Metalotermik redüksiyon ile molibden disilisit bileşimlerinin üretilmesi
Production of molybdenum disilicide compounds via metallothermic reduction method
Yazar: LEVENT DEMİRCİ
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bu yüksek lisans tez çalışması çerçevesinde Molibden disilisit alaşımlarının
geleneksel üretim yöntemleri dışında metalotermik redüksiyon yöntemi kullanırak
üretilmesi amaçlanmıştır. Böylece geleneksel üretim yöntemlerine kıyasla daha kısa
sürede çok daha basit üretim prosesleriyle üretilmesine imkan sağlayacağı
öngörülmüştür.
Tez çalışması kapsamında hedeflenen nihai ürün bileşimine ulaşmak amacıyla ilk
aşamada yanma sentezlerinden, kendinden ilerleyen yüksek sıcaklık yanma sentezi
yöntemi (SHS) kullanılmıştır. Bu yöntemde hedeflenen nihai ürüne ulaşabilmek
amacıyla oksit tozları ve redükleyici olarak seçilen metalin toz formunu içeren karışım
stokiyometrik oranlarda hazırlanmıştır. Karışımın bir pota içerisine yerleştirilip
tetiklenmesi ile aşırı ekzotermik reaksiyonun başlaması sağlanmıştır. Karışım üzerinde
bir dalga şeklinde ilerleyen reaksiyon çok kısa sürede tamamlanmasıyla istenilen
ürüne ulaşılmasına imkan vermektedir. Böylelikle klasik üretim yöntemlerinin uzun
işlem süresi, maliyetli ekipman gereksinimleri ve çevresel zararları gibi dezavantajları
elimine edilebilmektedir. Deneysel çalışmaların iİkinci aşamasında ise redüktant
olarak kullanılan Alüminyumun yapıya geçip metal kazanım oranlarını düşürmesi
sebebiyle daha az miktarlarda kullanılıp hacim yanma sentezi (VCS) yöntemiyle hedef
bileşime ulaşılması amaçlanmıştır. Bu yöntemde de oksit tozları ve redükleyici metal
tozları sitokiyometrik oranlarda hazırlanmıştır. Hazırlanan tozlar pota içerisine
yerleştirilip 800 0C de 1 saat boyunca tav fırını içerisinde bekletilerek reaksiyonun
gerçekleşmesi sağlanmıştır.
Bu çalışmada Molibden disilisit alaşımlarının metalotermik yöntem ile üretimi
gerçekleştirebilmek için çalışmalar yapılmıştır. Yapılan deneyler çerçevesinde
reaksiyona giren toz miktarı ve oranları FactSage 6.4 termokimyasal simülasyon
programı ile hesaplanmıştır. Bu sayede oksit bileşenler ve redükleyici metal istenilen
kompozisyonda son ürüne ulaşmak amacıyla daha doğru olarak hazırlanabilmiştir.
Reaksiyonun adyabatik sıcaklığı, redükleyici miktarının nihai ürüne etkisi ve
stokiyometrik hesaplamalar simülasyon program ile hesaplanmıştır.
Termodinamik hesaplamalar sonucunda Molibden disilist oluşumu için planlanan
reaksiyonun 298 K'deki entalpi değeri (ΔH298) -372kJ olarak tespit edilmiştir.
Adyabatik sıcaklık değeri ise 2423 oC olarak hesaplanmıştır ve bu değer reaksiyonun
kontrollü olarak ilerleyebileceği sınır değerler aralığında olduğu görülmüştür. Bir
diğer parametre olan spesifik ısı değeri (SI) hesaplamalar sonucunda 2293,6 J/g olarak
tespit edilmiştir. Elde edilen termodinamik verilerden hareketle metalotermik
uygulama şartları sağlandığı belirlenmiştir.
Deneylerde molibdenli hammadde olarak molibden oksit (MoO3), tungstenli
hammadde olarak tungsten oksit (WO3), silisyum kaynağı olarak metalik silis veya
silisyum oksit (SiO2), redükleyi olarak ise Alüminyum (Al) kullanılmıştır.
Gerçekleştirilen deneylerde her biri 150 gr olacak şekilde farklı sitokiyometrilerde
şarjlar hazırlanmıştır. SHS deneylerinden reaksiyon öncesi seramik potalar 20 dakika
boyunca 500 °C de ön ısıtmaya tabi tutulmuştur ve sonrasında tetikleyici ısıyı
sağlamak amacıyla hazırlanan şarjlara akım verilerek reaksiyonların başlaması
sağlanmıştır. VCS deneylerinde ise hazırlanan şarjlar pota içerisine yerleştirilmiştir ve
tav fırını içerisinde 1 saat boyunca 800 0C de bekletilerek reaksiyonun başlaması sağlanmıştır. Bu çalışmalar ile ticari bileşime yakın Molinden disilisit bileşimleri elde
edilmeye çalışılmıştır.
SHS ve VCS uygulamaları sonucunda elde edilen ürünlere karakterizasyon çalışmaları
amacıyla XRD analizleri uygulanmıştır.

Abstract:
The aim of this study was producing molybdenum disilisicide alloys by metallothermic
reduction via combustion synthesis such as VCS (volume combustion synthesis), SHS
(self propagating high temperature synthesis) which has many advantageous such as
considerably energy consumption, lower environmental effect, simple equipments and
low time requirement when compared with traditional production methods such as arc
remelting or mechanical alloying.
Many important properties of molybdenum disilicide such as high melting
temperature, relatively low density and high oxidation resistance, makes it's a proper
material for various industrial areas. Molybdenum disilisicide and molybdenum
disilicide based composites have many usage areas in various industries such as
heating elements, diesel engine plugs, industrial metal lances and glass industry. While
only molybdenum disilicide and it's different lattices can be found alone in some
applications such as heating elements, it can be alloyed with other compounds such as
Si3N4 in engine applications or it can be alloyed with Al2O3 in molten metal lance
industries. The first commercial usage of molybdenum disilicide is heating element
and its patented by Kanthal which is Sweden based heating element producer
company. The other application area of molybdenum disilicide is coating purpose and
it gives high corrosion resistance to the material which is coated by molybdenum
disilicide. Apart from that, high oxidation resistance at high temperatures of
molybdenum disilicide makes it's a proper material for aviation industry.
Metallothermic reduction is an alternative process to the conventional carbothermic
reduction smelting which is generally endhothermic and which requires external heat
to initiate to process. However, metallothermic is an exothermic reaction and doesn't
require any external heat to initiate reaction unlike the carbothermic process.
Therefore, metallothermic reaction doesn't require any external heat and it's process
is based on basic steps. Moreover, thanks to it's exothermic nature, it decrease thermal
energy input to produce liquid metal in reaction. As previously mentioned,
carbothermic process requires high power rating furnaces and these furnaces require
high capital expenditure. In contrast, metallothermic reactions use the energy which is
occurred by exothermic nature and the reaction generates adequate heat energy to
propagate to completion without any additional heat input post for ignition.The
additional energy required to promote self-propagation of metallothermic reaction can
be obtained through several methods. In certain cases, the reactants are preheated to
the relevant temperature to effect self-propagation. In other instances the use of highly
oxidic compounds which are simultaneously reduced with the charges are introduced
to the bulk charge. These oxidic compounds are commonly termed reaction boosters
as they generate high energy per mol reductant and therefore boost the overall reaction
enthalpy.
There are many drawbacks in producing of refractory non-oxygen and oxide
compounds and materials, semiconducting compounds, hybrides, intermetallics such
as enviromental pollution, high capital expenses, complex reactors and consuming
natural sources to generate heat. However, self propogating high temperature synthesis
has eliminated above problems owing to it's exothermic nature. One of major features
of SHS process is high purity which is obtained during sythesis by using all reagents.The technical potential of SHS is determined by the variety of the materials used and the
possibility of specific combinations of reactants and products not available for other
methods. The chemical nature of the reactions of combustion used for the synthesis is also
variable.
In combustion sythesis reactions ; VCS and SHS, there are various reaction paramateres
which can effect reactions and it's propagation rate.Establishing the optimum reaction
parameters for synthesizing a material is based on obtaining a fundamental understanding
of the controlling reaction mechanisms in each SHS reaction system. Two of major
parameteres are specific heat of reaction and thermal conductivy of mixture. Theoretically,
increasing thermal conductivity of the mixture will increase mixture's burning rate and
efficiency. The greated thermal conductivy provides more rapidly ignition and propagate
from the reaction zone to unreacted area. In case of greater specific heats, the greater the
heat required to raise the next layer of composition to the ignition temperature.The
temperature of the reaction zone is reduced, and the burning rate lowered. At highest
thermal conductivity and heat capacity, ignition difficultly is achieved, and normally an
increased intensity or duration of the ignition stimulus is required.
In this study, many experimental studies have been completed to produce aimed
heating element which is based molybdenum disilisicide and it's alloy with tungsten
disilicide by metallothermic reduction. In the scope of experimental studies, reactant
powder quantities are calculated through FactSage 6.4 thermochemical simulation
programme to obtain reaction's specific heats and adiabatic temperatures. Thus,
correct amount of reactans are prepared to reach desired end product's chemical
composition. Adiabatic temperature, reductant material effects on end product and
stoichiometric calculation are simulated through with the programme. Oxide forms of
raw materials and reductant metals are prepared as powder form with specific
stoichiometries in order achieve aimed chemical composition of heating element. At
the first stage of experimental studies, SHS (self propoagating hight temperature
synthesis) method is applied to reach aimed material's chemical composition. Mixture
charged were placed in a ceramic ladle and ignited by an ignition is supplied by a
battery and extremely exothermic reaction has initiated. Reaction is spreaded on mixed
charge and finished in very low time. Thus, desired product is achieved in a short time
period and many disadvantegous of conventional methods such as high energy
requirement, environmental effect, complicated equipments and high cost were
eliminated. However, highly exothermic character of SHS reactions, many spreaded
material problems were occurred, caused end product loss and metal distrubition rates
are decreased in final product. Therefore, at the second stage of experimental studies,
VCS (volume combustion synthesis) method is applied and less reductant powders are
used in experimental studies in order to eliminate excess aluminium which will cause
a decrease in metal recovery rates by applying to final product and which will cause
high specific heat and metal splashes. Thus, less splashed materials are aimed and high
metal recovery rates are aimed. In these methods, charge powder are prepared
according to calculated sitociometries by Factsage 6.4 and placed in a ceramic can and
left it to 800 0C for one hour in a furnace.
According to results of thermodynamic calculations, enthalpy value for the first
reaction is identified as -4,39x105 at 298 Kelvin and adiabatic reaction is identified as 2305 oC, enthalpy value for the second reaction identified as -4,17x105 and adiabatic
reaction is identified as 1998 oC, enthalpy value for the third reaction identified as -
5,56x105 and adiabatic reaction is identified as 2060 oC and enthalpy value for the
fourth reaction is identified as -7,56x105 and adiabatic reaction is identified as 2444 oC.
According to above thermodynamic datas, reactions cover requirements of
metallothermic application conditions and these reactions are suitable to iniate by
themselves and propagate.
In experimental studies, molybdenum oxide (MoO3) as molybdenum source, tungsten
oxide as tungsten source (WO3), metallic silicon or silicon oxide (SiO2) and
Aluminium (Al) as reductant are used.
XRD analysis are applied to end product which is obtained after metallothermic
reactions under characterization studies.