R. Orrù - Academia.edu (original) (raw)

Papers by R. Orrù

Eurasian Chemico-Technological Journal, 2021

The fabrication by Spark Plasma Sintering (SPS) of bulk high entropy ceramics from powders obtain... more The fabrication by Spark Plasma Sintering (SPS) of bulk high entropy ceramics from powders obtained by Self-propagating High temperature Synthesis (SHS) is addressed in this work. The effect produced by the introduction of 1 wt.% of graphite to the powders before SPS is investigated under different temperature conditions. The final density and composition of sintered (Hf0.2Mo0.2Zr0.2Ti0.2Ta0.2)B2 and (Hf0.2Mo0.2Zr0.2Ti0.2Nb0.2)B2 ceramics are found to be negatively affected by the presence of oxide impurities in the powders. While product composition can be progressively improved when the temperature is increased from 1800 to 1950 °C, residual porosities remain relatively high if using additive-free powders. In contrast, the introduction of 1 wt.%C markedly allows for oxides elimination by carbothermal reduction mechanism. Products consolidation is correspondingly enhanced so that relative densities of about 97% are attained. Other than the latter effect, surface oxides removal also...

Eurasian Chemico-Technological Journal, 2012

Space colonization and exploitation of extra-terrestrial natural resources could help humanity in... more Space colonization and exploitation of extra-terrestrial natural resources could help humanity in facing various Earth problems. In this regard, production of energy and materials starting from Moon and Mars natural resources as well as the transportation of humans in space could be considered the long term remedy to issues such as overpopulation, depletion of fossil fuels, climate change as well as reduction of available natural resources. Along theses lines, two recently filed patents related to use of novel technologies for the in situ exploitation of natural resources available on Moon and Mars have been developed.

Ceramic transactions, Jul 21, 2010

Scripta Materialia, 2019

A single high-entropy phase material with hexagonal structure is produced by a two-steps processi... more A single high-entropy phase material with hexagonal structure is produced by a two-steps processing method. Elemental reactants are first remarkably converted by Self-propagating High-temperature Synthesis (SHS). The completion of the chemical transformation to the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase and its concurrent consolidation up to 92.5% relative density is achieved by processing the SHS powders at 1950°C via Spark Plasma Sintering. It is clearly evidenced that the use of the SHS technique is extremely beneficial to promote the formation of high-entropy ceramics, as compared to the time consuming ball milling treatment alternatively adopted.

Journal of the European Ceramic Society, 2017

Hydroxyapatite-based composites (HA-C) with bioglass as second phase are usually produced by hot-... more Hydroxyapatite-based composites (HA-C) with bioglass as second phase are usually produced by hot-pressing or pressureless sintering. However, using such methods 2 thermal levels which exceed the crystallization temperature of the glass are generally required, with possible negative effects on the bioactivity of the final system. Spark plasma sintering (SPS) is a powerful consolidation technique in terms of both processing time and temperature. In this work SPS has been employed, for the first time, to obtain HA-C with an innovative bioglass as second phase. Such glass was specifically designed to be used whenever a thermal treatment is required, thanks to its low tendency to crystallize. A systematic study is conducted in order to identify the optimal sintering conditions for preparing highly dense composites and, at the same time, to minimize the crystallization of the glassy phase. The obtained samples are highly bioactive and display higher compactness and hardness with respect to the counterparts produced by conventional sintering methods.

Ceramics International, 2016

Abstract Dense and crack free six-layered functional graded materials were successfully produced ... more Abstract Dense and crack free six-layered functional graded materials were successfully produced by Spark Plasma Sintering by combining 3 mol% Y 2 O 3 -partially stabilized ZrO 2 (3Y-PSZ) and 316L stainless steel. All the sintered products consisted of a steel free layer on one side and a cermet composite containing 50 vol% of both constituents on the opposite side. Conversely, the stainless steel concentration in the interlayers was progressively changed following diverse spatial profiles. It was found that the temperature interval from 1080 to 1180 °C required for the full consolidation from the 50 vol% composite layer to the 3Y-PSZ one, respectively, can be reached when adopting a specific die configuration where the cross section was varied from 30 to 28 mm, respectively. Correspondingly, the densification level of each layer, as well as the related hardness and fracture toughness properties, were highly enhanced with respect to the standard cylindrical die. In addition, a significant improvement of the material toughness was obtained when the material concentration exponent was decreased from 2 to 1, whereas this effect tends to vanish when such parameter was further reduced to 0.5.

The synthesis of dense NiTi starting from elemental powders was investigated utilizing the field-... more The synthesis of dense NiTi starting from elemental powders was investigated utilizing the field-activated, pressure-assisted technique. The effect of current intensity, time, and reactant particle size on the characteristics of the product was investigated. Reaction between Ni and Ti did not result in the complete conversion to the desired single-phase product (NiTi). The product contained NiTi 2 , Ni 3 Ti, and the metastable phase Ni 4 Ti 3. However, the number and the amounts of these secondary phases were significantly reduced with an increase in the applied current and synthesis time, or the use of smaller reactant particle sizes. A high degree of conversion was achieved in a relatively short time (20 min), with a current in the range of about 1300-1500 A (4585-5290 kA/m 2). Under these conditions, the dense product (99% relative density) contained NiTi as the major phase with small amounts (5.7-7.8 mol%) of NiTi 2 .

Ceramics …, 2009

In this work, the fabrication of bulk TiC 0.7 /TiB 2 nanostructured composites through metastable... more In this work, the fabrication of bulk TiC 0.7 /TiB 2 nanostructured composites through metastable transformation processing is investigated by taking advantages of two non-conventional powder metallurgy methods. First, the highly metastable TiC 0.7 /TiB 2 agglomerated powders are synthesized by the so-called self-propagating high-temperature synthesis (SHS), followed by rapid quenching. Then, the spark plasma sintering (SPS) method is adopted to consolidate the SHSed powders. A bulk ceramic composite with nanocrystalline microstructure characterized by a high-relative density is then obtained. Dwell temperature of 1400 8C, heating time of 3 min, and total processing time equal to 5 min, while applying a mechanical pressure of 20 MPa, are found to be the optimal SPS experimental conditions in order to obtain near-fully densified samples. The obtained TiC 0.7 /TiB 2 samples exhibit hardness HV5 as high as 24 GPa, modulus of elasticity of about 400 GPa, fracture toughness of about 5.6 MPa m 1/2 , and a compressive strength of about 2.9 GPa. A very low-wear rate (W v = 3.8 Â 10 À6 mm 3 /(N m)) and a good thermal shock resistance (DT c = 250 8C) are also displayed. In addition, a high-abrasive wear factor (AWF) equal to 1.84 is evaluated on the basis of the achieved mechanical properties. These results make the obtained TiC 0.7 /TiB 2 composite suitable for wear resistant parts as well as cutting tool materials.

Journal of the European Ceramic Society

ABSTRACT The effect of the heating rate during reactive SPS to obtain dense ZrB2 from elemental r... more ABSTRACT The effect of the heating rate during reactive SPS to obtain dense ZrB2 from elemental reactants is investigated. When the temperature is increased at 500 °C/min or faster, the synthesis reaction evolves under combustion regime. In contrast, if heating rates equal or lower than 200 °C/min are considered, a gradual solid-state diffusion mechanism is established. In both cases, the synthesis reaction goes to completion during the non-isothermal heating step, although samples have to be processed for 20 min at 1850–2000 °C and 50 MPa to reduce residual porosity. Despite of the combustion synthesis route allows to obtain pure dense products at milder temperature conditions, safety problems and product inhomogeneity make its exploitation not convenient. On the other hand, safety conditions are preserved during the gradual formation of ZrB2 by SPS. Correspondingly, 96% dense monolithic products can be obtained at temperature levels of about 2000 °C within 20 min dwell time.

Processing by Centrifugation, 2001

Eurasian Chemico-Technological Journal, 2013

In this work a brief overview of the most important technologies for space exploration, with part... more In this work a brief overview of the most important technologies for space exploration, with particular emphasis on the Moon missions, is presented. It is shown that the focus has been on the technologies to extract consumables (O 2 , H 2 O, N 2) for human life-support replenishment. The fact that the exploitation of extraterrestrial resources to obtain the desired materials during each ongoing mission, which has been the subject of several investigations since the sixties of the last century, is discussed. The paradigms ISRU (In Situ Resources Utilization) and ISFR (In Situ Fabrication and Repair) are then introduced. In particular, one of the most important process for the production of oxygen, i.e. the reduction of ilmenite by hydrogen is analyzed. In addition, the current iteration of the roadmap which identifies two feasible pathways for human missions after ISS (International Space Station) is addressed. Next, the fabrication of Lunar physical assets is taken into account, while focusing particularly on those processes where combustion-like reactions are exploited. The main results recently obtained in the literature in this regards are also summarized. In particular, the choice of the reducing agent and the influence of the most important processing parameters (composition of the starting mixture, gas pressure level, and gravity conditions) are examined in a systematic manner.

Journal of the European Ceramic Society, 2015

Sensors and Actuators B: Chemical, 2008

SrTi0.4Fe0.6O2.8 (STFO60) powders were prepared by self-propagating high-temperature synthesis (S... more SrTi0.4Fe0.6O2.8 (STFO60) powders were prepared by self-propagating high-temperature synthesis (SHS) and subsequent ball-milling (BM) treatment, and then deposited on an interdigited alumina substrate by screen-printing to fabricate a planar resistive oxygen sensor for direct injection (DI) diesel engines.The electrical characteristics and response to oxygen of the STFO60 sensor have been carefully examined first in a bench system with synthetic gases

Ceramic Transactions, 2010

Page 1. FABRICATION OF DENSE Zr , Hf AND Ta BASED ULTRA HIGH TEMPERATURE CERAMICS BY COMBINING SE... more Page 1. FABRICATION OF DENSE Zr , Hf AND Ta BASED ULTRA HIGH TEMPERATURE CERAMICS BY COMBINING SELF PROPAGATING HIGH TEMPERATURE SYNTHESIS AND SPARK PLASMA SINTERING Roberta ...

Zeitschrift für Naturforschung A, 1997

In this work we investigate the mechanism of product formation of the aluminothermic reaction of ... more In this work we investigate the mechanism of product formation of the aluminothermic reaction of Fe2O3 in the presence of Al2O3 using Ball Milling and Self-propagating High-temperature techniques. Results obtained by experiments under either argon or air atmosphere are analysed by X-ray diffraction and Mössbauer spectroscopy, together with microstructure observations. It is shown that ball milling products are strongly affected by the kind of atmosphere, while self-propagating high-temperature ones are only weakly influenced. Reaction mechanisms taking place in these cases are proposed. While ball milling involves only solid state reactions, the formation of a melt occurs under self-propagating high-temperature conditions.

A series of SrTi 1-x Fe x O 3-δ (STO or STFO) powders, with x ranging from 0 to 0.6, were prepare... more A series of SrTi 1-x Fe x O 3-δ (STO or STFO) powders, with x ranging from 0 to 0.6, were prepared by Self-propagating High-temperature Synthesis (SHS) starting from SrO 2 , Ti, TiO 2 and Fe. A ball-milling (BM) treatment was subsequently carried out for further structure refinement and size reduction. Morphological and microstructural characteristics of both untreated and ball-milled SHS samples were investigated by XRD and SEM. Screen-printed SrTi 1-x Fe x O 3-δ SHS powders on alumina substrate were investigated for resistive oxygen sensors operating at high temperature (450-650 °C). STFO powders have shown better performances with respect to the corresponding undoped STO ones. Moreover, when STFO powders were subjected to BM treatment, both temperature-independent resistance characteristics and sensor response greatly improved. On the basis of the reported data, it can be suggested that the ball milling treatment likely acts: i) stabilizing the formation of non-equilibrium structures; ii) decreasing the particle size, increasing surface defect and hence surface reactivity; iii) favouring the substitution of titanium by iron in the SrTiO 3 perovskite structure.

The research described in this report will increase the understanding of fundamental aspects of s... more The research described in this report will increase the understanding of fundamental aspects of self-propagating combustion synthesis processes and synthesis of materials that cannot be formed under normal gravity conditions. Self-propagating combustion synthesis processes involve several stages that are significantly affected by gravity: melting of reactants and products, spreading of the melt, droplet coalescence, diffusion and convection, buoyancy of solid