research observed even a minor amount of GPL addition to the structure considerably decreases the static bending deflection of the beam further, this consolidation increases as the weight proportion of GPLs increases. As previously mentioned, this study also highlighted the importance of the GPL distribution pattern in terms of enhancing the bending properties of the beam. Utilizing square GPLs with less single graphene layers and distributing more GPLs close to the top and the bottom surfaces of the beam instead of uniformly over the beam thickness is the most efficient method to effectively consolidate the stiffness and to decrease the deflection of the beam [39]. The normal stress break or disparity over the thickness direction of the beam drastically enhanced by increasing the overall number of layers. Utilization of ten layers can afford a decent approximation to the chosen GPLs dispersion, significantly decreased mismatch and comparatively less manufacturing cost. Arefi et al. [40] mentioned the nondimensional deflection of microplate is increased when the height-over-length ratio of the GPL is increased. Similarly, increasing the thicknessover-length ratio of GPLs when the volume of the graphene content kept constant, the stiffness of the plate decreases, which causes an increase in deflection. The increase in the GPL content increased the stiffness of the plate and decreased the interfacial strains. Increased porosity coefficients increased the axial stress. Additionally, increased porosity coefficients caused the increase in deflection, stress, and strains. Table 2 summarizes the effect of different parameters such as filler composition, filler size, filler weight distribution, and filler size on the operation performance of the final product in terms of impact toughness, buckling values, vibrational characteristics, elastic modulus, tensile strength, stiffness, and various stress properties [41].
2 Processing, fabrication, and applications of different porous nanocomposites

CMNCs
CMNCs are the mixing of one or several different ceramic phases to increase the wear resistance and thermal stability. Ceramics alone suffer from low toughness that results in brittleness, which avoids the utilization of ceramics in many industrial applications. However, low toughness and brittleness problems are tackled when CMNCs used to offer more efficient and longer operation time at the area of utilization [2]. The reason why CMNCs are much stronger than ceramics alone lies at their structure where energyabsorbing materials such as fibers or particles are included in the ceramic matrix to decrease the brittleness and improve the durability against fracture [42]. Aluminium oxide (Al 2 O 3 ) and silicon carbide (SiC) are the most common materials used in CMNCs. Examples of CMNCs can be given as Al 2 O 3 /SiO 2 and SiO 2 /Ni [5,43].

Porous CMNCs
Porous ceramic (nano)composites offer many benefits such as minor electrical and thermal conductivity, lightweight, low heat-to-mass ratio, increased specific surface area, reasonable hardness, resistance to wear, corrosion, and high temperature applications. The mentioned improved mechanical and material properties made porous ceramics significantly common in various engineering applications [44][45][46]. Processing of pores, fabrication of the porous matrix, and the physical dimensions of the pores have a great impact on the mechanical and material properties of the  [47][48][49].
Although there are many recent processing techniques of porous ceramics under development, the most obvious processing methods can be stated as partial or full sintering, replicas, sacrificial templates (fugitives), and direct foaming [47,50,51]. Although the processing techniques of the porous ceramics is slightly different from nonporous ceramics, manufacturing techniques are still the same. Especially, additive manufacturing (AM) techniques, that is, chemical vapor deposition (CVD) is the most common technique in terms of producing porous ceramic structures, which are given in Table 3.

Processing and synthesis of ceramic matrix nanocomposites (PCMNCs)
The most popular techniques utilized for PCMNCs and porous ceramics are powder, polymer precursor, spray pyrolysis, and chemical and physical vapor depositions. Tables 4 and 5 include visualization of some common strategies of manufacturing CMNCs. Additionally, Table  5 includes accessible schematics with labels of CMNC synthesis.
Some main synthesis methods, such as powder, sol-gel, precursoring, and pyrolysis techniques, have their unique way of processing methods. Table 6 has been included to emphasize the order of process as well as various stages of the relative operation.
Every engineering manufacturing technique has its own advantages and disadvantages. Table 7 has been included to state and explain the relative advantages and disadvantages of the synthesizing as well as processing systems of the CMNCs mentioned above in this article.

Fabrication of ceramic matrix nanocomposites (FCMNCs)
Fabrication of composites, nanocomposites, and porous and nonporous ceramic matrices are the same. The different ceramic materials are obtained at the synthesizing stage with different procedures (Tables 2 and 3). However, the final fabrication method, which can be implemented to mass production is the same for every ceramic composite branches. The most common fabrication techniques for CMNCs have been determined along with every process's individual reactant arrangement and technique specification, which are presented in Table 8. Table 8 mentions the main methods for fabrication of CMNCs. Additionally, Table 9 visualizes the above-mentioned fabricating strategies of manufacturing CMNCs. Accessible schematics with labels (if present for that process) of CMNC fabrication are presented below.

MMNCs
MMNCs are the mixing of ductile metal or alloy matrix and nanosized reinforcement materials. The cooperation between ductile metals and nanoparticles causes MMNCs to have an elevated ductility, toughness, strength, and modulus. Due to their superior materialistic and mechanical properties, MMNCs are widely utilized in automotive and aerospace industries [2]. Examples of MMNCs can be stated as iron-chromium (Fe-Cr)/Al 2 O 3 , nickel aluminium oxide (Ni/Al 2 O 3 ), and Co/Cr (cobalt chromium) [5].

Porous MMNCs
Highly porous metals are attractive due to the high flow stress and toughness, solid mechanical formability, resistance to thermal degradation, and significant electrical and thermal conductivity features [108]. Porous structured metals are classified as lightweight, improved mechanical and material properties, with an additional energy absorption feature [109]. To be able to produce porous metals effectively and useful for specific engineering applications, various processing techniques has been recently used in the sector. Powder metallurgy, melt foaming, metallic fiber sintering, gas injection to metallic sheets, infiltration, metal deposition, and hollow sphere sintering are the main methods used for processing of porous structured metals [110]. Furthermore, lately innovated techniques to process and produce porous metallic structures are casting foaming (Aluminium foam) processes, precursor foaming, LOTUStype foaming, and space holding processes. The most commonly utilized technique among these processes is the space holding process due to its ability in terms of allowing the regulation of the pore morphology of the porous metals [108][109][110][111][112][113][114][115][116][117][118][119]. Figure 11 shows the structure of porous aluminium (metal foam) obtained via space holder method. Additionally, this particular type of porosity is called a closed-cell structure, which can also be identified as metal foam. To extend, the closed-cell configuration in metals is more advantageous for many applications as it can endure under high pressure. Additionally, the closed-cell porous structures are nearly four times denser than the open-cell porous structures, which makes them more suitable for Sintering Synthesis process includes the reaction of relevant reactants with suitable precursors to obtain vapor phase nanoceramics [47,50,51] Replica templates -Utilizes either synthetic or natural template, which is infiltrated through a ceramic suspension [47,50,51] -Later, when the mix is dried off completely, the template is detached leaving a replica of the initial template morphology Sacrificial templates, that is, freeze casting -This method includes the so-called "pore former" or sacrificial to perform as a place keeper in the ceramic matrix [47,[50][51][52] -Once the ceramic matrix consolidates, sacrificial is detached leaving empty pores behind -To cite an example to this specific process, freeze casting uses ice crystals in ceramic matrix to form pores Direct foaming Utilizes gas bubbles that are intentionally trapped in the ceramic matrix during the slurry phase. When the ceramic slurry is dried off, the places occupied by gas bubbles take spherical pore shapes [47,48,50,51] Review of fabrication, preparation and application of FGP nanocomposite materials  329 -Addition of water and condensation reactions of an organic/inorganic molecular precursor dissolved inorganic solutions [53][54][55][56][57] -Three-dimensional (3D) polymers that contains metal-oxygen bonds are obtained at the end of mentioned reactions -The process is followed by ventilation operation to remove excess liquids and to obtain a solid material, which is then subjected to thermal operations for strengthening Powder Al 2 O 3 /SiC -Choice of materials that will be used in the process (mainly selected as powders mainly small dimensions, uniform, and purified) harsher operations. However, comparatively high density of closed-cell structures makes them infamous for lightweight engineering applications [120,121]. Nevertheless, the processing method utilized to attain porous structure on the metallic materials decides the nature of the porous matrix. Moreover, porous structure within any metallic materials plays an important role in deciding the ultimate physical and mechanical properties. Hence, categorization of porous metals leads to a separation of the above-mentioned processing techniques into following sets [108]: • Isolated porosity ○ Pores are isolated within the metal structure. Sometimes referred as "dilute porosity." • True metal foam ○ A gas phase generates a group of physically in contact bubbles divided by thin metal membranes. Hence, the natural structure of the specific porous system tends to be closely celled. • Foam precursor ○ An already located polymer foam (i.e., polyurethane) is utilized to produce the structure of the metal. zirconium diboride-SiC-zirconium carbide-zirconium silicide -The reacting substances will alter to resultants throughout the combustion reaction, which will be observed straight after the reaction initiates For nonoxide-oxide composites; For oxide-oxide composites -Another name of the process is solution combustion [65,74-77] Al 2 O 3 /zirconium dioxide; cerium(IV) oxide-metal x oxide y ; and metal oxide x -zinc dioxide; Gamma-iron(III) oxide-TiO 2 /ZrO 2 / magnesium aluminate -The process includes initiation and developing of automaintained heating reactions at an either aqueous or sol-gel atmospheres -Distinctive nanomaterials can be synthesized via this method -Control of reaction conditions greatly affects the yield Coprecipitation synthesis For oxide-oxide composites -The process is considered to be an alternative to similar sol-gel processing as it is also utilized chemical ways to mix oxides together  Schematics of powder processing for various materials [89] (Continued) 332  Ismail Barbaros et al. Schematic of the spray decomposition [91] Schematic of the coprecipitation synthesis [92] • Porosity created by packing ○ Individual porous elements are attached together to form an assembly, which has high pore content. • Porosity created by phase change ○ Phase changes of a single distinct phase to many phases, mainly gas, leads to the creation of a porous structure. • Regular lattices ○ A steadily porous structure, generally created by uniform beam elements, is recurred a lot of times to generate a material.
Additionally, processing and preparation of porous metal (nano)composites is achieved via combined processing or reprocessing of techniques mentioned above. To cite an example, redepositing or filling of porous metals with additional porous metals or porous alloys. Either welding and bonding operations are carried out to attach porous components together or porous assemblies are created via addition of distinctive metal powders, fibers, and other materials, which are then transferred to the processing unit for further reprocessing with the help of techniques stated above [108][109][110].

Processing and synthesis of metal matrix nanocomposites (PMMNCs)
The most popular techniques utilized for PMMNCs and porous metals are precursoring, pyrolysis, CVD/physical vapor deposition (PVD), infiltration and solidification, and sol-gel processes. Table 10 summarizes the procedure, arrangements, and operations of every process. Table 10 presents the main techniques during the synthesis of MMNCs. Additionally, Table 11 includes visualization of the most popular processing methods of MMNCs. Accessible schematics with labels of MMNC synthesis are also included in the table.
The following table summarizes the advantages and disadvantages of the above-mentioned processes, which are utilized for the synthesis of MMNCs. Both major and minor advantages and disadvantages are given and explained in Table 12.

Fabrication of metal matrix nanocomposites (FMMNCs)
Fabrication of composites, nanocomposites, and porous and nonporous metal matrices are the same. The different metallic materials are obtained at the synthesizing stage via different procedures previously mentioned (Table 10). However, the final fabrication method, which can be  -Mass production is challenging -Fast and easy process -Challenging to control the process parameters and variables that affect the outcome -No complex apparatus required -Low energy consumption -Suitable for magnetic metal oxide production processes Coprecipitation -Simple application -Application on uncharged particles is not possible -Slow process and time consuming -Low poisonous waste products -Cannot be applied if there is a significant mismatch on the precipitation rates of the solvents -Easy to control process parameters -Distribution of the particles is not efficient, which makes coprecipitation prone to aggregation -Modifications on final homogeneity and particle surface is available -Low energy and temperature application -No organic solvent is required -Elevated quantity of magnetic products can be obtained by using this method Mechanochemical -Suitable for nonstoichiometric halides synthesis at low temperatures -Unwanted products can be produced which can lead to contamination of the final product. Oxidation is inevitable [59,98,99] -Eliminates the risks of high temperature applications such as thermal decomposition and high pressure  Schematic of catalytic decomposition method [107] implemented to mass production is the same for every metallic composite branches. The following table illustrates the FMMNCs. A brief explanation of the process application as well as corresponding advantages and disadvantages is also depicted in Table 13.
In the previous section, the FMMNCs were given and explained in Table 13. Table 14 includes visual representation of the schematics with labels of the above-mentioned fabrication methods of MMNCs.

PMNCs
Fillers are the nanosized particles (nanofillers) used in PMNCs, which are categorized as 1Dlinear, 2Dlayered, and 3D powder. The link between polymer matrix and nanofiller at a molecular stage has an impact on the attraction between nanocomposites. As a result, supply of minor amount of nanofiller with dimensions smaller than hundred nanometers to matrix causes alteration on the overall composite material properties. Materialistic properties of PMNCs are elevated thermal stability, enhanced abrasion resistance and elevated barrier capacity (reduced gas permeability) [149].
To cite examples of PMNCs, thermoplastic/thermoset polymer/layered silicates, polyester/ Tin oxide can be given [5]. Usage areas of PMNCs in daily life and industry are packaging, power tool housing, fuel and solar cells, and fuel tank [2].

Porous PMNCs
Porous polymers are becoming one of the most promising material group which are being started to utilize in almost every engineering sector. This research interest on such materials are due to their ability to own properties of both porous materials and polymers in a sole structure. Porous polymers have large surface area, exceptional physiochemical properties, ease of production and processing. Furthermore, porous polymers can be dissolved within a solvent and directly processed while maintaining the porous structure which is not possible to achieve in any other porous structures [150,151]. Figure 12 illustrates the scanning electron microscope image of macro porous polyurethane. The method utilized to produce this type of porous structure is known as gas foaming, which is the most popular technique. Various gaseous products obtained during chemical reactions can be used at some point in the manufacturing of the porous polymer which can be removed later to construct porous structure. In the production of macroporous polyurethane shown in Figure 12 the porous structure was obtained by removing the carbon dioxide molecules produced during the chemical reactions. This specific porous structure can be named as an open-cell structure, which is proven to be less dense and more flexible than the closed-cell structures [121,152]. Moreover, FGP polymer structures can be specifically designed to illustrate responsive properties with an additional ability to alter the pore structure relative to the engineering application. Porous state of the polymers can be altered between closed and open state when environmental exposure is the case. Additionally, since the structure of the porous polymers are organic and includes light components, the overall mass of the structure is significantly reduced, providing additional advantage of light-weighting [150,153,154]. Porous polymers are generally categorized by their size, physical shape, pore shape and dimension, spread of interconnectivity, and ultimate amount of porosity. The application which the porous polymer structure is supposed to be used is a deciding factor in terms of what kind of porous structure and mechanical properties of the polymer will have [155]. To effectively consider the porous polymer structure and respective structure's preparation and fabrication techniques, it is vital to divide the porous polymer structures into intersections. IUPAC has produced a template to classify the porous polymers relative to their pore size which is previously mentioned in the porous CMNCs section of this report. The union has agreed to separate porous structures into three distinct classes named as; microporous, macroporous and mesoporous. Note that, the porous structure of the polymer has a strong correlation with its manufacturing technique.
• Microporous polymer structures (Ø microporous > 2 nm) ○ The most important property of such polymers is high flexibility. This unique feature leads to efficient adsorption as well as cohesion applications. In   Alternative name is mechanical alloying, which is a ball milling operation where a powder mixture located into a ball mill is exposed to energetic collisions sourced from balls. This process can lead to fine and good mechanical property materials -Milling of the introduced reinforcing powders until the essential size is produced, that is, nanosized particles [2,55,97,122] CVD/PVD Aluminium/molybdenum, copper/ tungsten, copper/lead CVD is an AM method, which is utilized to manufacture high quality and performance solids under vacuum conditions. The process includes chemical reactions occurring between a halide and an organometallic mixture to be placed and additional gases to obtain involatile thin solid films on substrates PVD: [2,55,97,122,126] PVD is also an AM method. However, differing from the CVD where the material changes from a condensed form to a vapor form and again return back to a thin solid film condensed form -Sputtering or evaporation of distinctive parts to obtain a vapor phase -Supersaturation of the vapor in a noble medium to encourage the condensation of the metal nanoparticles -Application of heat treatment in a noble medium to reinforce the nanocomposite CVD: -Gases are obtained via chemical reactions which are then solidified to required form.
The process includes hydrolysis of the precursor in either acidic or basic atmosphere followed by the polycondensation of the hydrolysis products creating a polymeric network where metal nanoparticles can be positioned -Metal pieces are utilized to lead the chemical reduction of salts in a solution Colloidal: - ○ Macropolymers are produced via cross-linking copolymerization of vinyl and divinyl monomers in a noble diluent. The diluent used during the mentioned reaction, which can be either linear polymer, nonsolvent or solvent, is the key factor in terms of determining the porous structure of the macro porous polymer. By being similar to microporous and mesoporous polymers, there is also a strong correlation between porous structure and the manufacturing method of macroporous polymers. To cite examples, freeze drying, porogenation, microemulsion formation, and gas blowing methods can be utilised to produce macroporous polymers. [150,151].
According to the Berro et al. [151], the most common preparation methods of porous polymers are; gas foaming, phase (immersion precipitation, chemically and thermally reaction driven phase separation) separation, small liquid drops templating (emulsion, bicontinuous micro emulsion and breath figures templating), colloidal crystal templating, templating via self-assembled structures, molecular imprinting and biotemplating utilizing natural biological templates. These methods are tabled in the following section. Production of pores on polymer structures, corresponding advantages and disadvantages are included in Table 15.

Processing of polymer matrix nanocomposites (PPMNCs)
The previous section explains the processing and preparation of porous PMNCs. Similarly, the following section is the categorization of the porous polymers. The main processing techniques are sol-gel, various polymerization methods, intercalation, and sacrificial template utilization. Table 16 summarizes the processing techniques of porous polymer nanocomposites with corresponding system arrangements and process procedures. For iron/gold containing nanocomposites: -Synthesis of the iron -Preparation of the secondary shell and dehumidification of the powders after second gold coating -Squeezing of the mixture to obtain the required material Schematic of liquid metal infiltration process [129] Schematic of rapid solidification with ultrasonics [125] (Continued) Review of fabrication, preparation and application of FGP nanocomposite materials  341 The previous section explains the processing techniques of PMNC systems. The following section summarizes the advantages and disadvantages about the mentioned PPMNCs in Table 17.

Fabrication of polymer matrix nanocomposites (FPMNCs)
Fabrication of polymeric, nanocomposites, and porous and nonporous ceramic matrices are the same. The different polymeric materials are obtained at the synthesizing stage with different procedures (Tables 15 and   16). However, the final fabrication method, which can be implemented to mass production is the same for every polymeric composite branches. Table 18 illustrates different FPMNCs. A brief explanation of the process is available together with the relative advantages and disadvantages of every manufacturing process. The previous section explains the main fabrication methods of PMNCs with brief explanation as well as advantages and disadvantages. The next section is the visuals of the above-mentioned manufacturing processes utilized during the manufacturing of PMNCs. Table 19 depicts the schematics of the processes explained previously in Table 18.

Applications of porous nanocomposites in various engineering sectors
The application of porous nanocomposites offers improved mechanical and material properties of the engineering assemblies. Distinctive engineering sectors including biomedical, electrical and electronics, aerospace, marine, mechanical, and energy storage are greatly demanding constant development and innovation on the material usage in many engineering applications for improved efficiency and longer service life. To cite, Pinkert et al. [186] studied the spatial separation of the metal  -Costly and complex -Improved mechanical and material properties -Challenging to coat complex structured shapes -Compatible with all inorganics and most organics -Relatively low yield -Eco-friendly process Sol-gel, colloidal (chemical processes)

Sol-gel:
-High porosity [2,55,127] -Efficient, effective, and versatile -Reduced wear resistance -High-purity final products -Low internal bonding strength -Low-temperature application -Produces large and stable surfaces Colloidal: -Controlling porosity is challenging -Simple process -High permeability -High chemical homogeneity hydroxide nanoparticles within the porous carbon matrix. Efficient distribution of the nanoparticles within the functionalized porous carbon network revealed improved energy density levels, which leads to an efficient supercapacitor utilization in electronics. Moreover, porous magnetic nanocomposites used in biomedical engineering showed enhanced mechanical properties especially higher tensile strength. scanning electron microscope (SEM) images illustrated improved internal connection between macroporous and microporous structures, which is the reason of improved strength. Magnetic Fe 3 O 4 nanoparticles are utilised for abovementioned biomedical engineering applications due to its reasonable water absorption properties as well as healthy antimicrobial possessions. [187]. To sum up, different nanocomposite assemblies are used in different engineering sectors. To cite an example, porous MMNCs are highly preferred in medical, aeronautics, marine, transportation (land and air transport), and military protection applications [188]. Moreover, porous PMNCs are mainly used in food packaging, coating, adhesion, drug delivery, electric, and electronic applications [189]. However, porous CMNCs are mainly utilized in industrial, civil, and energy absorbing applications [190]. The differences among the application areas of three nanocomposites originates from the variations between morphological, atomical, and bonding structures. Table 20 has been added to illustrate distinctive porous nanocomposite materials along with their relative engineering sector.
3 Numerical, mathematical, and computational modeling review A research conducted by Ansari et al. [210] studied geometrically nonlinear static bending of FG-GPLRC porous plates. Four distinctive porous dispersion plan and patterns were selected, and material characteristics were deduced by using closed-cell Gaussian random field (GRF) scene, the Halpin-Tsai micromechanical model, and the rule of mixture [15,18]. To process the data on computer, authors used virtual work principle and higher-order shear deformation theory (HSDT) derivations in matrix form. Variational differential quadrature (VDQ) and finite elemental methods (VDQ-finite element modeling [FEM]) numerical approach was used. The problem domain was divided into finite elements and VDQ selection method was exploited for every finite element model. Authors used the mentioned procedure to investigate the impact of porosity and GPL dispersions. Resulting matrix equations were solved with the help of the pseudo arc-length continuation algorithm.  Schematic of disintegrated melt deposition [146] Schematic of powder metallurgy [147] (Continued) This study highlighted the tendency of VDQ-FEM propose efficiency for solving problems. Another study carried out by Yas and Rahimi [15] examined the variation of thermal buckling of FGP nanocomposite beams under distinctive temperatures by using generalized differential quadrature method (GDQM). Different nanofillers, scattering patterns, and porosity allocations were studied. GRF was used together with closed-cellular solids to obtain Poisson's ratio and correlation among porosity coefficient and mass density. Halpin-Tsai micromechanics modeling was utilized to determine elastic modulus of the nanocomposite. Arshied et al. [211] utilized the GRF model method for closed-cell cellular FSP grooving process, (1) filling of the powder material, (2) pinless FSP, (3) pinned FSP (4) [148].
Review of fabrication, preparation and application of FGP nanocomposite materials  347  [150,157] -The manipulation of the meso/ macroporous structure is possible -Complex and costly for scaled up production -Functional grading is achievable -Not very efficient pore production -Cannot be utilized effectively for hard materials Block-copolymer selfassembly Space obtained via removal of sacrificial component, morphology regeneration, and vesiculation -Customized pore structure and arrangement -Not a very effective method for mass production [150,158] Two different polymers bonded covalently to self assemble to form a specific structure in nanoscale-customized pore size -Suitable for complex structures such as stimuli responsive assemblies -Expensive -Absence of micro or nanopores Direct synthesis

Microporous
Links between monomers are hyper-cross linkable -High surface area of this structure enables permanent porosity -There is only one way that the monomers for this process can be produced [150,159] -Pore structure can be customized by customizing the monomer structure used in the porous assembly is challenging [150,161] -Honeycomb structure can be obtained solids for the porous matrix characteristics and effective features of porous nanocomposite was obtained by Halpin-Tsai along with extended rule of mixture micromechanics model. Shear deformation impacts were considered by using the first-order shear deformation theory (FSDT). Energy method was used to derive equations and solved by generalized differential quadrature (GDQ).
To understand the thermoelastic manners of sandwich plates with porous polymeric core and CNT clusters/polymer nanocomposites, Safaei et al. [17] used total energy function along with mesh-free strategy together with two-plate theories to deduce the leading thermoelastic equations. To monitor the temperature dependency of the CNT clusters/ polymer nanocomposite, Eshelby-Mori-Tanaka's method was operated. Yang et al. [212] utilized FSDT to consider the transverse shear strain and Chebyshev-Ritz approach to discretize the displacement fields. Leading equations were extracted and then solved to deduce critical uniaxial, The process utilizes monomers, which includes layered particles. Polymerization of the monomers is started. Structures obtained by this method is highly exfoliated due to the allocation of monomers at both inside and outside of the interlayers [162][163][164] Polymethyl methacrylate Poly urethane Epoxy Intercalation/polymer from solution Clay with PCL This process uses distributed nanofillers in a solvent and an additional soluble polymer. Process includes the absorption of the polymer by the delaminated sheets with a simultaneous evaporation of the solvent. After all the solvent has been evaporated, the sheet formation is adjusted to trap the polymer chains between the layers. Thus, multilayered structure can be achieved by this method [162,163] PLA High-density polyethylene PEO PVA Polyvinyl pyrolidone biaxial, and shear-buckling stresses, as well as the natural frequency of the plates with distinctive porosity and GPL dispersions. Moreover, to scrutinize material properties of graphene-reinforced nanocomposites, similarly, Liu et al. [18] used Halpin-Tsai and rule of mixtures. In addition, free vibration and bending equations of nanocomposite were derived with the help of 3-D elasticity theory together with the state space method. Jalal et al. [213] proposed usage of big data approach for an efficient design of composite structures, which mainly considered functionally graded carbon nanotube-reinforced nanocomposites. The materialistic features of the nanocomposites were deduced via Eshelby-Mori-Tanaka method. Followingly, researchers compared two methods of using big data approach, which are mesh-free method and optimized neural network (ONN). Robust mesh-free technique was utilized to extract vibrational frequencies, impact of CNT alignment and aggregation. Six parameters (geometry dimensions, composite core, nanocomposite layer, volume fraction of CNTs and clusters, volume fraction of changing exponent) included a total of 15,625 data sets, which are followingly analyzed by ONN. ONN results were found to be very consistent and confirmed the suitability of ONN's usage for enormous data analyzing. Furthermore, utilization of ONN is nearly a thousand times faster relative to mesh-free method with negligible amount of simulation error [214]. Yaacoubi et al. [215] studied loading distributions and shifting in sandwich plates, which reinforced with functionally graded nanocomposite face sheets was tested by FSDT built on mesh-free strategy. The assembly was treated as Winkler-Pasternak elastic model, and the nanocomposite was consolidated with three distinctive CNTs. Molecular dynamics (MD) study at nanoscale and Halpin-Tsai were utilized to deduce the elastic constants of the assembly. The adjustment of boundary conditions was vital. This was accomplished by utilizing moving least squares (MLSs) to approximate the displacement field and the transformation technique.
Temperature plays an important role on the nonlinear free vibration of edge-cracked graphene nanoplatelet (GNP)-reinforced composite laminated beams [216]. Supposing GNPs fillers were dispersed randomly and thermal field was distributed uniformly, material properties of the GPLRNC was found via micromechanical models. Loading and intensity coefficients were deduced via FEM. Karman-type geometric nonlinearity with respect to FSDT was used to derive crack motion equations. The bending stiffness of the cracked section was modeled via massless rotational spring model, and finally, differential quadrature technique was utilized to extract both linear and nonlinear natural frequencies of the ruptured GPLRNC beams   [25,[217][218][219][220]. Nguyen et al. [39] proposed an effective numerical method to investigate and regulate geometrically nonlinear responses of the FGP plates consolidated with GPLs integrated with piezoelectric segments. The methodology was the utilization of iso geometric analysis (IGA) based on the Bezier extraction and the C zero type HSDT (C 0 -HSDT). By using the Bezier extraction, the original nonuniform rational B-Spline (NURBS) control meshes were converted into the Bezier elements, which lead to receive the standard numerical process such as the finite element method (FEM). The mechanical shift field was estimated based on the C 0 -HSDT, whereas the electric potential was supposed to be the function of the thickness of every single piezoelectric sublayer. The FG plate contains the inner pores and GPLs distributed in the matrix either uniformly or nonuniformly rendering distinctive patterns along the thickness of the plate. Moreover, to manipulate dynamic feedbacks, two piezoelectric layers were attached to the top and the bottom faces of the plate. The geometrically nonlinear equations were solved by the Newton-Raphson iterative technique and the Newmark's time integration scheme. Furthermore, a steady shift and velocity response control methods were implemented to effectively monitor both nonlinear dynamic and static feedbacks of the plate. With the help of this strategy, structural damping, based upon a closed loop control with piezoelectric instruments, was scrutinized. Set of tables below has been included to categorize various modeling of nanocomposites, which have been extensively discussed above. Distinctive categorization has been done for different nanocomposite types. To cite an example, Table 21 illustrates the modeling  parameter nanocomposite beams. Similarly, consequent  Tables 22 and 23 illustrate modeling parameters for nanocomposite plates and for nanocomposite shells, respectively.
Categorization of nanocomposite in terms of analysis model, mathematical model, numerical approach, equations derivation method, and computational algorithm (if used in the study). Table 22 depicts the modeling classification of nanocomposites plates.
Grouping of nanocomposites in terms of analysis model, mathematical model, numerical approach, equations derivation method, and computational algorithm (if used in the study). Table 23 depicts the modeling classification of nanocomposites shells. Table 24 includes the summary of the key assumptions and the main distinctions of mechanical models, which have been previously discussed in Tables 21-23 in detail. -High fiber volume fractions Review of fabrication, preparation and application of FGP nanocomposite materials  353 Table 19:

Continued
Schematics of filament winding process [180,181] Schematics of fiber placement technology [177,182] (Continued) Review of fabrication, preparation and application of FGP nanocomposite materials  355 Table 19:

Conclusion
The research analyzed the synthesis, processing, preparation, and fabrication as well as elastic properties of FGP materials, along with the mathematical, numerical, and computational modeling. Manufacturing processes of every material differs naturally. One thing should be emphasized is that every material type including ceramics, metals, and polymer has a different processing technique for relative porous structures. However, the final fabrication method of both porous and nonporous structures is still the same after the raw material has been obtained via preprocessing. Additionally, sol-gel processing was found to be most popular processing technique for both porous and nonporous structures and it can be utilized for every type of nanocomposites (ceramic, metal, polymer). Metal nanocomposite structures are costly due to high temperature applications and production, whereas polymers are the most famous and chosen material for engineering application unless the application is an extreme temperature application. The addition of nanoreinforcing agents to the above-mentioned materials takes place during the synthesis and preparation, which were proven to enhance the mechanical and materialistic properties of the assemblies. The effect of nanofillers of nanofillers were discussed and classified. The research highlighted the importance of nanofiller parameters on the elastic properties of the functionally graded nanocomposites. The tendencies found are as follows: • Increasing the nanofiller composition in the structure increases the impact toughness of the nanocomposite and C/MoS 2 Novel electrode material for supercapacitors [191] Ag/carrageenan-gelatine hybrid hydrogel Biological applications including tissue engineering, regenerative medicine, antimicrobial, anticancer, and drug delivery [192] Fluoroalkyl end-capped oligomeric Biomedical, pharmaceutical, coatings, electronics, optics, and diagnosis [193] Polyhedral oligomeric silsesquioxanes (POSS)based polyamide Thermosets, thermoplastics, drug delivery, solid polymer electrolytes, membrane applications (desalination, gas separation), food packaging, and automobile (fuel tanks) [194] POSS-based biocompounds Dental applications, drug delivery, and tissue engineering [194]  Honeycomb-like porous zinc carbodiimide-based nanocomposites Electronic applications. Used to manufacture asymmetric supercapacitor cells [205] High porosity-reduced graphene oxide/NiCo 2 S 4 Electrode materials for electrochemical applications (reduced graphene oxide being anode active and NiCo 2 S 4 being cathode active material) [206] Multifunctional Fe 3 O 4 /N 2 doped-porous carbon nanocomposite Used as catalyst during purification/separation reactions. Used in water treatment and medicine [207] Porous FeMnO Supercapacitor electrode applications [208] Copper-porous silicon (Cu/PSi) Can be used as an electrode as well as sensor for identifying formaldehyde during electrochemical applications [209] Review of fabrication, preparation and application of FGP nanocomposite materials  357   • Increasing the number of layers in laminated (sandwich) structures enhances the stress distribution. • The physical proportion as well as the geometrical shape of the nanofiller greatly influences the bending properties. Increasing the height-over-length ratio of rectangular nanoporous filler increases deflection, increasing the thickness-over-length ratio of rectangular nanofiller decreases stiffness as well as deflection resistance.
The studies conducted to analyze various properties of porous nanocomposites includes broad mathematical and mechanical modeling assumptions. Modeling review highlighted some worth mentioned tendencies, which are specified below.
• Halpin-Tsai approach is the most common modeling type utilized in many applications including beams, plates, shells for many kinds of analysis including stress distribution, static bending, thermal buckling, thermoelastic, shear strain, vibrational, bending stiffness, and frequencies.
○ Rule of mixtures for composites, GRF, and micromechanical models are the methods used along with Halpin-Tsai. • The most common numerical method used in FGM sector is FEM among reviewed studies. • HSDT and FSFDT were the most common equation derivation methods.
○ In some cases, Chebyshev-Ritz method, total energy function, two-plate theory, energy functional method, 3D elasticity with state-space method, Karman type geometric nonlinearity, and Bezier extraction were used as well.
○ VDQ and GDQ approaches were the most common techniques to solve derived equations. Hamilton's principle Assumes a system, which obeys Newtonian route and changes state from time to time [276] Suitable for vibrational analysis. Used to derive elasticity and dynamic equations Rule of mixtures Utilized to predict properties of composite materials. Assumes properties of composites are functions of the volume-weighed mean of the matrices or distributed phase's properties [277] Halpin-Tsai Utilized to estimate the elastic properties of composites by considering the topography and the arrangement of the filler (reinforcement) and the composite matrix. This method relies on Hartree-Fock system, which is also known as selfconsistent system [278,279] Mori-Tanaka An efficient field theory, which relies on Eshelby's elasticity approach for both inhomogeneous and infinite media. This method can be utilized to deduce mean internal stresses within a composite structure, which has inclusions due to strain variation. To be able to calculate the modulus of the nanocomposites, both matrix and the filler are required to have 3D elastic parameters [279] Runge-Kutta An efficient strategy to resolve the initial value problems of DE. This approach can be utilized to develop higher order numerical method without requiring high-order derivatives [280] GDQM Used to solve governing equations obtained from the mechanical modeling [281] Produces efficient solutions especially for vibrational studies FSDT Used to perceive the impact of shear deformation on the structures. Assumes shear strain is constant throughout the thickness. Suitable for thinner structures [282] HSDT Used to perceive the impact of shear deformation on the structures. Suitable for analysis of both thick and thin structures • Pseudo-arc length continuation algorithm, mesh-free strategy, layer-wise analytical approach, and MD approach were computer-aided methods used for analysis. • NURBS control meshing optimization technique can be used along with iso geometric numerical method to control the nonlinear response of piezoelectric in FGP nanocomposite plates. Apart from modeling and nanofiller properties, some other vital parameters were found that can greatly influence the operational lifetime and properties of the nanocomposites. These factors are given below.
• Elevated temperature and moisture led to the decrease in stiffness, natural frequency, and critical buckling load of porous nanocomposites. • Functional grading of the core decreases deflection, which decreases flexibility. • Elastic properties of the structure are greatly dependent on the CNF content distribution along the thickness direction of the assembly. • Thermal residual stresses occurred during manufacturing, due to shrinkage and high temperature difference between operations, decreases the overall operational lifetime performance by increasing the interlaminar stresses. • FGP nanocomposites with higher CNF content exhibits better flexural properties especially, higher stiffness.
○ Structures with higher CNF content have more tolerance to bending (flexible); however, toughness of such structures is naturally lower. • Nongraded porous nanocomposites exhibit higher fracture load. • Flexural strength and modulus are highly dependent on the direction of the exerted load.
○ Spherical nanoparticles offer higher flexural strength. ○ Nanorods offer higher flexural modulus to the structure.
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